builds new traits from old ones. Darwin himself noted that “an organ rendered, during changed habits of life, useless or injurious for one purpose, might easily be modified and used for another purpose.”
But even when we’ve established that a trait is vestigial, the questions don’t end. In which ancestors was it functional? What was it used for? Why did it lose function? Why is it still there instead of having disappeared completely ? And which new functions, if any, has it evolved?
Let’s take wings again. Obviously, there are many advantages to having wings, advantages shared by the flying ancestors of flightless birds. So why did some species lose their ability to fly? We’re not absolutely sure, but we do have some powerful clues. Most of the birds that evolved flightlessness did so on islands—the extinct dodo on Mauritius, the Hawaiian rail, the kakapo and kiwi in New Zealand, and the many flightless birds named after the islands they inhabit (the Samoan wood rail, the Gough Island moorhen, the Auckland Island teal, and so on). As we’ll see in the next chapter, one of the notable features of remote islands is their lack of mammals and reptiles—species that prey on birds. But what about ratites that live on continents, like ostriches? All of these evolved in the Southern Hemisphere, where there were far fewer mammalian predators than in the north.
The long and short of it is this: flight is metabolically expensive, using up a lot of energy that could otherwise be diverted to reproduction. If you’re flying mainly to stay away from predators, but predators are often missing on islands, or if food is readily obtained on the ground, as it can be on islands (which often lack many trees), then why do you need fully functioning wings? In such a situation, birds with reduced wings would have a reproductive advantage, and natural selection could favor flightlessness. Also, wings are large appendages that are easily injured. If they’re unnecessary, you can avoid injury by reducing them. In both situations, selection would directly favor mutations that led to progressively smaller wings, resulting in an inability to fly.
So why haven’t they disappeared completely? In some cases they nearly have: the wings of the kiwi are functionless nubs. But when the wings have assumed new uses, as in the ostrich, they will be maintained by natural selection, though in a form that doesn’t allow flight. In other species, wings may be in the process of disappearing, and we’re simply seeing them in the middle of this process.
Vestigial eyes are also common. Many animals, including burrowers and cave dwellers, live in complete darkness, but we know from constructing evolutionary trees that they descended from species that lived aboveground and had functioning eyes. Like wings, eyes are a burden when you don’t need them. They take energy to build, and can be easily injured. So any mutations that favored their loss would clearly be advantageous when it’s just too dark to see. Alternatively, mutations that reduced vision could simply accumulate over time if they neither helped nor hurt the animal.
Just such an evolutionary loss of eyes occurred in the ancestor of the eastern Mediterranean blind mole rat. This is a long, cylindrical rodent with stubby legs, resembling a fur-covered salami with a tiny mouth. This creature spends its entire life underground. Yet it still retains a vestige of an eye—a tiny organ only one millimeter across and completely hidden beneath a protective layer of skin. The remnant eye can’t form images. Molecular evidence tells us that, around 25 million years ago, blind mole rats evolved from sighted rodents, and their withered eyes attest to this ancestry. But why do these remnants remain at all? Recent studies show that they contain a photopigment that is sensitive to low levels of light, and helps regulate the animal’s daily rhythm of activity. This residual function, driven by small
But even when we’ve established that a trait is vestigial, the questions don’t end. In which ancestors was it functional? What was it used for? Why did it lose function? Why is it still there instead of having disappeared completely ? And which new functions, if any, has it evolved?
Let’s take wings again. Obviously, there are many advantages to having wings, advantages shared by the flying ancestors of flightless birds. So why did some species lose their ability to fly? We’re not absolutely sure, but we do have some powerful clues. Most of the birds that evolved flightlessness did so on islands—the extinct dodo on Mauritius, the Hawaiian rail, the kakapo and kiwi in New Zealand, and the many flightless birds named after the islands they inhabit (the Samoan wood rail, the Gough Island moorhen, the Auckland Island teal, and so on). As we’ll see in the next chapter, one of the notable features of remote islands is their lack of mammals and reptiles—species that prey on birds. But what about ratites that live on continents, like ostriches? All of these evolved in the Southern Hemisphere, where there were far fewer mammalian predators than in the north.
The long and short of it is this: flight is metabolically expensive, using up a lot of energy that could otherwise be diverted to reproduction. If you’re flying mainly to stay away from predators, but predators are often missing on islands, or if food is readily obtained on the ground, as it can be on islands (which often lack many trees), then why do you need fully functioning wings? In such a situation, birds with reduced wings would have a reproductive advantage, and natural selection could favor flightlessness. Also, wings are large appendages that are easily injured. If they’re unnecessary, you can avoid injury by reducing them. In both situations, selection would directly favor mutations that led to progressively smaller wings, resulting in an inability to fly.
So why haven’t they disappeared completely? In some cases they nearly have: the wings of the kiwi are functionless nubs. But when the wings have assumed new uses, as in the ostrich, they will be maintained by natural selection, though in a form that doesn’t allow flight. In other species, wings may be in the process of disappearing, and we’re simply seeing them in the middle of this process.
Vestigial eyes are also common. Many animals, including burrowers and cave dwellers, live in complete darkness, but we know from constructing evolutionary trees that they descended from species that lived aboveground and had functioning eyes. Like wings, eyes are a burden when you don’t need them. They take energy to build, and can be easily injured. So any mutations that favored their loss would clearly be advantageous when it’s just too dark to see. Alternatively, mutations that reduced vision could simply accumulate over time if they neither helped nor hurt the animal.
Just such an evolutionary loss of eyes occurred in the ancestor of the eastern Mediterranean blind mole rat. This is a long, cylindrical rodent with stubby legs, resembling a fur-covered salami with a tiny mouth. This creature spends its entire life underground. Yet it still retains a vestige of an eye—a tiny organ only one millimeter across and completely hidden beneath a protective layer of skin. The remnant eye can’t form images. Molecular evidence tells us that, around 25 million years ago, blind mole rats evolved from sighted rodents, and their withered eyes attest to this ancestry. But why do these remnants remain at all? Recent studies show that they contain a photopigment that is sensitive to low levels of light, and helps regulate the animal’s daily rhythm of activity. This residual function, driven by small
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