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The Happening of Evolution

Is evolution a fact? In the previous post, we noted that a ‘fact’ in science is not apodictic, but always provisional—it may always be confounded by a new discovery.  Following Ruse’s proposition, we considered the three-fold division of evolution into (1) its happening, (2) its paths and (3) its mechanism. If each of these (admittedly interdependent to differing extents) elements can be satisfactorily established, then, yes, we may declare evolution a fact beyond reasonable doubt.

Darwin’s Theory

Darwin presented his theory of evolution as ‘one long argument,’[1] not in a dense, unwieldy tome solely within the purview of the cognoscenti, but in a work of popular science, which could be followed by the lay reader. In this work, the Origin of Species, Darwin sought to marshal evidence from a variety of disciplines in order to establish a Whewellian consilience of inductions: his theory of evolution explicating the evidence; the evidence, in turn, pointing to his theory.

The idea [of a consilience] is that somehow, if a hypothesis is true—tells us about the real world—then various facts or other claims follow from it, and will keep doing so. And there is a kind of feedback process here. As the hypothesis leads to new information, so its derivations themselves confer a kind of probability upon the hypothesis.[2]

It would help to briefly consider what Darwin’s theory actually was. According to Darwin, species were not immutable, but were the product of a gradual process of evolution, the primal (though not sole) cause of which was natural selection. Today, his theory is commonly referred to as ‘evolution by natural selection’; Darwin referred to his theory as ‘descent with modification’. Philosopher of biology Elliott Sober has argued that these characterisations do not accurately convey the essence of Darwin’s theory suggesting that Darwin’s theory be referred to as ‘common ancestry plus natural selection.’[3] This characterisation acknowledges the relative indepdendence of each of the two ideas, an independence which can be appreciated historically, for whilst Darwin did much to convince the scientific community of the first idea, he failed to convince even some of his closest ‘disciples’ of the second.[4] Nevertheless, the thesis of common ancestry is absolutely integral to Darwin’s theory. If common ancestry can be demonstrated, then species cannot be immutable and one is forced to propose a mechanism to account for the process.

Sober and Orzack note that ‘the best evidence of common ancestry comes from neutral or even deleterious features.’[5] As Gould frankly puts it, ‘ideal design is a lousy argument for evolution.’[6] According to an evolutionary hypothesis, adaptation is the result of natural selection. But if adaptation is perfect, then an evolutionary hypothesis becomes less likely, as perfect adaptation is more suggestive of intelligent design. If, on the other hand, adaptation is not perfect, or if there are non-adaptive or maladaptive traits, then a hypothesis of intelligent design becomes less likely and an evolutionary hypothesis more likely. As Stephen Jay Gould writes, ‘you cannot demonstrate evolution with perfection because perfection need not have a history.’[7] However, ‘if organisms have a history, then ancestral stages should leave remnants behind.’[8] History is revealed by ‘the useless, the odd, the peculiar, [and] the incongruous’.[9] Natural selection effaces the marks of history. This is what makes evolutionary convergence (the evolution of similar features in independent lineages) such a nuisance for phylogeneticists. Is the feature shared by two groups homologous (due to common ancestry), in which case a close phylogenetic relationship is inferred? or is it due to convergence, in which case it would be a mistake to infer a close phylogenetic relationship?[10] Thus, ‘The more a trait’s distribution can be explained solely on the basis of natural selection, the less evidence the trait will provide for shared ancestry.’[11] One must look, then, to those non-adaptive or maladaptive traits in order to prove the happening of evolution.

Morphology

Homology provides perhaps the most striking confirmation of evolution. Darwin was convinced that the exquisite adaptations which pervade the living world were the result of natural selection ‘daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good’,[12] yet natural selection can only work with what is to hand. An intelligent designer could presumably fashion a novel structure of whatever materials required, but such a luxury is denied to selection. Selection can only modify existing structures, making do with whatever is to hand, as it were: ‘the parts may change to almost any extent in form and size, and yet they always remain connected together in the same order.’[13] Homologies are testament to common ancestry. Unlike convergences, ‘Entities are homologous if, in principle, they can be traced back to a single genealogical precursor.’[14]

Several homologous vertebrate forelimbs

Looking within our own subphylum, the vertebrate forelimb presents a striking example of homology. If you take a look at the diagram above, you can see that the limbs of humans, dogs, whales and birds are all variations on a common theme. One can see the humerus, the radius and ulna of the lower arm, as well as the digits (reduced to three in birds). Whilst the limb in each species is adapted for a particular function, it is apparent that the isomorphisms are due to the inheritance of the structure from an ancestor common to all four species. Homologies such as these only make sense in the light of evolution, and that is what makes them such good pieces of evidence. As Darwin wrote, uttering a common refrain throughout the Origin, ‘On the ordinary view of the independent creation of each being, we can only say that so it is; – that it has so pleased the Creator to construct each animal and plant.’[15]

Biogeography

Just as only ‘descent with modification’ can adequately explain homology, so can only evolution adequately explain the other classes of facts Darwin considered. To take a second example used by Darwin we look to biogeography; a subject to which he devoted two chapters in the Origin. This area provided perhaps the most cogent proof of evolution; both Alfred Russel Wallace and Joseph Hooker came to accept evolution on account of the geographical distribution of species. Biogeography was also cenral in Darwin’s conversion to and discovery of evolution. Richardson writes that ‘Ultimately, biogeography offered the only sufficiently elaborated body of knowledge on the relationship of species to their environment, and to changes in that environment, that could constitute a valid test case for Darwin’s developing theoretical speculations on transmutation and his search for an efficient cause of change.’[16] Why should it be that, despite highly similar environmental conditions prevailing in regions of distant continents the species that one finds there invariably bear close resemblance to the species of that area and not of the distant area?

If we look to the islands off the American shore, however much they may differ in geological structure, the inhabitants, though they may be all peculiar species, are essentially American…We see in these facts some deep organic bond, prevailing throughout space and time, over the same areas of land and water, and independent of their physical conditions.[17]

What could account for the similarity between neighbouring species if the environmental conditions appeared to be irrelevant? If one Australian species would have been just as well adapted to the environment of an American species, why should the two species be so dissimilar yet bear such peculiar affinities to the species of their homeland? ‘This bond, on my theory,’ wrote Darwin, ‘is simply inheritance, that cause which alone, as far as we positively know, produces organisms quite like, or, as we see in the case of varieties nearly like each other.’[18] It is migration that is responsible for the similarities between mainland and offshore species, and it is restrictions to migration which are responsible for the differences between species inhabiting identical environments on opposite sides of the globe. At the close of the second chapter on biogeography, Darwin writes, ‘I think all the grand leading facts of geographical distribution are explicable on the theory of migration…, together with subsequent modification and the multiplication of new forms.’[19]

Geographical distribution of Galápagos tortoises

Embryology

In a letter to Joseph Hooker, Darwin referred to the section on embryology as his ‘pet bit’[20] of the Origin. Darwin had written that ‘certain organs in the individual, which when mature become widely different and serve for different purposes, are in the embryo exactly alike.’[21]If one looks at, say, the early embryo of a human and the early embryo of a fish the two embryos are remarkably similar; even though the two go on to develop into very different organisms. And the early stages of the human embryo resemble the adult forms of ancestral forms. Darwin goes on to write that ‘The embryos, also, of distinct animals within the same class are often strikingly similar: a better proof of this cannot be given, than a circumstance mentioned by Agassiz, namely, that having forgotten to ticket the embryo of some vertebrate animal, he cannot now tell whether it be that of a mammal, bird, or reptile.’[22] As evolution proceeds, newer developmental instructions are layered on top of more ancient instructions, forming a kind of developmental palimpsest.

