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Consider a biological structure which no longer benefits an organism, such as the eyes of an organism whose population now lives in total darkness. I can think of three reasons why such a structure might disappear:

0) Random changes to the structure over time wouldn't be corrected by selection favoring the functional version of the structure, leading to a wider variation where most versions of the structure no longer effectively function.

1) The resources the structure demands could be better spent on structures which are actually being used; e.g. human eyes require a lot of blood that could be used elsewhere.

2) Perhaps the existence of a very complex structure leads to biological problems which would no longer be an issue if the structure were not present; e.g. human breasts plus breast hormones frequently leads to cancer.

Are these three examples reasonable means by which a feature would disappear? Are there any other possible reasons?

Is there a general name for the phenomenon of evolutionary removal of vestigial features due to those features no longer being useful to a population?

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    $\begingroup$ Natural selection? $\endgroup$ – tel May 22 '15 at 20:50
  • $\begingroup$ I'm referring specifically to the loss of unused structures, because I've heard some biology students claim "it doesn't happen" which is overly broad. It would be useful if there were a name for when "it actually does happen". $\endgroup$ – Trixie Wolf May 22 '15 at 21:03
  • $\begingroup$ Your examples are fine, but in general you should try to avoid "just-so" stories when considering evolutionary events. The truth is always a combination of the kinds of reasons you listed. The most you can say for sure are post-hoc phenomenological observations, ie "The fitness of the population which lost its tails increased relative to the fitness of the population which retained them". $\endgroup$ – tel May 22 '15 at 21:05
  • $\begingroup$ I didn't mean to imply that those reasons were in any way mutually exclusive. $\endgroup$ – Trixie Wolf May 22 '15 at 21:08
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This phenomenon can be (and has been) described as regressive evolution (the loss of a phenotypic trait). There are several reasons why this occurs:

The eye degeneration example you chose is a good one because it is well studied in cavefish (which evolved from sighted surface fish and have degenerate eyes). Normal eye development is under the control of the Pax6 transcription factor. Expression of another transcription factor, Shh, reduces Pax6 expression. Shh expression along the embryonic midline is responsible for splitting the eye field bilaterally. Overexpression of Shh in surface fish leads to eye degeneration and, indeed, it was found that cavefish have an expanded Shh expression pattern.

Cavefish have also undergone a behavioural shift to bottom feeding and have become less aggressive to focus more on finding food. As it happens, the expanded Shh expression also causes a widening of the jaw and amplification of taste buds, both of which aid in scooping and sampling the river bottom. Furthermore, increased Shh expression during brain development influences a decrease in aggressiveness and a shift to foraging behaviour.

This is an example of pleiotropic antagonism: positive selection for jaw enhancements and behavioural changes via expanded Shh expression, which increase fitness in cave environments, can explain why the eye degenerated.

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    $\begingroup$ Thank you so much for your detailed answer. By describing the terminology and addressing the subject around it, I think you've helped me understand the underlying concept itself much better. $\endgroup$ – Trixie Wolf May 23 '15 at 23:20
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    $\begingroup$ @TrixieWolf You're welcome $\endgroup$ – canadianer May 24 '15 at 2:06
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So-called "vestigial" features are defined in a post-hoc fashion as features which do not have a positive correlation with a population's fitness under its current living conditions. There are no special names for the evolutionary processes that lead to their loss, since they are exactly the same evolutionary processes that act upon any feature that has a neutral/negative effect on fitness.

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