Simultaneous wing molt as a pre-adaptation for loss of flight

This molting strategy might speed up the evolution of flightlessness.

Numerous bird species have lost the ability to fly. Think, for example, about the many rail species that abandoned a flying lifestyle when they colonized new islands. Although the evolution of flightlessness is mostly connected with island species, there might be other factors that increase the likelihood of losing the ability to fly. In a recent study in The American Naturalist, Ryan Terrill investigated the influence of molting strategy on this evolutionary transition. Different bird species use different approaches to renew their plumage. Some species molt their wing feathers sequentially to maintain the ability to fly, while other species become flightless when they replace their wing feathers simultaneously. Because bird with a simultaneous molting strategy have to survive a period of flightlessness, they may transition more easily to a completely flightless life. In other words, simultaneous wing molt could function a pre-adaptation (or exaptation) for the loss of flight.

Rate of Evolution

To test this hypothesis, Terrill compared the evolutionary speed at which species lost the ability to fly for diverse molting strategies. Using different sets of phylogenetic trees, he consistently found “an elevated rate of evolution of flightlessness in birds with simultaneous wing molt”. This result supports the idea that this molting strategy can affect the evolutionary transition to a non-flying lifestyle. During the flightless molting period, these species have to acquire food and escape predators without flying. Morphological and behavioral adaptations to survive this period might predispose these species to evolve flightlessness at some point in their evolution. However, it is important to keep in mind that simultaneous wing molt is not a prerequisite for flightlessness, it can only speed up its evolution.

The connection between molting strategy and the evolution of flightlessness. Taxa in red undergo simultaneous wing molt whereas taxa in gray do not. About 68% of flightless species are in the 3% of birds that show simultaneous wing molt. From: Terrill (2020) The American Naturalist.

Exaptation

In the paper, Terrill uses the term “pre-adaptation” to describe the potential influence of simultaneous wing molt on the evolution of flightlessness in birds. To laypeople, this term seems to imply some predetermined purpose. As if evolution is preparing a species for an important adaptive change. This is obviously not the case. Evolution is a blind process without any foresight. Science writer Carl Zimmer put it nicely: “There’s no foresight involved, though—simply the lucky coincidence that a feature that evolved to do one thing may turn out later to do another thing even better.” To avoid this confusion, some biologists prefer the term “exaptation”, as proposed by Gould and Vrba. Whatever you want to call this interesting phenomenon, it is important to be aware that evolution does not follow a preprogrammed path. It is just the complex interplay between chance and necessity. And as evolutionary biologists, we are fortunate to work on unweaving this wonderful web of life.

References

Terrill, R. S. (2020). Simultaneous Wing Molt as a Catalyst for the Evolution of Flightlessness in Birds. The American Naturalist, 196(6), 775-784.

Featured image: Titicaca Grebe (Rollandia microptera) © Tsirtalis | Wikimedia Commons