We cannot, for instance, suppose that in the embryos of the vertebrata the peculiar loop-like course of the arteries near the branchial slits are related to similar conditions,—in the young mammal which is nourished in the womb of its mother, in the egg of the bird which is hatched in a nest, and in the spawn of a frog under water. We have no more reason to believe in such a relation, than we have to believe that the same bones in the hand of a man, wing of a bat, and fin of a porpoise, are related to similar conditions of life… In two groups of animal, however much they may at present differ from each other in structure and habits, if they pass through the same or similar embryonic stages, we may feel assured that they have both descended from the same or nearly similar parents, and are therefore in that degree closely related. Thus, community in embryonic structure reveals community of descent.[23]

Molecular Genetics

Homology, biogeography and embryology provided three lines of evidence which Darwin wove into his consilience. A fourth line of evidence which was not available for Darwin but which provides eloquent confirmation of the happening of evolution comes from molecular biology. It is a striking fact that all organisms share the same genetic material. Not only do all organisms share the same genetic material, they also share the same genetic code. This is an instance of a ‘universal homology’.[24] A triplet code is written in the four-letter nucleotide alphabet of DNA codes for an amino acid (the constituent building block of a protein or polypeptide). For example, the triplet code GGC (guanine·guanine·cytosine) codes for the amino acid glycine. With a triplet code composed of four letters, there are 43 possible triplets to code for only 20 amino acids (as well as punctuation marks), so that there is a great deal of redundancy in the code, so that, for example, GGG (guanine·guanine·guanine) also codes for glycine. As Sober notes,

As long as there are multiple codes that each would work, a shared code is evidence for common ancestry. And the more such codes there are, the stronger the evidence that the near-universality of the code provides for common ancestry. This point holds even if the shared code we observe in the life around us turns out to be optimal.[25]

It is not inconceivable that different groups of organisms should have different codes, and for this reason the universality of the code (with its minor variants) testifies to the happening of evolution. The most satsisfying explanation for the universality of the code is descent with modification.

~

The pieces of evidence presented here have provided confirmation of the first element of evolution: its happening. Only if descent with modification—that is, evolution—has occurred can we satisfactorily account for the facts before us. The theory of evolution wields extraordinary explanatory power, accounting for a plethora of otherwise disparate facts. The happening of evolution can be known without knowing the causal mechanisms responsible. In the next post, I shall consider the second element of the theory of evolution: its paths.

[1] C. R. Darwin, On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life, (London: John Murray, 1859), p.459.

[2] M. Ruse, Darwinism and its discontents, (Cambridge: Cambridge University Press, 2008), p.38.

[3] E. Sober, ‘Did Darwin write the Origin backwards?’, Proceedings of the National Academy of Sciences, 106 (1): 10048-10055, (2009), p.10050.

[4] See P. J. Bowler, The non-Darwinian revolution: reinterpreting a historical myth, (Baltimore, ML: The Johns Hopkins University Press, 1988).

[5] E. Sober and S. Orzack, ‘Common ancestry and natural selection’, British Journal for the Philosophy of Science, 54: 423-437, (2003), p.427.

[6] S. J. Gould, ‘The panda’s thumb’, in The panda’s thumb, chap.1, (New York: W. W. Norton & Co., 1980), p.20.

[7] S. J. Gould, ‘Senseless signs of history’, in The panda’s thumb, chap.2, p.28.

[8] Ibid.

[9] Ibid.

[10] On evolutionary convergence, see S. Conway Morris, Life’s solution: inevitable humans in a lonely universe, (Cambridge: Cambridge University Press, 2003); also see http://mapoflife.org/index/.

[11] Sober and Orzack, ‘Common ancestry and natural selection’, p.428.

[12] Darwin, On the origin of species, p.84.

[13] ibid., p.434.

[14] M. Ghiselin, The triumph of the Darwinian method, (Mineola, NY: Dover Publications, Inc., 2006), p.110.

[15] Darwin, On the origin of species, p.435.

[16] R. A. Richardson, ‘Biogeography and the genesis of Darwin’s ideas on transmutation’, Journal of the History of Biology, 14 (1): 1-41, (1981), p.6-7.

[17] Darwin, On the origin of species, p.350.

[18] Ibid.

[19] Ibid., p.408.

[20] C. R. Darwin, Letter to Joseph Dalton Hooker, 14 December 1859, http://www.darwinproject.ac.uk/entry-2583.

[21] Darwin, On the origin of species, p.439.

[22] Ibid., p.439-440.

[23] Ibid., p.449.

[24] M. Ridley, The problems of evolution, (Oxford: Oxford University Press, 1985), p.10.

[25] Sober, ‘Did Darwin write the Origin backwards?’, p.10053.

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Wallace Online

Alfred Russell Wallace (1823-1913)

Charles Darwin, as everybody knows, discovered natural selection, presenting his theory of evolution in 1859 in the Origin of Species. And yet far fewer are aware of the co-discoverer of natural selection, Alfred Russel Wallace. Although the Origin was published in 1859, Darwin had discovered natural selection far earlier, late in the 1830s, soon after his return from the second voyage of HMS Beagle. Whilst Darwin’s discovery preceded Wallace’s, Wallace has been unjustly eclipsed by Darwin’s aggrandised figure. On 8 June 1858 Darwin received Wallace’s paper on selection. Darwin fretted when he first read through the young explorer’s outline of his theory, although it is suspected that Darwin read more into the paper than was actually there. Worried, Darwin wrote to his friend Charles Lyell, ‘if Wallace had my M.S. sketch written out in 1842 he could not have made a better short abstract!’ It was arranged, however, by Lyell and Joseph Hooker that Wallace’s paper should be read before the Linnaean Society along with excerpts from Darwin’s earlier drafts of his theory so that the two might receive recognition, although Darwin would receive rightful antecedence. The settlement was amicable, devoid of acrimony or bitterness. Wallace wrote to Hooker in the autumn of 1858,

I cannot but consider myself a favoured party in this matter, because it has hitherto been too much the practice in cases of this sort to impute all the merit to the first discoverer of a new fact or a new theory, & little or none to any other party who may, quite independently, have arrived at the same result a few years or a few hours later.

Wallace then goes on to say that

It would have caused me much pain & regret had Mr. Darwin’s excess of generosity led him to make public my paper unaccompanied by his own much earlier & I doubt not much more complete views on the same subject, & I must again thank you for the course you have adopted, which while strictly just to both parties, is so favourable to myself.

Some years later, Wallace wrote to Darwin of the theory of natural selection, ‘I shall always maintain it to be actually yours & your’s only.’

In a letter to Wallace in January 1859, Darwin wrote,

Permit me to say how heartily I admire the spirit in which they are written. Though I had absolutely nothing whatever to do in leading Lyell & Hooker to what they thought a fair course of action, yet I naturally could not but feel anxious to hear what your impression would be.

–adding graciously that

Everyone whom I have seen has thought your paper very well written & interesting. It puts my extracts, (written in 1839 now just 20 years ago!) which I must say in apology were never for an instant intended for publication, in the shade.

In later years, their views on evolution diverged. Wallace became a panselectionist, believing that all traits were the result of natural selection, whilst Darwin was far more of a pluralist, using his theory of sexual selection to explain such apparently maladaptive traits as the peacock’s flamboyant tail as having been preferentially selected by the peahen. Despite Wallace’s criticisms Darwin would ‘not give up’ sexual selection. Wallace also became interested in spiritualism and curiously asserted that the human intellect could not be the product of evolution. Darwin worried to Wallace, ‘I hope you have not murdered too completely your own and my child’. Whilst the two men drifted apart, they did not become embittered, however. In 1870, Darwin wrote to Wallace,

I hope it is a satisfaction to you to reflect,–& very few things in my life have been more satisfactory to me–that we have never felt any jealousy towards each other, though in one sense rivals.

~

This slight historical preamble leads me to the exciting news that, just last week, Wallace Online is making Wallace’s writings (including the 1858 selection paper) freely available. This project, directed by John van Whye (also director of Darwin Online), is greatly exciting, and will do much to reverse Wallace’s partial eclipse. Both the Darwin and Wallace resources are veritable treasure troves and the quality and volume of the scholarship is inspiring. I would seriously recommend that you head over to each, if you have not already done so.

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Is Evolution a Fact?

