Barrier Valleys, Trait Divergence, & the Split of Species

What one exemplary valley tells us about the drivers of avian speciation
Take a good look at the image above. Dry, prickly, windswept, and vast. This is part of the valley carved into the Peruvian landscape by the Marañon River. Long considered the source of the Amazon River, this ancient tributary has left an indelible mark on both the landscape and the birds that live there.
In the tropics, the exquisite diversity of life dwells upon a scaffold of geographical features like the Marañon. Every feature tells a story to those who listen, whispering clues of its role in organizing the diversity of life we see today. In one locale, we find one unique cluster of species, while in the next, we observe a different cluster. Investigating this pattern often starts with studying geography. In doing so, we come closer to understanding not only why life is organized as it is in the tropics, but also to understanding why it's so diverse in the first place.

There are many explanations of why and how the tropics are so diverse. Some claim it's the wide variety of resources there, providing the fodder populations need to split into many species. Others point to the climatic stability of tropical regions, where events like glaciation have never brought about high extinction rates. Without these extinctions to open up resources, natural selection favors more and more adaptation to a particular resource. As species go through this process, they benefit from more efficient use of their resource-specialty. But like a picky child eating only their preferred foods off their dinner plate, specialized species leave more resources unused the more they specialize. As species subdivides resources at a finer scale, allowing species to subdivide as well, making it possible for more species to coexist in a given area without risking competition.

As is often the case in ecology and evolution, these explanations are intertwined. But at least with birds, geography stands out among them as consistent factor. Indeed, the surest way to get two (or more) species from one is to subdivide them into different geographic areas.

This is exactly what the Marañon Valley does, dividing a homogeneous tropical landscape — and the organisms that live there —in two with an inhospitable arid valley.
A typical, nonmigratory tropical bird: Speckled Tanager. How did geography help lead to its speciation? Photo by Nick Athanas of antpitta.comIt is necessary to choose a visual aid that is appropriate for the material and audience.
Of course, this valley isn't inhospitable for all organisms. There are certainly plants there, and these plants produce energy that sustain other organisms. How can the valley have the power to split bird populations that can fly? Around this valley exists humid montane rainforest, filled with a myriad of ecologically picky bird species. These picky, often sedentary species are not likely to disperse far from their favored habitat, let alone across a broad and dry valley. To them, it is inhospitable, appearing other-worldly in comparison to the rainforest they call home. A key is that when birds aren't crossing the valley, neither are their genes. Without interbreeding, populations on either side of the valley are free from homogenizing gene flow, and can proceed to differentiate into distinct species. This is why the Marañon Valley is an effective biogeographic barrier. It isolates habitat on either side that sustains similar bird communities, and acts as a near perfect stage for observing the process of differentiation.
Rarely, though, dispersal does occur. Bird populations are tricky in that dispersal — the propensity to spread into new areas — changes idiosyncratically. Depending on the climate, a given bird species may disperse frequently, only to become practically sedentary a millennium or two later.

These rare dispersal events have occurred randomly throughout history, leading to bird species that have been split by the Marañon Valley for various amounts of time. Some populations (and their genes) have been isolated from their sister population on the other side for a long time, whereas others have been isolated for a short amount of time. Those that have remained separated for long enough have accumulated enough differences to diverge into two species. Those that have been experiencing dispersal across the valley may remain similar enough to be considered the same species.
This pattern is obvious around the Marañon Valley. Many species on one side of the valley share a common ancestor with a species on the other side—two species that were one until the valley separated them. Inhabiting similar habitat on both sides of the Marañon Valley, they face similar challenges, benefit from the same adaptations, and partake in similar if not identical ecosystems. They still behave similarly, feed on the same things, nest in similar ways, and displaying similar reproductive strategies.
And yet, they look different.
But wait — if they are still ecologically similar to each other, why do they look different? This, my friends, is the key question, and is why the Marañon Valley and its divided avifauna are so interesting. We will employ an exciting recent study to answer it.
Enter a 2015 study by Benjamin Winger, of University of Michigan, and John Bates, of Chicago's Field Museum of Natural History titled The tempo of trait divergence in geographic isolation: Avian speciation across the Marañon Valley of Peru. The study relied on the Marañon Valley's capacity as a natural experiment, where populations can be observed at multiple stages in the process of differentiation.
The authors sampled three species from eight genera, 24 species in all. Within each genus, one of the three species had to appear roughly the same on both sides of the Marañon Valley (called the "monotypic" species). The next two species had to be each other's "sister", once one species that was later divided by the Marañon Valley. Presumably, the monotypic species would have low genetic divergence (how different their DNA is) to match their similar appearances. The sister species, on the other hand, were expected to have been separated for longer, with more genetic divergence to match any differentiation in their appearance. To ensure that the results weren't skewed by the adaptation that new environments demand, all sampled species occupied humid montane forest, where species tend to be ecologically picky and to have limited dispersal across the valley. This way they would be certain to learn something about why species evolve different appearances even when their environments provide no reason to.

