My hot-dog student, Daniel Matute, has published a new paper in PLoS Biology, and although I say the paper is from “our lab,” it’s entirely Daniel’s work.
The paper is about a phenomenon whose importance in speciation is contested: reinforcement. Reinforcement describes a form of natural selection that can actually increase the reproductive barriers between a pair of species that descend from a common ancestor.
As I’ve mentioned on this site, most evolutionists define “species” as “groups of populations separated from other such groups by reproductive isolating barriers (RIBS, an appropriate Chicago acronym).” Those barriers are all the genetically-based traits of organisms that prevent them from exchanging genes where they co-occur in nature. RIBS include things like mate discrimination, a preference for living in different niches (so that members of different species don’t encounter each other), different mating seasons, or the sterility or lethality of any hybrids that are formed.
Most of us think that these barriers are simply the byproduct of evolutionary divergence that occurs when geographically isolated populations of a single species begin to evolve in different directions. I give examples of how this works in WEIT, but the discussion is too long to repeat in this short post.
In this view of life, species are simply evolutionary accidents. That is, nature doesn’t somehow favor the production of species, nor does natural selection favor the erection of reproductive barriers between groups per se. Rather, physically isolated populations evolve in different directions due to natural selection or random genetic drift, and when that divergence has proceeded to a certain extent, reproductive barriers appear simply as a byproduct. For example, if two species adapt to two different niches, their physiologies may diverge to the point that the combination of both species’ genes in a single hybrid makes it unfit or inviable.
But there is one form of selection that can favor the evolution of reproductive barriers, and that’s reinforcement. Reinforcement begins with two physically isolated populations that have been apart long enough to evolve some barriers to gene flow, but barriers that are not quite complete—so that they can’t qualify as “full” species. For example, two populations of flies might produce hybrids that are partially (but not completely) sterile, so if they meet there is still the possibility of some gene flow between them through the bridge of semi-fertile hybrids.
Now assume that these populations expand their ranges and, though previously isolated physically, now encounter each other. (This happens all the time in nature as geographic barriers like rivers, forest, and the like disappear or change their positions.) Members of the two populations will encounter each other, and some of them will mate. But if you mate with members of the “wrong” population, some of your offspring will be sterile or partially sterile.
There is thus an evolutionary disadvantage to mating with the wrong group, and an advantage to mating with the “right” one. Under these conditions, natural selection will favor individuals with genes that favor their mating with the right group, because those genes leave more copies than genes that don’t promote discrimination—”nondiscrimination” genes tend to wind up in sterile hybrids!
Thus, over time, a partially isolated species can become a fully isolated one through natural selection, and that’s how reinforcement works. It’s a process that directly favors the erection of reproductive barriers, and can, we think, put the finishing touches on speciation.
The problem is that though this sounds nice, and can indeed work if you make mathematical models, it’s hard to demonstrate in nature. To do so, you’d need to show that, in a pair of species that have some geographical overlap, the degree of mate discrimination, or of other reproductive barriers that prevent the formation of hybrids, is higher between populations in the area of overlap than it is between populations taken from areas where they don’t overlap (these tests are done in the laboratory).
There are a few promising examples of this (one was published in 1995 by my ex-student Mohamed Noor), but the frequency of reinforcement remains unclear. (When I was young, I was taught, for example, that it was always the final step in speciation. That of course can’t be true, because speciation can go to completion between groups that never contact each other in nature, for example species like the Galápagos tortoise that evolved after an ancient colonization of a remote island.)
Daniel noted that the species of Drosophila on the small African island of São Tomé, where we’ve worked for some years, have a distribution that looks promising for studying reinforcement. One species, Drosophila yakuba (“yak”), lives not only on the island but also on the African mainland. The other species, D. santomea (“san“) is endemic only to the island, and presumably evolved after the island was colonized by the ancestor of yak (we estimate this happened about 400,000 years ago).
The situation is propitious for studying reinforcement because, on the island, which is basically a volcanic mountain, the species have overlapping distributions: yak lives at sea level and up to about 1400 meters on the mountain, while san is found between 1100 and 1400 meters. There is thus an area of overlap between 1100 and 1400 meters, and in this area you can find a few hybrids (about 5% of the individuals). These hybrids are at a disadvantage, evolutionarily, because half of them—all the males—are sterile.
Thus we have two species with overlapping ranges, some hybridization in the area of overlap, and the possibility of gene flow through the fertile female hybrids. This is just the situation we need to look for reinforcement. So, is the mating discrimination between yak and san stronger when they come from the area of overlap than from the areas where they don’t overlap? (Remember, we do these tests in the lab and so can examine any pair of strains.)