Evolution is a fact. Beyond reasonable doubt, beyond serious doubt, beyond sane, informed, intelligent doubt, beyond doubt evolution is a fact.1

Richard Dawkins

But what does this actually mean? What does it mean to say that evolution is a ‘fact’? Is evolution truly incontestable, or are there legitimate doubts which should discourage our assent? If one viewed the statistics for the acceptance of evolution, particularly in America, one would be forgiven for asking whether evolution really is an indisputable fact. In a Gallup poll taken this May, 46% of those Americans surveyed believed that ‘God created humans in present form’.2 With what assurance, then, can evolutionists speak of evolution as a fact?

Evolution is clearly an important idea. Theodosius Dobzhansky, a central figure in the formulation of the neo-Darwinian (or ‘modern’) synthesis, famously stated that ‘Nothing in biology makes sense except in the light of evolution’.3 Echoing Dobzhansky, Richard Dawkins has said that ‘Without evolution, biology is a collection of miscellaneous facts.’4 And Graham Cairns-Smith has gone so far as to define biology as ‘the study of the causes and effects of evolution’.5 But what do we mean by ‘evolution’? If evolution is a fact, what exactly is purported to be a fact?

Philosopher and mathematician William Dembski, a leading exponent of so-called Intelligent Design (ID) theory, contends that the Darwinian establishment is guilty of ‘equivocation’ over the term ‘evolution’:

The fallacy of equivocation is the fallacy of speaking out of both sides of your mouth. It is the deliberate confusing of two senses of a term, using the sense that’s convenient to promote one’s agenda.6

Agendas aside, are there multiple senses of the term ‘evolution’, and, if so, what are they and are they true? Dembski asks, ‘Is it a fact that organisms have changed over time? There is plenty of evidence that appears to confirm that this is the case. Is it a fact that the panoply of life has evolved through purposeless naturalistic processes? This might be a fact, but whether it is a fact is very much open to debate.’7 Dembski’s implication is that Darwinians employ the broad term ‘evolution’ in discussion so as to assert the factuality of those disputable elements of the theory by disingenuously sneaking them in under the cover of those elements which are not disputable. If we are to say that evolution is a fact, then, we must break down the term and see whether each of its constituent elements may be confirmed. If each facet can be considered veridical, then we can justifiably proclaim evolution (in a broad Darwinian sense) an established fact–a fact beyond reasonable, serious, sane, informed, intelligent doubt.

Darwinians are quite prepared to distinguish the elements of evolution. Michael Ruse proposes a three-fold division:

First, there is what we might call the putative fact or happening of evolution, the claim that organisms did not arrive here on earth miraculously, but by a process of descent. Second, there is the question of the paths taken in the process, what evolutionists call phylogenies. Did birds evolve via the dinosaurs, or directly from more primitive organisms? Third, there is the matter of the mechanism of evolution: the causes behind the process.8

This division is helpful, although clearly the elements are interdependent. Ruse notes elsewhere that ‘any such division is somewhat artificial. You could hardly have a path of evolution without its being a fact, and mechanisms which take us nowhere are surely not all that evolutionary.’9

One last question remains, however, and that is what do we mean by ‘fact’? It is an apodictic fact (that is, absolutely and eternally true) that 2 + 2 = 4; however, if we are to speak of evolution as a fact we clearly cannot do so in an identical manner. Scientific theories, unlike mathematical equations, are not absolutely certain,10 and, unlike mathematical theories, they are vigorously challenged and contested. And neither are they permanent; theories are revised, modified, supplemented and, where necessary, abandoned and supplanted. A scientist does not aspire to establish a theory as incontrovertible–such an aspiration is denied by the nature of the enterprise. The scientist aims to construct a theory which adequately accounts for the observed phenomena and which yields testable predictions. And they are aware that the theory will not remain unchanged due to experimental disproof or explanatory inadequacy. In science, facts, like species in the living world, are not immutable. In that case, when evolutionists speak of evolution as a ‘fact’ they do so bearing in mind that our current understanding will probably be subject to revision. Thus Stephen Jay Gould wrote,

Evolutionists make no claim for perpetual truth… In science, “fact” can only mean “confirmed to such a degree that it would be perverse to withhold provisional assent.”11

So, then, using Ruse’s terminology, we have (1) the happening of evolution, (2) the path of evolution and (3) the mechanism of evolution. I propose to examine each of these three elements in an attempt to confirm, or disconfirm, them. Some of the elements may be more certain than others, and none will be apodictic, but if we can satisfactorily establish each of these constituents, then, yes, we may declare evolution a fact.

~

In the next post in this series I shall discuss the first element of the theory of evolution: the ‘happening’ of evolution.

References & Notes

  1. R. Dawkins, The greatest show on Earth: the evidence for evolution, (London: Bantam Press, 2009), p.8.
  2. Gallup poll: ‘Evolution, creationism, intelligent design’, available at http://www.gallup.com/poll/21814/Evolution-Creationism-Intelligent-Design.aspx.
  3. T. Dobzhansky, ‘Nothing in biology makes sense except in the light of evolution’, American Biology Teacher, 35: 125-129, (1973).
  4. R. Dawkins, ‘The joy of living dangerously: Sanderson of Oundle’, in A devil’s chaplain, ch.1.8, (London: Weidenfeld & Nicholson, 2003), p.58.
  5. A. G. Cairns-Smith, Seven clues to the origin of life, (Cambridge: Cambridge University Press, 1985), p.1.
  6. W. A. Dembski, Intelligent design: the bridge between science and theology, (Downers Grove, IL: InterVarsity Press, 1999), p.115.
  7. ibid.
  8. M. Ruse, ‘Is there a limit to our knowledge of evolution?’, BioScience, 34 (2): 100-104, (1984), p.100.
  9. M. Ruse, Taking Darwin seriously, 2nd ed., (New York: Prometheus Books, 1998), p.1.
  10. Immanuel Kant wrote, ‘Pure natural science cannot altogether refuse and dispense with the testimony of experience; hence with all its certainty it can never, as philosophy, rival mathematics.’ (Prolegomena to any future metaphysics that will be able to come forward as science, translated by P. Carus, revised by J. W. Ellington, [Indianapolis: Hackett Publishing Company, 1977], §40, p.69.)
  11. S. J. Gould, ‘Evolution as fact and theory’, in Hen’s teeth and horse’s toes: further reflections in natural history, chap.19, (New York: W. W. Norton & Co., 1983), p.255.
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Genes and Organisms: The Nature of Selection (Part 2)

In the previous post, we saw that in the ‘struggle for existence’ those organisms best adapted to their environment are more likely to survive and produce a greater number of offspring than their lesser adapted competitors. These adaptive characteristics are likely to be inherited by their offspring. As Darwin put it,

Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring.1

Genes are the particulate units of heredity which exert phenotypic effects (speaking broadly, organisms) by instructing embryological development. Selection feeds upon variation which is supplied primarily by genetic mutation. Mutations have phenotypic consequences which may be adaptive (resulting in increased reproductive success) or maladaptive (resulting in decreased reproductive success). (Many mutations happen to be neutral, but these are of no interest to selection.) As selection works inexorably on, ‘rejecting that which is bad, preserving and adding up all that is good,’2  a given gene pool becomes populated with those genetic variants (alleles) which produce organisms better adapted to their environments than their competitors. Those alleles which produce the best adapted organisms find themselves represented in the greatest frequencies within the population because the best adapted organisms survive to produce the greatest number of offspring, increasing the representation of those alleles in the next generation. As Dawkins writes, ‘The world tends automatically to become populated by germ-line replicators whose active phenotypic effects are such as to ensure the successful replication.’3

Because selection is a cumulative process, whereby slight adaptive variations accrete within a lineage, the unit of selection must possess ‘some minimum degree of longevity/fecundity/fidelity’.4 Because phenotypes and genotypes are transient combinatorial entities, we saw that these entities could not be the unit of selection. The unit of selection ‘must have a high degree of permanence and a low rate of endogenous change, relative to the degree of bias (differences in selection coefficients).’5 We thus came to see the unit of selection as the active germ-line replicator.

