I will break up my discussion of the paper below into two parts that will appear today and tomorrow. This is because I want to avoid a single long post that may put off readers. I give references to all the papers mentioned at the bottom of the post.
All evolutionists agree, and the data show, that nearly all new species form as descendants of what were populations of a single ancestral species. (Occasionally new species, especially in plants, form after hybridization of two pre-existing species.) One of the biggest controversies in my own field of speciation is this: can new species form in one area without any geographic isolation of populations (“sympatric speciation”), or is a period of partial or full geographic geographic isolation necessary (“parapatric” or “allopatric” speciation, respectively)? (I’ve simplified the meaning of these terms a bit.) While theory shows that geographic isolation facilitates the development of reproductive isolating barriers between populations (sexual isolation, hybrid sterility, and so on) that are the sine qua non of speciation for most biologists, some theory also suggests that geographic isolation is not necessary: under special conditions, new species can form in one area in situ.
The data, summarized in my book Speciation with Allen Orr (now a decade old), suggest that geographic isolation is usually necessary, but there are a few cases implying speciation without any geographic isolation. These are hard to demonstrate unequivocally, largely because closely related species that now live in the same area could have speciated in allopatry (different areas), and then come into secondary contact after the reproductive barriers evolved in isolation. Since it’s harder to form species sympatrically, to demonstrate this process one must rule out that ancestral populations were ever isolated geographically. Since that’s hard to do (speciation takes thousands to millions of years to complete), convincing cases are rare.
In Speciation (pp. 142-143), Allen and I laid out criteria for showing convincing cases of sympatric speciation. They include the presence of sister species (each other’s closest relatives) in the same area; the demonstration that these are indeed “good” species (i.e., they are distinct groups that never or rarely exchange genes); the demonstration that their status as each other’s closest relatives does not come from hybridization between more distantly-related species (this would homogenize their genomes and make them look closely related when they really aren’t); and the hardest bit: showing that these those sisters species descend from populations that were never geographically separated. That’s the biggest issue, because when you see two closely related species living in the same area, how can you convincingly show that their ancestors always lived in the same area?
Allen and I decided that one of the best situations for meeting these criteria occur on islands: either oceanic islands (islands like Lord Howe or St. Helena that were formed without life on them, usually as volcanoes that rose above the sea), or “habitat islands”: isolated patches of habitat that have existed for a long time. (Landlocked lakes are one example.) If you could show, for instance, that on one such island you find two or more sister species that do not occur elsewhere (i.e., are endemic to that island), then that would be pretty strong evidence that those species had formed sympatrically on the island, descending from a common ancestor that invaded the area long ago.
Trevor Price and I tested this theory by looking for endemic sister species of birds on oceanic islands. In a paper published in Evolution in 2000, we found not a single such case on 46 isolated oceanic islands, implying that sympatric speciation was rare in birds. (If it didn’t occur on islands, it is unlikely to occur on continents.) Further work by Yael Kissel and Tim Barraclough in 2010 showed the same situation in several other groups, including lizards, mammals, and flowering plants. Sister species on islands were observed only when the islands were so large that geographic barriers were likely to have been present. This further implies that sympatric speciation is rare, at least in those groups studied.
In contrast, though, work by Papadopulos et al. on the flora of Lord Howe Island (a small oceanic island between Australia and New Zealand, with an area of about 15 square kilometers) shows the existence of sister species in about nine groups of plants, most notably two species of endemic palm trees that are wind pollinated. Since the sister species in these groups are found nowhere else, they likely formed on the island. This, too, seems a pretty good case of sympatric speciation.
But in vertebrates we have only a couple of cases—all involving fish—that point to sympatric speciation. These cases involve species living in small lakes that fill the craters of extinct volcanoes—”crater lakes”. In 1994, Ulrich Schliewen and his colleagues described groups of closely related cichlid species, each group descending from a single common ancestor, that inhabited crater lakes in Cameroon. (Such cases have since been described in lakes in Nicaragua as well). Lake Barombi Mbo, only 2.3 km across, contains a group of 11 “monophyletic” cichlid species (descended from a single invader), while Lake Bermin, only 0.7 km across, has a monophyletic group of 9 cichlids. Here are the two lakes:
And here are the putatively monophyletic species flocks, shown in the paper of Martin et al. mentioned below. (There are only 10 species shown for Lake Barombi Mbo because the authors sampled only 10 of the 11 for genetic markers.)
