Why do only some species look different?
A jaunt into a fascinating evolutionary inquiry
Even since before the times of Charles Darwin, naturalists have been engaged in dogged pursuit of what produces species. Encompassed in this question is a rich treasure trove of potential research questions: what produces variation in a population? What causes species to look and/or act differently from other species? How are these things affected by landscape, climate, and the presence of other species?
As you might be able to tell by reading through this publication, these questions are on my mind a lot. And I'm not alone. The origin stories of species and their many unique characteristics engages academics at almost every major institution, and serves as a muse to students considering a research career for themselves.

Among these academics are Emily Hudson, a Ph.D. candidate at University of Nebraska — Lincoln, and Trevor Price, a Professor at University of Chicago. Price's research career has basically addressed the problem of species: why do we have them? why are there so many? how quickly do they evolve? And of course, given that their work would show up on this publication, they both use birds as their study system.

These two researchers, one at the outset of her career and the other well-established and highly respected, published a paper in 2014 titled, Pervasive Reinforcement and the Role of Sexual Selection in Biological Speciation. In it, they sought to uncover how sexual selection, rather than natural selection, has an influence on speciation (Speciation, as a reminder, is when two populations that were once unified evolve differences. Among these differences is one critical difference: they don't mate much if at all with each other. In this writing, I'll call populations that are on the way to becoming species "potential species"). Hudson and Price, like many other ornithologists, observed something peculiar: some closely related species evolve to look very different, while others remain similar looking even after they have speciated. They also noticed that a lot of the traits that do tend to evolve differences between species are related to reproduction. The shiny blue patch on a male's head, the dazzling red color on the throat, or the intriguing black-and-white zebra stripes along the flanks. All these traits–secondary sexual traits–are subject to sexual selection.
Figure 1 from Hudson & Price 2014, with illustrations by Emiko Paul. It depicts a couple exemplary groups of where sexual selection appears to have led to the evolution of multiple species. On the left, we have bellbirds: clockwise from top left, we have 3-wattled, White, Bare-throated, and Bearded Bellbirds; males look different, but females have remained similar. On the left, we have a similar system in the subspecies of Golden Whistler. Note that ALL of these species/subspecies are essentially allopatric.
With this in mind, their question was this: does differentiation in these sexually selected traits affect whether or not two populations will mate with each other? In other words, do differences in appearance affect whether two potential species will hybridize?

First, a little background. These questions are tied intimately to whether two potential species exist in the same range (in sympatry), or if they are geographically separated (in allopatry). If two potential species are found in the same places, it is much more likely that they will hybridize, and eventually fuse back into one species.

It turns out that this sympatry problem is a pretty insurmountable one, at least in birds. Instead, most speciation happens in allopatry. Geographically separated, two potential species can evolve species-level differences without the homogenizing interference of genes flowing from the other potential species.
Figure 3 from Hudson & Price 2014, with illustrations by Ian Lewington. On the left, we have two species of Seicercus warbler. In spite of the fact that they are sympatric and look near-identical, they have remained distinct species. The same goes for the two species minivet on the right. How is this possible?
Hudson and Price's paper discusses one critical detail in this process: while in allopatry, both potential species' mate preferences and ability to recognize members of their own species diverge. Their so-called "windows of recognition" change, eventually leaving them only able to recognize members of their own species as potential mates.
This is where differences in appearance comes in. If the two potential species evolve differences in appearance while in allopatry, those differences become intimately connected to their window of recognition, i.e. if a bird doesn't exhibit a given set of traits, it won't be recognized as a potential mate.

But REMEMBER: all of this evolves. Species, which are made of many individuals, don't pass from one state to the next in immediate changes. Differences in appearance evolve gradually through time, as do changes in the window of recognition.

After some period of time, environmental and/or climatic changes will cause the allopatric potential species to again make contact, a phenomenon called secondary contact. Here, the two potential species are put to the test: will they have evolved different enough appearances &/or different enough windows of recognition to remain distinct? Or will they still be similar enough that they will fuse back into one species with pervasive reproduction between populations?

The answers to these questions depends on the system we're looking at, and on how much time the potential species spent in allopatry.
What Hudson and Price wanted to know was this: in species that remain distinct during secondary contact, what matters more: Divergence in appearance, or divergence in windows of recognition?
Just because a bird looks distinct in traits that sexual selection acts on does not mean it is a more distinct species.
If the former is the case, we would expect that species that look very different are always the more distinct species. But this just isn't the case in birds. Many groups (Scytalopus tapaculos, Phylloscopus Leaf Warblers, Certhia creepers, to name a few) contain species who have been distinct for a very long time, and yet remain near-identical, at least to humans.

After doing phylogenetic, morphological, and bioacoustic analyses, Hudson and Price discovered that divergence in secondary sexual traits is poorly correlated with completed speciation. Just because a bird looks distinct in traits that sexual selection acts on does not mean it is a more distinct species.

Instead, Hudson and Price think that it's the window of recognition that matters most. As long as a female bird can tell who is and who isn't a potential mate, it doesn't matter how similar the species are. Divergence in appearance isn't always a given, which lines up with what we observe in the world's diverse avifauna.
If the windows of recognition have diverged far enough and are narrow enough, sexual selection can lead to reinforcement, when two potential species are naturally selected to remain separate. If the window of recognition is what matters, then we have a fascinating mechanism of reinforcement: hybrid males may be just different enough that females in either of their parental species won't recognize him as a potential mate (we'd call this a narrow window of recognition). This puts hybrids at a selective disadvantage, and simultaneously reinforces speciation. Alternatively, if the window of recognition is too broad, females in either of the potential species have no problem mating with males from the other population. Hence, the populations fuse, and any potential they had to become distinct species is lost.

Pretty cool, eh? It's in thinking about things in this evolutionary fashion that we realize that appearances coevolve with abilities to perceive. By continuing to investigate how perception coevolves with appearance, we may finally understand what has brought about the extravagant diversity in birds. Along the way, we march ever forward to see whether our explanations with be refuted, or reinforced.
Citation:
Hudson, Emily J., and Trevor D. Price. "Pervasive reinforcement and the role of sexual selection in biological speciation." Journal of Heredity 105.S1 (2014): 821–833.
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