If replicators exist that are active, variants of them with certain phenotypic effects tend to out-replicate those with other phenotypic effects. If they are also germ-line replicators, these changes in relative frequency can have long-term, evolutionary impact.6

This conclusion was challenged by the late Stephen Jay Gould, however. Gould maintained that the gene cannot be the unit of selection because selection is blind to the gene, seeing only well adapted or poorly adapted organisms:

No matter how much power Dawkins wishes to assign to genes, there is one thing that he cannot give them—direct visibility to natural selection. Selection simply cannot see genes and pick among them directly. It must use bodies as an intermediary. A gene is a bit of DNA hidden within a cell. Selection views bodies. It favours some bodies because they are stronger, better insulated, earlier in their sexual maturity, fiercer in combat, or more beautiful to behold.7

This is almost exactly right. So, where is the problem? The problem is due to Gould’s inability to see both the gene and the organism as two sides of the same coin. The existence of a replicator does not preclude the existence of a vehicle, or vice versa. Gould writes that ‘Dawkins will need another metaphor: genes caucusing, forming alliances, showing deference for a chance to join a pact, gauging probable environments. But when you amalgamate so many genes and tie them together in hierarchical chains of action mediated by environments, we call the resultant object a body.’8 And yet these are almost Dawkins’ words. As Dawkins has said, he ‘had already done precisely what Gould later recommended’9:

However independent and free genes may be in their journey through the generations, they are very much not free and independent agents in their control of embryonic development. They collaborate and interact in inextricably complex ways, both with each other, and with their external environment.10

Dawkins also makes clear just how genes ‘caucus’ and ‘form alliances’:

Selection favours those genes which succeed in the presence of other genes, which in turn succeed in the presence of them. Therefore mutually compatible sets of genes arise in gene-pools. This is more subtle and more useful than to say that ‘we call the resultant object a body’.11

Yet Gould continues,

Moreover, Dawkins’ vision requires that genes have an influence upon bodies. Selection cannot see them unless they translate to bits of morphology, physiology, or behaviour that makes a difference to the success of an organism… Not only do we need a one-to-one mapping between gene and body… we also need a one-to-one adaptive mapping… It may be that many, if not most, genes work equally well (or at least well enough) in all their variants and that selection does not choose among them.12

Here things have become much more confused. Dawkins’ vision does not require that there is a one-to-one mapping between a gene and a phenotypic trait. Such an arrangement would only be possible if the genotype was a blueprint, where one item of the blueprint (gene) corresponds to one item of the represented structure (phenotypic trait), and it is for this reason that Dawkins employs the metaphor of the genotype as a recipe, a series of instructions to be followed in embryology. Dawkins is emphatic that ‘you cannot reconstruct an individual’s genome by inspecting his body, any more than you could reconstruct William Shakespeare by decoding his collected works.’13 It is surprising, then, to find that it is actually Gould (1980, p.85) who appropriates the blueprint metaphor: ‘Genes are blueprints for organisms…’!14 This confusion is due to a conflation of genetics with embryology. Dawkins writes that ‘Mendelism is a theory of particulate inheritance, not particulate embryology.’15

When we speak of a gene (a1) ‘for’ a phenotypic trait (T) we do not mean that T is due only to a1. T will be due to a host of genes and environmental conditions, but if allele a2 is substituted for a1 then T will not arise. This is what it would mean to say that a1 is a gene ’for’ T. Dawkins writes that ‘It is a fundamental truth… that whenever a geneticist studies a gene ‘for’ any phenotypic character, he is always referring to a difference between two alleles.’16 He goes on to say that ‘However complex the genetic basis of features that all members of a species have in common, natural selection is concerned with differences. Evolutionary change is a limited set of substitutions at identifiable loci [the positions on a chromosome occupied by genes and their alleles].’17 Williams writes that

A gene is one of a multitude of meiotically dissociable units that make up the genetic message. No constant phenotypic effect need be associated with a particular gene… No matter how functionally dependent a gene may be, and no matter how complicated its interactions with other genes and environmental factors, it must always be true that a given gene substitution will have an arithmetic mean effect on fitness in any population.18

Genes are not independent units; they exist within an environment containing the genes with which they find themselves in company. A genotype is a collaboration of genes; however a gene at a given locus will be favoured or disfavoured by selection relative to the effects of its alleles. ‘One allele can always be regarded as having a certain selection coefficient relative to another at the same locus at any given point in time.’19

Gould’s argument—that genes are not the unit of selection because they are invisible to selection—may be valid if it were not for adaptation. If organisms were not adapted to their environments—if the relation between organisms and environments were completely ‘random’, that is, there were no ‘fit’—then we might suppose that genotypes do not, in fact, influence phenotypes. However, because of adaptation, because the relation between organisms and environments is highly non-random, then the only explanation we can muster to account for this relation is the cumulative selection of random variations, the substitution of alleles at identifiable loci. Any Darwinian adaptation—that is, an adaptation produced by natural selection—must have a genetic basis: ‘Unless natural selection has genetic variation to act upon, it cannot give rise to evolutionary change. It follows that where you find Darwinian adaptation there must have been genetic variation in the character concerned.’20

Genes are allowed to exert their normal effects on development. Their developmental consequences—phenotypic effects—feed back on those genes’ chances of surviving, and as a result gene frequencies change in succeeding generations in adaptive directions… the relationship between a gene and its phenotypic effect is not an intrinsic property of the gene, but a property of the forward developmental consequences of the gene when interacting with the consequences of many other genes and many external factors.21

Sterelny makes clear the role of the replicator in development:

Replicators are designed mechanisms; their biofunction is to contribute to the process through which phenotypes and genotypes reduplicate themselves. Replicators play a privileged role in the total developmental matrix because they are designed copying mechanisms. Some parent-offspring similarities result from elements of the developmental matrix that have been selected to produce those similarities.22

We can now understand the nature of the selection process and understand the relation between genes and organisms. Organisms are the product of genes which instruct the development of the organism in embryology. Organisms are unique, mortal combinations of, at least potentially, immortal genes, constructed to aid the propagation of the coercive replicators. The genes depend on the flamboyantly indirect route of the organism for their survival. As such, we understand that adaptations are the result of cumulative selection, the non-random survival of alleles, and are designed for the benefit of the active germ-line replicator, the gene. Selection, in determining the survival of organisms, determines the survival of the organisms’ genes. The most successful genes are those which produce the most successful vehicles. We thus see genes and organisms not as mutually exclusive options, but as two sides of the same coin. One does not choose between replicators and vehicles, as they are both facets of the same phenomenon. However, one must realise that it is the replicators that survive and are propagated, whilst they depend on vehicles for their survival and propagation. From the macroscopic level of the vehicle we see organisms struggling to survive and reproduce, complexly interacting with their environments, and from the microscopic level of the replicator we see much the same thing—genes struggling to survive and reproduce, complexly interacting with their own environments.

References & Notes

  1. C. R. Darwin, On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life, (London: John Murray, 1859), p.61.
  2. ibid., p.84.
  3. R. Dawkins, The extended phenotype, (Oxford: Oxford University Press, 1983), p.84.
  4. ibid., p.87.
  5. G. C. Williams, Adaptation and natural selection: a critique of some current evolutionary thought, (Princeton, NJ: Princeton University Press, 1974), p.23.
  6. R. Dawkins, The extended phenotype, p.84.
  7. S. J. Gould, ‘Caring groups and selfish genes’, in The panda’s thumb, p.85-92, (New York: W. W. Norton & Co.), p.90.
  8. ibid., p.91.
  9. R. Dawkins, The selfish gene, 30th anniversary edition, (New York: Oxford University Press, 2006), p.262.
  10. ibid., p.36-37.
  11. R. Dawkins, The extended phenotype, p.117.
  12. S. J. Gould, ‘Caring groups and selfish genes’, p.91.
  13. R. Dawkins, The extended phenotype, p.175-176.
  14. S. J. Gould, ‘Caring groups and selfish genes’, p.85.
  15. R. Dawkins, The extended phenotype, p.116.
  16. ibid., p.92.
  17. ibid., p.93.
  18. G. C. Williams, Adaptation and natural selection, p.56-57.
  19. ibid., p.57.
  20. R. Dawkins, The extended phenotype, p.20.
  21. ibid., p.176.
  22. K. Sterelny, ‘Understanding life: recent work in philosophy of biology’, The British Journal for the Philosophy of Science, 46 (2): 155-183, (1995), p.165.
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Genes and Organisms: The Nature of Selection (Part 1)

Natural selection is the differential survival of individuals in populations; those individuals best adapted to their environment are more likely to survive and produce offspring than other, not so well adapted, members within the population. In order for evolution by natural selection to occur there must be: (1) reproduction, (2) variation and (3) heredity. An organism (phenotype) is constructed, in development, according to the genetic instructions (genotype) inherited from its parents.

