Wings serve most birds for flying, or (as in penguins) for swimming. But ostriches, which exclusively use only their legs for locomotion, still have wings. Why?
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3$\begingroup$ Watch Planet Earth II - Grasslands. There is apparently a funny use for those wings :) $\endgroup$– A. SteinerCommented Dec 27, 2016 at 21:05
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$\begingroup$ Related: Why don't mammals have more than 4 limbs?. $\endgroup$– Remi.bCommented Dec 28, 2016 at 15:21
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3 Answers
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There is a common misconception that a selective process is needed to remove a feature from a population. However, the correct approach is quite the opposite: selective processes normally maintain a feature in a population: if a given feature stops promoting selective advantage, its corresponding genes start to "erode" and the feature disappears from the population (and that can happen in different rates, depending on several factors: have in mind that maladaptative features do exist).
That being said, the maintenance of wings in ostriches indicates us that there must be a reason for it.
It has been hypothesised that wings:
- Improve balance when running;
- Helps regulating temperature;
- Serve as mating display.
(source: University of Washington)
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$\begingroup$ What do you mean "by genes start to erode"? $\endgroup$ Commented Dec 27, 2016 at 4:50
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1$\begingroup$ Imagine that the mutation rate is the same along all the DNA. We have 2 genes, one that gives advantage and one that stopped giving any selective advantage. Thinking about the population as a whole, the first gene remains almost the same: mutants are removed. But the second gene starts accumulating mutations. That's the "erosion" in my answer. It's not a common term in evolution, I borrowed it from Sean Carroll in "The making of the fittest". $\endgroup$– user24284Commented Dec 27, 2016 at 4:57
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$\begingroup$ this doesn't answer the question, the op is asking why they have them, not why they haven't lost them. $\endgroup$– montelofCommented Jun 6, 2017 at 18:56
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$\begingroup$ And that's precisely what I answered. $\endgroup$– user24284Commented Jun 6, 2017 at 22:50
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They are vestigial structures. Vestigiality refers to genetically determined structures or attributes that have lost some or all of their ancestral function in a given species, but have been retained during the process of evolution.
The emergence of vestigiality occurs by normal evolutionary processes, typically by loss of function of a feature that is no longer subject to positive selection pressures when it loses its value in a changing environment. The feature may be selected against more urgently when its function becomes definitively harmful. Typical examples of both types occur in the loss of flying capability in island-dwelling species.
In the case of an ostrich, its ancestors used wings to fly, but the development of faster running abilities, keener eyesight and a paucity of capable predators allowed the bird to no longer receive positive selective pressure to have " functional" wings.
It's late. I'll add more detail in the am.
answered Dec 27, 2016 at 7:46
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2$\begingroup$ I am not sure vestigial is the correct term in this case. Since the wings have acquired (or never lost) another function during male display. Furthermore I think they help balance while running. $\endgroup$– have funCommented Dec 28, 2016 at 8:51
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2$\begingroup$ @have fun : I thought about that, but really vestigiality can be used in instances in which an organism's feature has lost "SOME or all" of its "ancestral function." In this case, the ostrich has lost SOME of its wing functionality (flying). So I think vestigiality still applies. $\endgroup$ Commented Dec 29, 2016 at 18:53
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Why not? Since the wing on a ostrich does not harm the animal, there is no selection pressure to remove it from the population. And male ostriches do use their wings for mating displays. So it isn't completely without use.
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1$\begingroup$ Or to put it a bit differently, ostriches with wings the size they are are not at a competitive disadvantage to those with smaller (or larger) wings, so when/if random size changes happen, there's no preferential selection. At least not in the time span we can look at: it's possible that if we came back in a few million years, ostrich descendants would have smaller wings, or they might not be visible at all. (Though the genes might be there, like the "sonic hedgehog" gene in snakes: news.nationalgeographic.com/2016/10/snakes-grow-legs-evolution/… ) $\endgroup$– jamesqfCommented Dec 27, 2016 at 5:33