by Paul Braterman
Evolution has nothing to do with progress. Most evolution doesn't even have anything to do with adaptation, and it is perfectly possible for a change that is worse than useless to spread through a population. Paradoxically, however, such non-adaptive change may be a necessary prelude for major adaptations.
This post was inspired by a recent opinion piece (open access here1) in BMC Biology, entitled "Splendor and misery of adaptation, or the importance of neutral null for understanding evolution" (I will explain what "neutral null" means later). The paper itself is in parts highly technical, with 86 references to the original scientific literature, but I will try here to give a general overview of some of the main conclusions, and to place them in context.
Darwin and Wallace both thought that evolution was driven by selection. If so, then whenever we find a feature in an organism, it makes sense to ask what function it serves. The function may for example be help in survival (natural selection, in the narrowest sense of the term), or help in obtaining mating opportunities (sexual selection).
L: The recurrent laryngeal nerve passing under the aortic arch. Illustration by Jkwchui after Truthseeker-2004, via Wikipedia
Because the evolution of a species is constrained by its history, there will be features that are themselves non-adaptive, but come about as side-effects of more important adaptive changes. Such incidental maladaptions include the tortuous paths of major nerves and arteries, which have arisen as the unwanted by-product of changes in body plan since our fish-like ancestors. One well-known example is the recurrent laryngeal nerve, which loops under the aorta near the heart and back up again on its way from the cranium to the larynx and oesophagus. In a fish, its path is more or less a straight line, but as the heart has moved down in the body, and the aorta with it, the nerve has been forced into this contorted pathway.
Likewise, we can expect to find vestigial organs, which once had a function, and are now redundant, but have not yet completely disappeared. An example is the pelvis of the whale, inherited from its four-legged ancestors. Such vestigial organs often acquire secondary functions, in the phenomenon known as exaptation. The bones in the mammalian ear, related to bones in a reptile's flexible jaw, illustrate this. [Insert diagrams: whale pelvis; jaw-to-ear] And indeed whales use their pelvis and femur relics in sexual embraces.
R: Sperm whale with drawing of skeleton, NOAA via Wikipedia
Adaptationism is the view that all aspects of an organism are, directly or indirectly, the result of selection. So every feature needs to be explained, either in terms of its own function, or as an incidental relic or side-effect of more directly functional features. This is a natural enough assumption, but like all assumptions it requires justification. Otherwise it is merely a "Just So" story.
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