The chief source of novel variations in populations arises by random genetic mutation—errors in the copying process of the genetic instructions—which may, though they may not, affect the phenotype in some way. (Mutations are described as ‘random’ because they are not directed; that is, a mutation will arise regardless of its effect on an organism’s reproductive success.) Mutations vary in magnitude, from point mutations (the alteration of one nucleotide base from, say, adenine to guanine) through to the fusion of entire chromosomes. Because mutations are random and because there are many more ways of worsening something than improving something, the vast proportion of mutations are deleterious. Those mutations which are advantageous, however, which increase reproductive success by better adapting an organism to its environment will be favoured by selection. Those individuals best adapted to their environment are likely to produce the greatest number of offspring, their offspring inheriting their successful characters, and so, over many generations, these successful variations will come to predominate within the population. This is natural selection.

Because heredity is particulate, as revealed by Mendel, cumulative selection can occur; successful variations can accrue within a lineage over time. This cumulative process must be gradual because large mutations, or ‘macro-mutations’, are almost guaranteed to be harmful as they are more likely to disrupt development and be maladaptive than they are to improve adaptation. If we imagine a multi-dimensional biospace in which we can locate all possible organisms, a vast proportion of the space will be empty, as only a tiny fraction of the locations in biospace are ‘workable’, that is, contain a feasible organism. (All organisms, past and present, necessarily occupy a region in biospace because they once existed.)

Because it is lineages that evolve and not organisms, a lineage would trace a trajectory through biospace, occupying its own workable zone. The workable zone of one lineage would be separated from that of its nearest neighbour by a gulf of empty biospace, much of which will be unworkable. The great majority of movements a lineage could make through biospace would not result in improved adaptation, although a small subset of the possible movements would. Because each movement through biospace is random, due to the randomness of mutation, the likelihood of a random movement (mutation) improving adaptation and increasing reproductive success is highly unlikely. It is far more likely that a random movement will be maladaptive. Small, tentative movements through biospace (slight mutational variations) are more likely to result in improved adaptation compared with taking large, bounding leaps (saltations) because the chance of successfully leaping from one workable zone to another (more successful) workable zone at random is highly unlikely. Darwin wrote that

natural selection can act only by taking advantage of slight successive variations; she can never take a leap, but must advance by the shortest and slowest steps.1

If a lineage is to navigate an adaptive trajectory through biospace all it must do is take small steps. Inheritance and selection will see to the rest.2

That selection chooses among individuals was well realised by Darwin. Those individuals that are poorly adapted to their environment are more likely to die before they produce offspring, unlike their better adapted competitors who will pass on their adaptive characters to their descendants. But whilst selection occurs at the level of the individual, choosing among alternative organisms, we must ask whether or not it is the organism that is the unit of selection, whether it is the organism that benefits from selection or whether it is actually some other entity. Adaptations are clearly for the good of something, but is that something the organism? Williams writes that

The essence of the genetical theory of natural selection is a statistical bias in the relative rates of survival of alternatives (genes, individuals, etc.)… the selected entity must have a high degree of permanence and a low rate of endogenous change, relative to the degree of bias (differences in selection coefficients).3

What are these ‘alternatives’? Phenotypes (identifiable as ‘individuals’ in the above passage) are transient entities which only appear once, never to appear again. So, too, are genotypes. Both phenotypes and genotypes are unique assemblages which shall only ever appear once. It is for this reason that phenotypes and genotypes cannot be the stuff of cumulative selection. The unit of selection ‘must have a high degree of permanence and a low rate of endogenous change, relative to the degree of bias…’ A suitable unit is the ‘replicator’, an entity of which copies can be made. Hull writes that

Replicators need not last forever. They need only last long enough to produce additional replicators that retain their structure largely intact. The relevant longevity concerns the retention of structure through descent…4

Dawkins writes that ‘a candidate should be regarded as an actual replicator only if it possesses some minimum degree of longevity/fecundity/fidelity (there may be trade-offs among the three).’5 There are different kinds of replicators, however. Dawkins distinguishes between ‘dead-end replicators’ and ‘germ-line replicators’, following August Weismann’s establishment of the independence of the germ-line. Somatic (body) cells, such as liver cells and skin cells, are constructed according to the instructions of replicators, but their replicators, whilst they may spawn a number of generations by mitotic division within an organism, shall never find themselves in the next generation. The replicators contained within the gametes, on the other hand—the germ-line replicators—are able to pass through to the next generation and are ‘potentially the ancestor[s] of an indefinitely long line of descendant replicators.’6 As well as being dead-end or germ-line, replicators may be ‘active’ or ‘passive’. Active replicators are able to affect the probability of their being copied, whilst passive replicators are unable to do so. The unit of selection must be the unit of heredity—it must be a germ-line replicator—and it must also be able to increase the chance of its being present in the next generation—it must be an active replicator. The unit of selection, then, is identified as the active germ-line replicator:

…wherever in the universe they may be found, they [active germ-line replicators] are likely to become the basis for natural selection and hence evolution… The world tends automatically to become populated by germ-line replicators whose active phenotypic effects [i.e. organisms] are such as to ensure their successful replication. It is these phenotypic effects that we see as adaptations to survival.7

The replicators within organisms are genes or ‘genetic replicators’. Williams defines the gene as ‘“that which segregates and recombines with appreciable frequency.”’8 In this case, the gene is construed as an informational unit capable of producing units with identical (or nearly identical) informational structures. It is information that is passed down from generation to generation, accumulating in a lineage over time thanks to the Mendelian nature of heredity and selection. Dawkins uses the metaphor of a ‘river of information’ flowing through geological time.9

So, if the gene is the unit of selection, then why organisms? Replicators are the units of genetic information which collaborate to create what Dawkins calls a ‘vehicle’ or, more evocatively, a ‘survival machine’ (what Hull calls an ‘interactor’), in which they reside. The selection process can then be seen as consisting of replicator selection and vehicle selection:

Replicator selection is the process by which some replicators survive at the expense of other replicators. Vehicle selection is the process by which some vehicles are more successful than other vehicles in ensuring the survival of their replicators.10

Replicators and vehicles (genes and organisms) can be seen as two sides of the same coin, however it is the gene (the active germ-line replicator) that is the unit of selection as it is the gene which is inherited and which instructs the assembly of an organism in development. Adaptations are the product of the cumulative selection (non-random survival and propagation) of alternative alleles and they exist for the benefit of the replicators which instructed their assembly.

~

In the next, and final, post I shall examine some of the criticisms of this conception of the selection process, particularly those advanced by the late Stephen Jay Gould.

References & Notes

  1. C. R. Darwin, On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life, (London: John Murray, 1859), p.194.
  2. For a similar, far better discussion of these principles, see R. Dawkins, The blind watchmaker, (Harlow: Longman, 1986), chap.3-4.
  3. G. C. Williams, Adaptation and natural selection: a critique of some current evolutionary thought, (Princeton, NJ: Princeton University Press, 1974), p.22-23.
  4. D. L. Hull, ‘Individuality and selection’, Annual Review of Ecology and Systematics, 11: 311-332, (1980), p.317.
  5. R. Dawkins, The extended phenotype, (Oxford: Oxford University Press, 1983), p.87.
  6. ibid., p.83.
  7. ibid., p.84.
  8. G. C. Williams, Adaptation and natural selection, p.24.
  9. R. Dawkins, River out of Eden, (London: Weidenfeld & Nicholson, 1995).
  10. R. Dawkins, The extended phenotype, p.82.
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The Excitement of Evolution

With the summer drawing to a close and having enjoyed some excellent holidays I am aware that my most recent posts have been sporadic and somewhat lengthy. Over the last few months I have had numerous ideas which shall hopefully in the near future come to fruition. One of my most central projects, currently in preparation, is an extended critique of mathematician and philosopher David Berlinski’s article The Deniable Darwin,1 in which I shall consider the various (and often outrageous, I might add) objections to evolutionary theory which he propounds. Alongside this are ideas (some partially completed, others not completed at all) for further forays concerned with particular issues and episodes in evolutionary biology, such as adaptationism, convergence and the Cambrian ‘explosion’. Also, mine and Joshua Gidney’s discussion on Intelligent Design shall hopefully resume sometime in the future.