Surprisingly, the answer, which we got eight years ago (Coyne et al. 2002), was no: mating discrimination, though strong, was equally strong no matter where you took your samples from. There didn’t seem to be any reinforcement. We don’t know why, but there are several explanations (perhaps, for example, there simply weren’t any mutations that sharpened mate discrimination.)
Daniel, however, went further, and found that there is indeed a form of reinforcement between these species, but not reinforcement of mate discrimination. Rather, it’s reinforcement of what we call postmating, prezygotic isolation or gametic isolation: barriers to gene flow that act after mating but before fertilization. One example is females of one species not using the sperm of males from the “wrong” species if they are inseminated by them.
This is what Daniel found:
1. Yak females from the area of overlap produce many fewer eggs (and progeny) when mated with san males than do yak females derived from outside the area of overlap. Thus, the reproductive barriers between the species were higher from places where they encountered each other. The effect was very strong and highly statistically significant. (Daniel’s Figure 2 shows this clearly). Daniel didn’t find this effect, however, when he looked at san females.
2. But what is the evolutionary advantage of producing fewer eggs/offspring when you’re inseminated by a male of the “wrong species”? For that would have to be the selective pressure that impelled the evolution of the “fewer-egg” syndrome in yak females who encounter san males.
One possibility is that if you live in the area of overlap, and sometimes mate with the wrong species, you have an evolutionary advantage if you can “recognize” this (I’m speaking of course of physiological recognition), dump those foreign sperm that produce semisterile offspring, and thus get a chance to mate sooner with the right species. (Species in this group tend to mate multiple times over their life.)
Sure enough, Daniel showed that when yak females were mated in the lab to san males, those females from the area of overlap lost sperm much faster than did yak females from outside the area of overlap. And he also showed that those females remated more quickly to their own males (yak) than did females who hadn’t used up their “wrong” store of sperm. In other words, there was an evolutionary advantage to dumping sperm when you’re inseminated by the wrong species, because by so doing you will, in the long run, produce more offspring with members of your own species. (We don’t know the mechanism by which females from the area of overlap get rid of the wrong sperm more quickly).
3. So, we find a pattern suggesting reinforcement and show that this pattern comports with how selection should operate. But can we mimic this whole process in the lab? Surprisingly, Daniel did, and it took only a few generations.
Daniel simply forced strains of the two species (taken from areas where they do not encounter each other) to live together in glass bottles in the lab. There were seven different pairs of strains to serve as evolutionary replicates. Each generation he removed the hybrids, mimicking complete sterility of the hybrids (of course, they’re not completely sterile in nature, but we wanted to avoid gene flow between the species, which makes it harder to tell them apart).
And, in only four generations (about two months), he observed that yak females had indeed evolved the “reinforcement” trait seen in the lab: when inseminated by san males, those females laid fewer eggs than did “control” yak females who were reared at the same density, but without the presence of the other species. Moreover, the degree of mating discrimination also increased, something that we didn’t observe as a pattern in the wild.
CONCLUSION: Well, the reason the study got published in PLoS Biology, I think, is because it’s not only a rare, pretty-well-documented case of reinforcement, but also because it’s the first case in which reinforcement in nature strengthened not mating discrimination, but a reproductive barrier acting after mating: gametic isolation. Most people hadn’t realized that postmating barriers could be also strengthened by selection. But they can be, and cases like this may be common. The reason why they might have been missed is, of course, because detecting the presence of a gametic barrier is much harder than simply measuring increased mate discrimination.
The next step is to allow some gene flow between the captive species, and see if we can still observe the evolution of gametic (and sexual) reinforcement under more realistic conditions. After all, in nature some fertile female hybrids are produced in the area of overlap, and presumably mate back to yak or san. That study is underway. And we’d like to understand why the evolution of higher mating discrimination occurs so readily in the lab, but isn’t seen in nature.
And a h/t to Daniel here, a terrific student who promises to be a star.
Matute D. R. (2010) Reinforcement of gametic isolation in Drosophila. PLoS Biol 8(3): e1000341. doi:10.1371/journal.pbio.1000341
Coyne J. A, Kim S. Y, Chang A. S, Lachaise D, Elwyn S (2002) Sexual isolation between two sibling species with overlapping ranges: Drosophila santomea and Drosophila yakuba. Evolution 56: 2424–2434. (Our earlier study failing to find increased sexual isolation between the species in the area of overlap).
Noor M. A (1995) Speciation driven by natural selection in Drosophila. Nature 375: 674–675. (One of the best examples of reinforcement in nature, done in my lab by Mohamed Noor fifteen years ago.)