This is a précis of an argument that I developed in an article called “Did Darwin Write the Origin Backwards?” The article was published in 2009 and may be found on my web set at http://philosophy.wisc.edu/sober/recent.html. An expanded version of the argument is the first chapter of a book that I’m publishing at the end of 2010 with Prometheus Books. The book has the same title as the 2009 article.
Although Darwin’s theory is often described as the theory of evolution by natural selection, most commentators recognize that common ancestry (the idea that all organisms now alive on earth and all present day fossils trace back to one or a few “original progenitors”) is an important part of the Darwinian picture. What has been less explored in Darwin studies is how these two parts of Darwin’s theory – common ancestry and natural selection — are related to each other. Ernst Mayr and others have noted that they are logically independent. But this leaves open how the two ideas are evidentially related. How does common ancestry affect the way in which evidence concerning natural selection should be evaluated? And how does natural selection affect the way in which evidence concerning common ancestry should be evaluated?
Darwin addresses one of these two questions very succinctly in a passage from the Origin:
… adaptive characters, although of the utmost importance to the welfare of the being, are almost valueless to the systematist. For animals belonging to two most distinct lines of descent, may readily become adapted to similar conditions, and thus assume a close external resemblance; but such resemblances will not reveal – will rather tend to conceal their blood-relationship to their proper lines of descent.
The fact that human beings and monkeys have tailbones is evidence for common ancestry precisely because tailbones are useless in humans. Contrast this with the torpedo shape that sharks and dolphins share; this similarity is useful in both groups. One might expect natural selection to cause the torpedo shape to evolve in large aquatic predators whether or not they have a common ancestor. This is why the adaptive similarity is almost valueless to the systematist who is trying to reconstruct patterns of common ancestry.
In this passage, Darwin is saying that to determine whether a trait shared by two species is strong evidence that they have a common ancestor, one must be able to judge whether there was selection for the trait in the lineages leading to each. In this sense, knowledge of natural selection is a prerequisite for interpreting evidence concerning common ancestry. However, there is a subtly different question that has a very different answer. Must natural selection have been an important influence on trait evolution for there to be strong evidence for common ancestry? Darwin’s answer to this question is no. A world in which organisms are saturated with neutral and deleterious similarities, while adaptive similarities are rare or non-existent, would be an epistemological paradise so far as the hypothesis of common ancestry is concerned. That’s the point that Darwin is making in the passage I just quoted. Inferring common ancestry does not require that natural selection has occurred.
What about the converse question – how does the fact of common ancestry affect the interpretation of evidence for natural selection? One of Darwin’s most famous arguments concerning natural selection does not depend one whit on common ancestry. This is Darwin’s Malthusian argument. If reproduction in a population outstrips the supply of food, the population will be cut back by starvation. If the organisms in the population vary with respect to characteristics that affect their ability to survive, and if offspring inherit these fitness-affecting traits from their parents, the population will evolve. The process of natural selection is a consequence of these conditions and it can and will occur even if no two species have a common ancestor.
All this is correct, but there is more to the Darwinian picture of natural selection than this. The Malthusian argument establishes that selection has occurred – that some traits changed frequency because of their influence on the viability of organisms. But which traits evolved by natural selection? Darwin doesn’t think that every trait we observe evolved because there was selection for it; recall his comment in the Origin that selection is “the main but not the exclusive cause” of evolution. And if a trait did evolve under the influence of natural selection, why was it favored by natural selection? It is these questions, which concern the detailed application of the hypothesis of natural selection to examples, that common ancestry helps to answer.
An interesting illustration of how Darwin uses the assumption of common ancestry to think about natural selection may be found in his discussion of why mammals in utero have skull sutures that allow them to pass through the birth canal:
The sutures in the skulls of young mammals have been advanced as a beautiful adaptation for aiding parturition [live birth], and no doubt they facilitate, or may be indispensable for this act; but as sutures occur in the skulls of young birds and reptiles, which have only to escape from a broken egg, we may infer that this structure has arisen from the laws of growth, and has been taken advantage of in the parturition of the higher animals.