This post shall be somewhat shorter than previous posts and shall not examine any particular issue, but shall rather portend the projects on which I hope to work in these coming weeks and months as here I hope simply to convey something of the excitement of evolution.

~

‘For sheer excitement, evolution, as an empirical reality, beats any myth of human origins by light-years.’2

Stephen Jay Gould

The story of life is a story of evolution. As Graham Cairns-Smith has put it, the study of biology has become ‘the study of the causes and effects of evolution…’3 Geneticist and key architect of the neo-Darwinian synthesis Theodosius Dobzhansky famously stated that ‘Nothing in biology makes sense except in the light of evolution’.4 While this may not be strictly true—one may understand some aspects of biology without knowing about evolution—it is surely true that if there is to be any unification in biology, if we are to account for the nature and diversity and complexity of life, only evolution can provide sufficient illumination. Darwin’s great discovery, natural selection, is the primary mechanism responsible for evolutionary change. Only Darwinian evolution, with selection as its causal force, can account for the astonishing phenomenon of life here on Earth in all its manifold and exquisite intricacy. Darwinism (shorthand for the modern theory of evolution) is also of deeply personal interest as it tells of how we ourselves came to be. As Richard Dawkins puts it,

You can live some sort of life and die without ever hearing the name of Darwin. But if, before you die, you want to understand why you lived in the first place, Darwinism is the one subject you must study.5

Only Darwinism can provide unification in biology, weaving what would otherwise be merely ‘a collection of miscellaneous facts’6 into an opulent explantory tapestry. With his discovery of natural selection, Darwin revealed the mechanism responsible for the emergence of biological complexity and transformed our understanding of evolution. There is now no doubt whatsoever that natural selection is of paramount importance.

So-called ‘alternatives’ to Darwinism, such as biblical creationism and Intelligent Design theory (ID), are not, in fact, alternatives at all.7 Neither of these propositions are scientific; there is simply no place for fundamentalist literalism and covert natural theology in serious science. They are not viable alternatives and do not merit attention, least of all in the school science curriculum. Not only do these theories do nothing for science, they also do nothing for theology, and probably do a great deal more harm. Is God a grand deceiver, or a shoddy engineer, or is he, rather, the author of Creation, worthy of praise and worship? (I know which I believe him to be.) As such, Darwinism tells the story of Creation, and as well as being true it is also genuinely explicable, which is more than can be said for creationism or ID whose opaque assertions convey only irresponsible assertion and cheap pseudoscience, respectively. Darwinism has proved a fertile research programme, wielding extraordinary explanatory power. The Darwinian theory of evolution lies at the heart of a powerful consilience of a host of inductions:

The idea [of a consilience] is that somehow, if a hypothesis is true—tells us about the real world—then various facts or other claims follow from it, and will keep doing so. And there is a kind of feedback process here. As the hypothesis leads to new information, so its derivations themselves confer a kind of probability upon the hypothesis.8

Darwin tied together otherwise disparate threads from areas such as anatomy, biogeography, embryology, instinct, palaeontology and systematics (to name a few), each of these evidencing his evolutionary hypothesis, his hypothesis, in turn, explicating each of these areas. These traditional fields have been supplemented most strikingly by a range of discoveries in the field of molecular biology. The evidence in favour of Darwinism is such that it should now be accepted as fact.

Darwinism has spawned remarkable discoveries, providing avenues of research for scores of scientists. The same simply cannot be said for its flabby and ineffectual ‘alternatives’. Evolutionary biology is not devoid of controversy; perhaps more than in any other scientific discipline, debates rage with gusto and even vehemence. In an important respect (and somewhat paradoxically) there is no one theory of evolution, and there never has been. Every evolutionist holds their own particular priorities and interpretations of the evolutionary process, each representing their own unique variety of evolutionary theory. That is part of what makes evolution so exciting. Differences abound, often engendering vociferous quarrels; the diversity of biologists’ personal varieties of Darwinism suitably reflects the diversity of biological varieties. However, despite their differences, there is  a shared commitment to uncovering the processes of life in the light of Darwin—processes driven by the intimate interaction of chance and necessity.

Evolutionary biology is a discipline so rich and various, so old and yet, in many ways, so young, that it is difficult not to become excited by it. Whilst this blog is not solely devoted to evolution, the subject preponderates due to my personal fascination with it. As I said at the beginning of the post, evolution is the story of life, telling the story of Creation. I have not had kind words for creationism or ID (admittedly, I think more must be said in the case of ID. Whilst I do believe that ID is non-science, it is simply otiose to attempt to dismiss its proponents as ‘creationists’, as if that would have done with the matter. ID’s exponents are active, efficient and organized, and there is a danger that some may take them seriously) but my motives were not irreligious. I would contend, rather, that the theist, particularly the Christian, loses nothing by abandoning these ideas, and that actually, by accepting evolution for the fact that it is, one can gain so much more. Francisco Ayala has described Darwin’s contributions as a ‘gift’ both to science and religion.9 Despite the fact that these ‘alternatives’ happen to be false, they simply do not do justice to the wonder of creation. In the words of Stephen Jay Gould, ‘Evolution is true–and the truth can only make us free.’10

Darwin well understood the excitement elicited by evolution and concluded the Origin with these famous and oft-quoted words:

There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.11

References & Notes

  1. David Berlinski, ‘The deniable Darwin’, Commentary, June, 101 (6), p.19-29, (1996).
  2. Stephen Jay Gould, ‘Darwin’s more stately mansion’, Science, New Series, 284 (5423), p.2087, (1999).
  3. A. G. Cairns-Smith, Seven clues to the origin of life: a scientific detective story, (Cambridge: Cambridge University Press, 1985), p.1.
  4. Theodosius Dobzhansky, ‘Nothing in biology makes sense except in the light of evolution’, American Biology Teacher, 35, p.125-129, (1973).
  5. Richard Dawkins, ‘Foreword to the Canto edition’, in: John Maynard Smith, The theory of evolution, Canto ed., (Cambridge: Cambridge University Press, 1993), p.xvi.
  6. Richard Dawkins, ‘The joy of living dangerously: Sanderson of Oundle’, in: A devil’s chaplain, chap.1.8, (London: Weidenfeld & Nicholson, 2003), p.58.
  7. For a comprehensive critical discussion of creationism and ID, see Robert Pennock, Tower of Babel: the evidence against the new creationism, (Cambridge, MA: MIT Press, 1999).
  8. Michael Ruse, Darwinism and its discontents, (Cambridge: Cambridge University Press, 2006), p.38.
  9. Francisco Ayala, Darwin’s gift to science and religion, (New York: Joseph Henry Press, 2007).
  10. Stephen Jay Gould, ‘Darwin’s more stately mansion’.
  11. Charles Darwin, On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life, (London: John Murray, 1859), p.490.
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Is Man an Ape or an Angel?

Figure 1
Benjamin Disraeli (1804-1881)

British politician, twice Prime Minister, Benjamin Disraeli quipped, in a speech at the Sheldonian Theatre, Oxford in 1864, ‘What is the question now placed before society with the glib assurance which to me is most astonishing? That question is this: Is man an ape or an angel? I, my lord, I am on the side of the angels. I repudiate with indignation and abhorrence those new fangled theories.’

