There’s more than one way to skin a cat, as the saying goes. Although this euphemism is a tired cliché, it applies to how morphological traits may evolve. To solve a functional ‘problem’, species may evolve different morphological means to reach the same functional end. This is called “many-to-one mapping” in evolutionary biology. My colleagues and I studied how a fascinating group of insects, stalk-eyed flies, might morphologically overcome impaired flight brought about by their ridiculously exaggerated eye stalks, the span of which can be longer than the fly’s body itself!
Male stalk-eyed flies use their eye stalks to size up other males during fights and to attract potential mates. This advantage of having long eye stalks has resulted in the evolution of a very elongated ornament. The problem is, the long eye stalks mess up the physics of flight for the males, making the energy required to turn in flight much higher. However, to counter this problem, male stalk-eyed flies have evolved changes in their wings to compensate for their potentially reduced flight performance. They have longer wings, and differently shaped wings compared to females and males of species without elongated eye stalks. Our recent study shows that different groups of stalk-eyed flies have modified their wings over evolutionary time via different means to reach the same end of compensating for the long eye stalks. Such many-to-one mapping of morphology is probably much more common in nature than we realize, and our study emphasizes that it can apply to a diversity of evolutionary scenarios.
The take-home message from this research, and other work like it, is that phenotypes change over evolutionary time via modifications of different combinations of pieces in different groups. That is, A and B might change in one group, whereas A and C might change in another, but with the same functional endpoint in both. Such is the subtlety and beauty of natural selection.