On the face of it, Darwin’s reasoning here is odd. If he wants to evaluate the hypothesis that mammals have skull sutures because this facilitates live birth, why does he consider the fact that nonmammals have the sutures but not the live birth? Let us hope that he isn’t thinking that if a trait T evolved because the trait facilitated X in one lineage, that T cannot be present without X in any organisms on earth. Penguins do not refute the hypothesis that wings evolved to facilitate flight in birds. And the hypothesis that a species of lizard evolved its green coloration because this color provided camouflage does not require that every green organism on earth gains protective coloration from its being green.
What Darwin is doing in this and in other similar passages is exploiting the fact of common ancestry to test hypotheses about natural selection. The reason that birds and reptiles are relevant to the question of why mammals have skull sutures is that all these organisms share a common ancestor. Common ancestry allows Darwin to infer what happened in the lineage leading to modern mammals. The fact that present day birds and reptiles have sutures but no live birth is evidence that sutures were present in the lineage leading to modern mammals before live birth evolved. If so, the sutures did not evolve because they facilitated live birth. On the contrary, live birth evolved after the sutures were already in place.
Darwin does not spell out the details of this inference, but modern evolutionary biologists will recognize it as an application of the principle of parsimony. Consider the phylogenetic tree shown in the accompanying figure. The tips of the tree represent modern mammals, reptiles, and birds. This is not the tree that a modern biologist would draw, but it may well have been the one that Darwin thought is true. As you move down the page, you are moving from present to past. The lines represent lineages; when two of them coalesce, you have reached a common ancestor. The tree says that mammals and birds are more closely related to each other than either is to reptiles; A2 is an ancestor of the first two, but not of the third. If you go sufficiently far into the past, you will find a common ancestor (A1) that unites all three of these present day groups.
The figure also indicates the traits (±skull sutures; ± live birth) that contemporary mammals, birds and reptiles exhibit. Given this tree, and the features exhibited by its tips,
what is the most reasonable inference concerning the characteristics of the ancestors A1 and A2? The most parsimonious inference is that A1 and A2 both have skull sutures but no live birth. This is the most parsimonious reconstruction in the sense that it requires fewer changes in character states in the lineages leading to the present than any other reconstruction. If this most parsimonious reconstruction is correct, we can deduce that skull sutures evolved before live birth made its appearance in the lineage leading to modern mammals; the mammalian lineage is represented in the figure by a broken line. This parsimony argument justifies Darwin’s statement that sutures now facilitate, or may even be indispensible for, live birth in mammals, but this is not why the sutures evolved.
The argument just described raises an interesting philosophical question: why should we think that the principle of parsimony is a good inferential rule? Why should we think that the most parsimonious hypothesis is true? I won’t pursue this enticing question here. Rather, the point of relevance is that in Darwin’s theory, and in the evolutionary biology of the present, common ancestry is not an unrelated add-on that supplements the hypothesis of natural selection. Instead, common ancestry provides a framework within which hypotheses about natural selection can be tested. In Darwinian biology, a lineage is like a mineshaft that extends from the surface of the earth to deep below, with multiple portholes connecting surface to shaft at varying depths. By peering into a porthole, we gain evidence about what is happening in the shaft. The more portholes there are, the more evidence we can obtain. Thanks to common ancestry, facts about the history of natural selection become knowable.
There is an asymmetry in how common ancestry and natural selection are related to each other in Darwin’s theory. To get evidence for common ancestry, natural selection need not have caused any of the traits we now observe. But to get evidence for natural selection, Darwin needs to be able to think of present day organisms as tracing back to common ancestors. Selection doesn’t make adaptations out of nothing; adaptations are modifications of the traits of ancestors. To know what those ancestors were like, we need to be able to infer their characteristics from what we now observe. It is common ancestry that makes those inferences possible.
If this is the right picture of how common ancestry and natural selection are related in Darwin’s theory, a puzzle presents itself: why did Darwin write the Origin by front-loading natural selection? Darwin does mention some ideas about common ancestry early in the book, but the big picture of there being one tree of life for the whole living world emerges only gradually, and later. On the whole, it is natural selection that comes first. Why is the book structured like this? Why didn’t Darwin begin by defending the idea of common ancestry and then gradually introduce natural selection as a secondary theme?