When one compares the achievements of man with the achievements of our fellow primates, one is met with a startling disparity. Our inventions of language, technology, art, science and novel mode of cultural inheritance present a seemingly unbridgeable gulf between those apes with whom, at first sight, one may have thought we shared so much. After all, where else in the animal kingdom do we find anything comparable to Shakespeare or the Sagrada Familia or special relativity theory? But, then, if we are prepared to ask where we may find evidence of achievements comparable to those of which we are most proud elsewhere in the animal kingdom, then we should also ask, with discomfiting reluctance, where else in the animal kingdom do we find the systematic destruction of habitats, severe xenophobia and genocidal slaughter, or abuse of toxic chemicals? If we are angels, then we must admit we have a dark side.

Just another Ape?

It is obvious that humans are unlike all animals. It is also obvious that we are a species of big mammal, down to the minutest details of our anatomy and our molecules.1

- Jared Diamond

We are most certainly distinct from other species for the reasons listed above, and it is important that we realise this. But it is also important that we realise just how we are distinct, in what ways we are distinct, and appreciate how it was that we came to be distinct. In his book The rise and fall of the third chimpanzee,2 Jared Diamond seeks to answer the question of whether our defining traits are without animal precursors. Diamond demonstrably concludes that the answer is, largely, and often emphatically, negative.

Figure 2 Frontispiece to T. H. Huxley’s Man’s Place in Nature (1863)

Biologically speaking, in answer to Disraeli, we are apes through and through. We are not just like apes; we are apes. Our lineage diverged from that of the chimps3 around 6-7mya4 (see Figure 3, below). In all, we share around 98% of our DNA with the chimps. This seemingly tiny discrepancy is responsible for the manifold differences between us and our closest cousins, brains and all. Although, to put it like this may be slightly misleading. As geneticist Steve Jones writes, ‘A chimp may share ninety-eight per cent of its DNA with ourselves but it is not ninety-eight per cent human at all – it is a chimp.’5 The fact that humans and chimps share approximately 98% of their genetic sequences does not mean that they should be, organismically, 98% the same. Genetics is a far more complicated business than that. Genetic mutations–from insertions and deletions in base sequences, through to the fusion of entire chromosomes6–have resulted in the ‘individuality’ of humans and chimps. Humans are not 98% chimps, and vice versa. Humans are 100% humans. We shared a common ancestor with the chimps just a few million years ago before our lineages diverged and continued to evolve separately; the differences we observe today achieved in a geological eye-blink.

Figure 3 Phylogenetic tree of primates

Both humans and chimps are referred to as ‘great apes’, along with gorillas and orangutans. Gibbons are referred to as ‘lesser apes’, although I am sure this is nothing personal. The Hominidae family includes two sub-families: Ponginae and Homininae. Orangutans belong to Ponginae, whilst gorillas, chimps and humans belong to Homininae. The Homininae sub-family then yields two further sub-sub-groupings, referred to as Gorillini and Hominini: gorrilas belong to the former, chimps and humans to the latter. The rationale for this natural classification can be followed in Figure 3: those species which share the closest groupings are those which share a more recent common ancestor.

In the Origin, Darwin realised that evolution, or ‘descent with modification’ as he dubbed it, would render this hierarchical nesting of groups intelligible, reasoning

that the natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike.7

The Trouble with Species

This also raises the issue of what a species actually is: ‘Not only does life present itself to our observation in the form of discontinuous quanta called individuals, but individuals usually occur in discontinuous arrays.’8 A human life, albeit longer than any other primate’s, when compared with the vast totality of geological time is but a fraction of an eye-blink. Although we can watch evolution happen, for example in populations of Escherichia coli or Drosophila, the kind of large-scale (macroevolutionary) transformations responsibe for the development of humans and chimps cannot be witnessed in our lifetimes, as the generational periods of primates are far longer than those of E. coli and Drosophila. Because evolution must occur gradually--Natura non facit saltum, as Darwin repeatedly emphasised in the Origin9–and cumulatively, over many generations, we can glimpse only a frame or two of the evolutionary motion picture. The slight section of a spatiotemporally continual phylogeny (evolutionary tree) which we glimpse in our lifetime does not allow us to appreciate the terrific, though quite stately, transformations lineages undergo over evolutionary time.

In 1942, ornithologist and eminent architect of the neo-Darwinian synthesis (or ‘modern synthesis’) Ernst Mayr provided a workable definition of ‘species’ which achieved wide acceptance:

Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.10,11

Figure 4 Plato (left) and Aristotle (right) in The School of Athens (1509-11), a fresco painted by Raphael

This populational view of species contrasted with the typological view which had prevailed for centuries prior to the emergence of the modern theory of evolution. Daniel Dennett writes, ‘The taxonomy of living things Darwin inherited was… a direct descendant, via Aristotle, of Plato’s essentialism.’12 Plato’s theory of Ideas maintained that objects in the world are imperfect copies of eternal archetypes (or Forms), exemplars of entities which exist in a notional realm. This theory was handed down by Plato to his student Aristotle, who held that these intrinsic essences possessed by objects could be discovered and defined by scientific investigation. An essentialist taxonomy sought to provide such absolute definitions for species, however, due to the development of evolutionary theory, ‘Aristotelian definition had to be abandoned both for species names and for “species”.’13 A theory of evolution, founded on the principle of descent with modification from a common ancestor, determines that species must assume their positions in a sweeping continuum–a phylogeny. For this reason, ‘classification must have some systematic relationship to phylogeny and… the unit of classification must be the unit of evoluton.’14 Once it is realised that species are not immutable, ‘unlimited change becomes not only possible but even necessary. Species will no longer have well-defined limits, classification becomes a purely arbitrary exercise, and any species may easily be transformed into another.’15 The trouble with species begins with the attempt to ascribe a fixed name and definition to a variable series. The great theoretical biologist John Maynard Smith wrote that

any attempt to group all living things, past and present, into sharply defined groups, between which no intermediates exist, is doomed to failure… [yet] even though it is not possible to devise a fully consistent classification into species, the attempt must for practical reasons be made.16

Figure 5 A series of skulls belonging to relatives of living humans. From left to right: Adapis (50mya), Proconsul (23-15mya), Australopithecus africanus (3.5-2.5mya), Homo habilis (2.3-1.7mya), Homo erectus (1.8mya-30,000ya), Homo sapiens (92,000-year-old skull discovered in Israel) and Homo sapiens (22,000-year-old skull discovered in France)

It is important to realise that the only reason we can recognise one species as distinct from another in the first place is that the intermediaries are no longer extant–and, further, that they are invisible. Richard Dawkins writes, ‘On the evolutionary view of life there must have been intermediates, even though, conveniently for our naming rituals, they are today usually extinct.’17 According to evolution, there are no leaps and no interpositions; only an uninterrupted succession of forms. Dawkins invites us to imagine standing at the equator, on the shore of the Indian Ocean, in southern Somalia, facing north, and holding in our left hand the right hand of our mother, our mother holding the right hand of her mother, and so on, forming a long chain, our ancestors stretching back into the heart of the continent, all the way back to the common ancestor we shared with the chimps. How far before we reach the common ancestor? The distance is surprisingly short: allowing one yard per person the common ancestor stands just 300 miles away.18

Daughters would resemble mothers just as much (or as little) as they always do. Mothers would love daughters, and feel affinity with them, just as they always do. And this hand-in-hand continuum, joining us seamlessly to chimpanzees, is so short that it barely makes it past the hinterland of Africa, the mother continent.19

Figure 6 Skull of Sahelanthropus tchadensis, the earliest fossil hominid, dug up in western Chad and dated 6-7mya

There was no moment in history, around 200,000 years ago, when an ancestor of a quite different species gave birth to anatomically modern Homo sapiens. In Dawkins’ thought experiment nowhere along the line would we be able to mark an individual as so unlike its parent, that they should be classified as ‘distinct’. Rather, as we moved along the chain we would find that our ancestors gradually began to appear more and more different to modern humans, but the graduation would be seamless, one form gradually blurring into another.20 The only reason we can make distinctions at all is that the intermediate forms have disappeared. Extinction and the necessary preconditions which make fossilization such a rare occurrence have taken chunks out of the continuum, so that what we observe are distinct islands. But we know that these islands are illusory. As Darwin wrote, they all

fall into one grand natural system; and this fact is at once explained on the principle of descent… That the extinct forms of life help to fill up the wide intervals between existing genera, families, and orders, cannot be disputed… I apprehend that in a perfectly natural classification many fossil species would have to stand between living species, and some extinct genera between living genera, even between genera belonging to distinct families.21