Examples of sister species and monotypic sample species from Winger and Bates 2015 in the tanager genus Iridosornis
The authors sampled three species from eight genera, 24 species in all. Within each genus, one of the three species had to appear roughly the same on both sides of the Marañon Valley (called the "monotypic" species). The next two species had to be each other's "sister", once one species that was later divided by the Marañon Valley. Presumably, the monotypic species would have low genetic divergence (how different their DNA is) to match their similar appearances. The sister species, on the other hand, were expected to have been separated for longer, with more genetic divergence to match any differentiation in their appearance. To ensure that the results weren't skewed by the adaptation that new environments demand, all sampled species occupied humid montane forest, where species tend to be ecologically picky and to have limited dispersal across the valley. This way they would be certain to learn something about why species evolve different appearances even when their environments provide no reason to.

With these 24 species as their study system, Winger and Bates began collecting data. They sampled a part of their mitochondrial DNA known to be useful as a molecular clock. They measured plumage color using spectrophotometry and a scoring system. They measured body proportions, and analyzed vocalizations using bioacoustics software. With these measurements from each species, Winger and Bates' goal was to determine how long it takes species to evolve differences in appearance around Marañon Valley, not to mention why they evolve differences at all.
It seems intuitive to think that populations would evolve more differences before the process of splitting into different species. After all, what we call different species tend to look very different. Don't those differences lead to speciation, when populations that were once one species lose their propensity to interbreed?
In some cases, yes. The opportunity of new resources results in the kind of differentiation that occurs before speciation. If resources are available, natural selection favors populations that subdivide to seize these new opportunities. As these sub-populations adapt to the new resources, they change in appearance, and eventually speciate. A classic example of this is the adaptive radiation of Darwin's Finches, each adapting to their own niche after arriving on the Galapagos.

But in the case of the Marañon Valley, Winger and Bates found that bird populations evolved differences after they were split by the valley. Here, we return to our big question:
If two species are still ecologically similar to each other, why would they evolve to look different?
On their way to a possible answer, the Winger and Bates determined a threshold amount of time for separated species to start looking different: around two million years. It takes around this long for the Marañon Valley to split one species into two and for these two species to begin looking different in spite of their ecological similarity.

Next, take a good look below. On the y-axis is a measure of phenotypic differentiation (differentiation in traits that make up their appearance), while on the x-axis is genetic divergence. The expectation is that a certain level of genetic distance — to qualify as different species — will only exist above a certain level of phenotypic differentiation. This is exactly what we see with plumage (A), where the horizontal dotted line marks the cutoff. All the split sister species are above it in black, whereas all the monotypic species, with low genetic and phenotypic divergence, are below it. But we don't see this in body proportions (B) or in voice (C). This means that either something was wrong with Winger and Bates's analysis, or that morphology and voice have not changed after speciation.
Why wouldn't the expected pattern be observed in body shape or voice? The authors suggest that because body proportions tend only to change when adapting to different ecological conditions, it is expected that they would find little change when all their sample species are in the same ecological conditions. Additionally, the authors suggest that the lack of pattern may be an artifact of their analysis, and not a legitimate result.

But what of plumage? A bird's feathers are a means of visual communication, used to fend off rivals, attract mates, and maintain social groups. They are intimately tied to behavior, and to interactions with other individuals of the same species. What we are seeing is plumage — a trait associated with social behavior like signaling — diverging faster than traits associated with adaptation to new habitats, like body proportions. What does this suggest? I'll let Winger and Bates answer:
""[O]ur study suggests that social selection plays an important role in driving phenotypic differentiation across the Marañon."
Yes, sociality may explain why different species start to look different in spite of living in the same habitat. It appears that in the absence of new environmental challenges, changes in birds' aesthetic preferences can act as an evolutionary force, but this takes time. It takes time — around two million years, to be precise — for mutations to provide new variation in appearance. This variation in appearance may be in male plumage ornaments that attract females in social dominance signals like the black bib on male House Sparrows, or in the bright colors that allow individuals to compete for territories.

All of these traits depend on unique social preferences: females who prefer the slightly-more-extravagant, males that are intimidated by the slightly-more-reddish, juveniles that are more submissive to the more extensive black bib. And these preferences, like human pop culture, seem never to stop changing.

The point is this: in analyzing trait divergence across the Marañon Valley, Winger and Bates have shown that social selection takes around two million years to produce two species from one (when a biogeographical barrier is in place). When populations remain in one piece, kept the same by gene flow, its social preferences also remain in one piece. Regardless, it takes either different resources or geographic isolation to get the process of speciation going. But once it does, social selection may be one mechanism that drives birds to be more different, to be more diverse, and to develop novel social trait preferences.

So in nature, given enough time and the right circumstances, sociality has the power to create new lineages, fill the landscape with more diversity, and, most wonderfully, to enrich the tapestry of life with novel colors and patterns.

And in that way, we see sociality among the diversity of nature mirroring our own.

Or perhaps we are mirroring the rest of the natural world? I'll leave it to you on that one.
Citations:
Gill, Frank B. "Species taxonomy of birds: Which null hypothesis?." The Auk 131.2 (2014): 150–161.

Ho, Simon YW, et al. "Biogeographic calibrations for the molecular clock."Biology letters 11.9 (2015): 20150194.

Winger, Benjamin M., and John M. Bates. "The tempo of trait divergence in geographic isolation: Avian speciation across the Marañon Valley of Peru." Evolution 69.3 (2015): 772–787.


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