Origin Ch.14: Explanatory power

This post is part of a series on The Origin Of Species.  It was originally posted on the old blog in feb 2009, during the Darwin 200 celebrations.

It’s getting late in the book, and for a while now Darwin has not so much been explaining and testing his theories, as sitting back and letting his theories do some explaining and testing of their own. It seems to require constant repeating that a scientific “theory” is not a “guess”. A “theory” is a general principle which is supported by a significant and diverse body of data and observations, which stands up to testing, which makes predictions which can be shown to be true, and which has explanatory power. The explanatory power of evolution permeates all branches of biology.

The classic line is that evolution turned biology from a “stamp collecting” science into a modern and productive field. Nevertheless, taxonomists have not been dissuaded from their stamp collecting. In chapter 14, though, Darwin did give meaning to many of their practices and trends. Linnaeus, father of the taxonomic system, was found that there were intuitive ways to place species in groups, and that the relationship between those species was somehow something more than just their shared traits. Darwin showed that this was because taxonomy had intuitively developed into grouping species roughly by how closely they are related by descent, and that evolution, by explaining the patterns of descent, thereby explained many of the quirks of taxonomy. “Community of descent is the hidden bond that naturalists have been unconsciously seeking.”

Evolution explains why we see great variation on the same fundamental body plans, and how those slight morphological variations underpin classification. And so evolution explains why, to the naturalist, it feels right to classify a marsupial mouse amongst its fellow marsupials, even though it superficially resembles the placental mouse. And evolution explains large scale trends, such as why some groups contain so many more species or sub-groups than others.

And evolution explains why, even though there are real distinct groups of species and real distinct groups of those groups, the system of calling those groups orders, sub-orders, families, sub-families, and genera is all so arbitrary — the observable divergence of characteristics and the diversification of groups is highly variable depending on the nature of those groups and their situation, and the non-uniform nature of extinction further complicates things. He uses an analogy to the evolution of language, and it is often claimed (though I am not sure whether it is true or a myth) that English has changed and diversified much more rapidly and extensively than French. (And molecular biologists and genomicists will be pleased that this analogy can also incorporate horizontal transfer.)

And evolution explains mimicry: why two butterflies which, based on the finer details of their anatomy and life cycle, should be placed in very distant groups, even though they look so similar that one can not tell them apart while they swarm. These mimics occur when one species has evolved a defense against predation — foul taste, poison, or a sting — and another takes advantage of the predator’s fear of that species, or rather, any individual which looks like it belongs to that species. And evolution explains why the mimic is always the less dominant species in terms of population than the mimicked. And evolution explains why mimicry is so much more common in insects than in other major animal groups.

And evolution explains why such diverse species as the vertebrates share such similar patterns of early development, and why vertebrate body plans are not entirely different from those of other animals. Today we can take this last principle even further: the field of genomics combines evolution with molecular biology to further explain quirks of classification, such as why bacteria are so hard to classify, why traits can stick together, and how we can be sure that all of the groups of life on earth can, ultimately, be classified into one group, related by descent.

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