The Tree of Life

It is still amusing that Darwin appropriates a Christian metaphor when he speaks of history as a ‘‘Tree of Life,’’ with us all today at the outer tips of the branches.22

- Michael Ruse

Humans were not specially created, distinct from other species. We have evolved, along with macaques, mangroves and MRSA. In the sixth edition of the Origin, Darwin wrote,

It is indeed manifest that multitudes of species are related in the closest manner to other species that still exist, or have lately existed; and it will hardly be maintained that such species have been developed in an abrupt or sudden manner. Nor should it be forgotten, when we look to the special parts of allied species, instead of to distinct species, that numerous and wonderfully fine gradations can be traced, connecting together widely different structures.23

The present biosphere is the product of nearly four-billion years of evolution, and we are late entrants in this magnificent drama. If the whole history of the earth were squeezed into a single day, our species emerged one minute and seventeen seconds before midnight.24 We are another bud on the luxuriant Tree of Life, sharing a common ancestry with all other species. Evolution reveals our unity with the Creation, in all its astonishing wonder and complexity. In the Origin, Darwin wrote,

As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.25

Figure 7 Ernst Haeckel’s Tree of Life in his Generelle Morphologie der Organismen (1866)

We are a unique species, but look at, and inside, ourselves and we behold the evidence of our tangled past, uncovering our unequivocal relations to all other species, past and present. In one of his most famous essays, Lewis Thomas wrote, ‘We carry stores of DNA in our nuclei that may have come in, at one time or another, from the fusion of ancestral cells and the linking of ancestral organisms in symbiosis. Our genomes are catalogues of instructions from all kinds of sources in nature, filed for all kinds of contingencies.’26 As we learn more of our evolutionary history, as we scrutinise and untangle the branches of Life’s great tree, we learn more of our intimate relations with all life. Thomas continued, ‘I cannot feel as separate an entity as I did a few years ago, before I was told these things, nor, I should think, can anyone else.’27

We are another branch in the primate cluster on Life’s great tree. And yet we feel that something sets us apart. We must also remember that how we came to be so distinguished was by natural, knowable, explicable means: evolution by natural selection.28 And yet there is a feeling that we have somehow transcended our biology in a way that other species have not. It is from these feelings that Disraeli’s question obviously springs. And whilst we are apes, undeniably a product of evolution, this feeling may persist, this yearning to join with the Psalmist:

When I consider thy heavens, the work of thy fingers, the moon and the stars, which thou hast ordained; What is man, that thou art mindful of him? and the son of man, that thou visitest him? For thou hast made him a little lower than the angels, and hast crowned him with glory and honour. Thou madest him to have dominion over the works of thy hands; thou hast put all things under his feet: All sheep and oxen, yea, and the beasts of the field; The fowl of the air, and the fish of the sea, and whatsoever passeth through the paths of the seas. O Lord our Lord, how excellent is thy name in all the earth!29

But that’s a whole other story.

References & Notes

  1. Diamond, J. M. The rise and fall of the third chimpanzee: how our animal heritage affects the way we live. (London: Vintage, 1992). p.1.
  2. See above note for reference.
  3. The two species belonging to genus Pan are Pan troglodytes and Pan paniscus. Pan troglodytes is known as the common chimpanzee and Pan paniscus is known as the bonobo. When I refer to ‘the chimps’ I am referring to both species.
  4. mya = million years ago.
  5. Jones, S. The language of the genes: biology, history and the evolutionary future, revised ed. (London: Flamingo, 2000). p.35.
  6. Humans have 23 pairs of chromosomes, whereas the other apes have 24 pairs. In hominid evolution, following the split with the chimps, the telomeres of two ancestral ape chromosomes fused to produce human chromosome 2. The authors of an article in Nature wrote, ‘Chromosome 2 is unique to the human lineage of evolution, having emerged as a result of head-to-head fusion of two acrocentric [where the position of the centromere on the chromosome is such that one chromosomal arm is shorter than another] chromosomes that remained separate in other primates.’ (Ladeana W. Hillier, et al. ‘Generation and annotation of the DNA sequences of human chromosomes 2 and 4′, Nature 434, 2005, p.724-731. p.729.)
  7. Darwin, C. R. On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. (London: John Murray, 1859). p.420.
  8. Dobzhansky, T., Ayala, F. J., Stebbins, G. L. and Valentine, J. W. Evolution. (San Francisco: W. H. Freeman and Company, 1977). p.166.
  9. ‘It is, no doubt, extremely difficult even to conjecture by what gradations many structures have been perfected, more especially amongst broken and failing groups of organic beings; but we see so many strange gradations in nature, as is proclaimed by the canon, “Natura non facit saltum,” that we ought to be extremely cautious in saying that any organ or instinct, or any whole being, could not have arrived at its present state by many graduated steps.’ (Darwin, On the origin of species. p.460).
  10. Mayr, E. Systematics and the origin of species from the viewpoint of a zoologist. (New York: Columbia University Press, 1942). p.120.
  11. This was a more succinct version of a definition he provided in 1940: ‘A species consists of a group of populations which replace each other geographically or ecologically and of which the neighboring ones intergrade or hybridize wherever they are in contact or which are potentially capable of doing so (with one or more of the populations) in those cases where contact is prevented by geographical or ecological barriers.’ (Mayr, E. ‘Speciation phenomena in birds’, The American naturalist, 74 [752], 1940, p.249-278. p.256.)
  12. Dennett, D. C. Darwin’s dangerous idea: evolution and the meanings of life. (London: Penguin Books, 1996). p.36-37.
  13. Hull, D. L. ‘The effect of essentialism on taxonomy–two thousand years of stasis (I)’, The British journal for the philosophy of science, 15 (60), 1965, p.314-326. p.318.
  14. –. ‘The effect of essentialism on taxonomy–two thousand years of stasis (II)’, The British journal for the philosophy of science, 16 (61), 1965, p.1-18. p.1.
  15. Coleman, W. ‘Lyell and the “reality” of species: 1830-1833′, Isis, 53 (3), 1962, p.325-338. p.326.
  16. Maynard Smith, J. The theory of evolution, Canto ed. (Cambridge: Cambridge University Press, 2000). p.216-217.
  17. Dawkins, R. ‘Gaps in the mind’, in A devil’s chaplain: selected essays by Richard Dawkins. (London: Weidenfeld & Nicholson, 2003). p.21.
  18. ibid. p.23-24.
  19. ibid. p.24.
  20. For an interactive timeline of hominid evolution, visit the Smithsonian Museum of Natural History website at http://humanorigins.si.edu/evidence/human-evolution-timeline-interactive; also, for an excellent and concise introduction to human evolution, see Zimmer, C. Where did we come from? the essential guide to human evolution. (East Sussex: Apple Press, 2006).
  21. Darwin, On the origin of species. p.130.
  22. Ruse, M. ‘On the Origin of species‘, in Defining Darwin: essays on the history and philosophy of evolutionary biology. (New York: Prometheus Books, 2009). p.20.
  23. Darwin, C. R. On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life, 6th ed. (London: John Murray, 1872). p.202.
  24. Bryson, B. A short history of nearly everything. (London: Doubleday, 2003). p.297.
  25. Darwin, On the origin of species, 1st ed. p.128.
  26. Thomas, L. The lives of a cell: notes of a biology watcher. (New York: The Viking Press, 1981). p.4-5.
  27. ibid.
  28. Whilst there are other mechanisms responsible for evolution, natural selection is the only inherently non-random mechanism capable of actual design by increasing the statistical probability of certain (adaptive) configurations.
  29. Psalm 8:3-9, King James Version.

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