How Did Charles Darwin Prove That All Animals Have A Common Ancestor

This is a précis of an argument that I developed in an article chosen "Did Darwin Write the Origin Backwards?" The article was published in 2009 and may be found on my web ready at http://philosophy.wisc.edu/sober/recent.html . An expanded version of the argument is the first chapter of a book that I'g publishing at the stop of 2010 with Prometheus Books. The book has the aforementioned championship as the 2009 article.

Although Darwin's theory is often described as the theory of development past natural selection, near commentators recognize that mutual ancestry (the idea that all organisms now alive on world 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 ii 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 up how the two ideas are evidentially related. How does common ancestry impact the way in which testify concerning natural selection should be evaluated? And how does natural selection bear upon 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 near valueless to the systematist. For animals belonging to two near distinct lines of descent, may readily become adapted to like conditions, and thus presume a shut external resemblance; but such resemblances will not reveal – will rather tend to muffle their claret-relationship to their proper lines of descent.

The fact that man beings and monkeys have tailbones is evidence for common ancestry precisely because tailbones are useless in humans. Dissimilarity this with the torpedo shape that sharks and dolphins share; this similarity is useful in both groups. One might expect natural option to crusade the torpedo shape to evolve in big aquatic predators whether or not they have a mutual 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 past two species is stiff bear witness that they have a common antecedent, one must be able to judge whether there was option for the trait in the lineages leading to each. In this sense, knowledge of natural choice 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 accept been an important influence on trait development for there to be potent 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 and then far as the hypothesis of common ancestry is concerned. That's the bespeak that Darwin is making in the passage I only quoted. Inferring common ancestry does non require that natural selection has occurred.

What virtually the converse question – how does the fact of common ancestry affect the interpretation of bear witness for natural selection? Ane of Darwin's most famous arguments concerning natural selection does not depend 1 whit on common beginnings. This is Darwin'southward Malthusian argument. If reproduction in a population outstrips the supply of food, the population volition be cut back by starvation. If the organisms in the population vary with respect to characteristics that impact their ability to survive, and if offspring inherit these fitness-affecting traits from their parents, the population will evolve. The process of natural option is a consequence of these weather condition and it can and will occur even if no 2 species have a common ancestor.

All this is correct, but there is more to the Darwinian picture show of natural selection than this. The Malthusian argument establishes that selection has occurred – that some traits inverse frequency because of their influence on the viability of organisms. But which traits evolved by natural option? Darwin doesn't call up that every trait we find evolved because in that location was pick for it; recall his comment in the Origin that pick is "the chief but not the exclusive cause" of development. And if a trait did evolve under the influence of natural selection, why was it favored past natural selection? Information technology is these questions, which business organization the detailed application of the hypothesis of natural selection to examples, that common beginnings helps to answer.

An interesting illustration of how Darwin uses the assumption of common beginnings to remember virtually natural selection may exist constitute in his discussion of why mammals in utero have skull sutures that allow them to laissez passer through the birth canal:

The sutures in the skulls of immature mammals have been advanced as a cute adaptation for aiding parturition [live birth], and no doubt they facilitate, or may be indispensable for this deed; simply as sutures occur in the skulls of young birds and reptiles, which take 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 college animals.

On the face of information technology, Darwin'southward reasoning here is odd. If he wants to evaluate the hypothesis that mammals have skull sutures because this facilitates alive nascence, why does he consider the fact that nonmammals have the sutures but not the live birth? Let us promise 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 Ten in whatsoever organisms on globe. Penguins do not refute the hypothesis that wings evolved to facilitate flying in birds. And the hypothesis that a species of lizard evolved its green coloration considering this color provided camouflage does non crave that every greenish organism on earth gains protective coloration from its being greenish.

What Darwin is doing in this and in other like passages is exploiting the fact of common ancestry to test hypotheses about natural choice. 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 24-hour interval birds and reptiles have sutures but no live birth is evidence that sutures were present in the lineage leading to mod mammals earlier live nascence evolved. If so, the sutures did not evolve because they facilitated live birth. On the opposite, alive birth evolved later on the sutures were already in place.

Darwin does non spell out the details of this inference, but mod evolutionary biologists will recognize it every bit an awarding 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 non the tree that a modern biologist would draw, but it may well have been the one that Darwin idea is true. As you motility downwards the page, you are moving from nowadays to past. The lines represent lineages; when 2 of them coalesce, you have reached a mutual 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 beginning two, simply not of the third. If you become sufficiently far into the past, you will find a common ancestor (A1) that unites all 3 of these present day groups.

The figure also indicates the traits (±skull sutures; ± live birth) that contemporary mammals, birds and reptiles showroom. Given this tree, and the features exhibited by its tips,

what is the nigh reasonable inference apropos the characteristics of the ancestors A1 and A2? The most parsimonious inference is that A1 and A2 both accept skull sutures but no live birth. This is the nearly parsimonious reconstruction in the sense that it requires fewer changes in grapheme states in the lineages leading to the nowadays than any other reconstruction. If this most parsimonious reconstruction is right, we tin can deduce that skull sutures evolved before alive nascency 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 argument that sutures now facilitate, or may even be indispensible for, alive birth in mammals, only 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 skillful inferential rule? Why should we think that the almost parsimonious hypothesis is true? I won't pursue this enticing question hither. Rather, the point of relevance is that in Darwin'southward theory, and in the evolutionary biological science of the present, common ancestry is not an unrelated improver that supplements the hypothesis of natural selection. Instead, common ancestry provides a framework inside which hypotheses about natural selection can be tested. In Darwinian biological science, a lineage is like a mineshaft that extends from the surface of the earth to deep beneath, 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 than portholes at that place are, the more evidence we tin can obtain. Thanks to common beginnings, facts about the history of natural selection become knowable.

There is an disproportion in how common ancestry and natural selection are related to each other in Darwin'due south theory. To go show for common ancestry, natural option demand non have acquired any of the traits we now discover. But to get evidence for natural selection, Darwin needs to exist able to think of nowadays day organisms equally tracing dorsum to mutual ancestors. Selection doesn't make adaptations out of nix; adaptations are modifications of the traits of ancestors. To know what those ancestors were like, nosotros demand to exist able to infer their characteristics from what we now detect. 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'due south theory, a puzzle presents itself: why did Darwin write the Origin past front-loading natural selection? Darwin does mention some ideas about common beginnings early in the book, but the big picture of there being one tree of life for the whole living world emerges simply gradually, and later. On the whole, it is natural choice that comes kickoff. Why is the volume structured like this? Why didn't Darwin begin past defending the idea of common ancestry and so gradually introduce natural choice as a secondary theme?

Source: https://nationalhumanitiescenter.org/on-the-human/2010/06/common-ancestry-and-natural-selection-in-darwin%E2%80%99s-origin/

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