The evolution of feathers: How sexual selection set the stage for natural selection

Sexual selection might create bridges across the adaptive landscape.

Evolution is sometimes depicted as a journey across a hilly landscape: valleys correspond to low fitness areas while hills represent adaptive optima. Natural selection will push populations uphill, culminating in beautifully adapted phenotypes. But what happens when a population reaches such an adaptive peak? Natural selection won’t allow a slow descent, so the population is effectively stuck on the peak even though there might be other (perhaps better) adaptive peaks nearby. In a recent Evolution paper, Scott Persons and Philip Currie provide a possible solution for this conundrum: sexually selected bridges. They illustrate this concept with the evolution of feathers.

Ajancingenia_reconstruction.png

A male Ajancingenia yanshini with a tail-feather fan displaying to female. © Sydney Mohr | Wikimedia Commons

 

A Journey Across the Adaptive Landscape

The fossil record of non-avian Dinosaurs contains several examples of early feathers: simple filaments or plumaceous structures (Stages 1 and 2 in the figure below). The consensus among paleontologists is that these “feathers” primarily functioned as insulators. In terms of the adaptive landscape, early feather-like structures allowed these Dinosaurs to climb the adaptive peak of insulation.

feathers

The different stages of feather evolution © Emily A. Willoughby | https://emilywilloughby.com

Later on, more complex arrangements of feathers evolved, such as the tail fans of oviraptosaurs. One can easily see how such elaborate feathers could function in sexual displays, similar to modern birds. Hence, these more complex feathers could be influenced by sexual selection. These feathers might have been a nuisance in terms of survival: think of the difficulties long-tailed peacocks encounter when trying to escape from predators. Hence, the complex feathers provide low fitness in the perspective of natural selection, but high fitness in a sexual selection context. However, the sexual selection hypothesis is still quite speculative. The authors state that “paleontology still awaits the single most conclusive piece of affirmative evidence: the recognition of sexual dimorphism in dinosaur feather morphology.”

If we return to the adaptive landscape metaphor, the Dinosaurs transversed a valley with the aid of sexual selection. The journey across this valley eventually brought the Dinosaurs at the base of an unexplored adaptive peak: the hill of aerial locomotion. Here, natural selection started pushing the populations uphill, resulting in the feathered and flying birds we know today.

evo13795-fig-0004-m.jpg

The hypothesized journey across the adaptive landscape of feather evolution. Sexual selection provided a bridge between isolation (left peak) and aerial locomotion (right peak). From: Persons & Currie (2019) Evolution

 

A Hybrid Bridge

This study provides a nice example of how sexual selection can provide the raw material for natural selection to work with. Given that this website mainly concerns hybridization in birds, you might be wondering what this blog post has to do with avian hybrids. Honestly, nothing. I just found this Evolution paper so cool that I decided to write about it.

Although… Hybridization could provide another way to bridge two adaptive peaks. The exchange of genetic material between species might allow populations to explore different areas of the adaptive landscape. I have covered these ideas in previous blog posts on combinatorial speciation and hybridization as the engine of adaptive radiations.

 

References

Persons, W. S., & Currie, P. J. (2019). Feather evolution exemplifies sexually selected bridges across the adaptive landscape. Evolution, 73(9):1686-1694.

A big thanks to Emily A. Willoughby for permission to use the feather artwork. Be sure to check out her website (https://emilywilloughby.com) for more amazing artwork.

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