Because the lakes are no longer connected to rivers that could carry in fish (lake Bermin has an outflow but no inflow); because they are small and uniform (so that raising or lowering the lake levels would not create isolated pools that could facilitate allopatric speciation); and because each small lake harbors a group of species from one putative invading ancestral species, this situation fulfills all four criteria we proposed for sympatric speciation. When we wrote our book, Allen and I considered this perhaps the best case of sympatric speciation in nature.
But that’s now in question. A new paper in Evolution by Christopher H. Martin et al. (reference and link below) genetically examined the radiations in these lakes and finds that sympatric speciation isn’t that likely after all. Today I gave you the background; tomorrow I’ll show you the results.
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Coyne, J. A. and H. A. Orr (2004). Speciation. Sunderland, MA, Sinauer Associates.
Coyne, J. A. and T. D. Price (2000). Little evidence for sympatric speciation in island birds. Evolution 54(6): 2166-2171.
Kisel, Y. and T. G. Barraclough (2009). Speciation has a spatial scale that depends on levels of gene flow. Amer. Natur. 175: 316-334.
Martin, C. H., et al. (2015). Complex histories of repeated gene flow in Cameroon crater lake cichlids cast doubt on one of the clearest examples of sympatric speciation. Evolution 69(6): 1406-1422.
Papadopulos, A. S., et al. (2011). Speciation with gene flow on Lord Howe Island. Proc Natl Acad Sci U S A 108(32): 13188-13193.
Schliewen, U. K., et al. (1994). Sympatric speciation suggested by monophyly of crater lake cichlids. Nature 368: 629-632.
There are about 15 Lake Malawi cichlids swimming around just over my shoulder. They are eagerly waiting tomorrow’s results concerning their brethren from lakes Bermin and Barombi Mbo.
Would the ecological speciation in sticklebacks in Canadian lakes as described by Dolph Schluter in his book The Ecology of Adapative Radiation be considered examples of sympatric speciation?
No, and in fact Dolph has evidence, both geological and genetic, that they didn’t speciate sympatrically. First, the lakes were alternately connected to the sea and separated, so the two morphs probably resulted from double invasions (the latest produced the “limnetic” form), and genetic evidence shows that the two forms (limnetic and benthic) in each lake did not have a common ancestor that invaded the lake.
Very lucid. Thank you.
Not a biologist but does anyone know (or have a good guess) how the founder species get into the lakes in the first place?
I believe there were river inflows into both at some time in the past (cutting through the crater wall).
But only one species? Seems kind of fishy.
That’s part of the point of the new paper. There’s more than one group of fish in the crater lakes, ergo more than one ancestor invaded.
Is it possible that fish eggs were delivered on the feet of duck-like water birds?
I am eagerly awaiting tomorrow’s installment.
Thanks for writing about this. I’m eager to see the results tomorrow.
Can DNA analysis shed any light on how the speciation occurred ?
This will be tomorrow’s installment.
Thank you for this lucid and compelling article. I await tomorrow’s instalment with interest.
Intriguing. I do like the idea that sympatric speciation is a real but less common phenomenon, but the number of well supported cases appears to be shrinking. If this one is going down, then the other cases of cichlid sympatric speciation in freshwater lakes may not be far behind.
An area where this form of speciation still holds up, at least in theory, is with chromosomal based speciation such as polyploidy.
Yes, but of course most speciation in animals is NOT polyploid, as that’s a special genetic mechanism that’s far more common in plants. Also, some people feel that the initial polyploid has to find some isolated niche lest it be “mated to death” by its diploid ancestors.
They are other chomosomal potential mechanisms than polyploidy, e.g. centric fusions, like in mice and shrews. They can theoretically induce speciation without geographic isolation, but in practice, both mechanisms are intermingled – if only because populations of small mammals never are really continuous over large areas, but rather a metapopulational system.
Yes. But similar to what Jerry says it would be more likely to go to fruition if the individuals carrying the recombinant chromosomes were isolated for a time.
Fascinating, thank you. I have long followed the debate and there was a time when I felt sympatric speciation was probable, but I now doubt it very much (at least in vertebrates).
Does anyone know of any reference to sympatric speciation by the religious in an attempt to discredit the theory of evolution e.g. the likes of Ken Ham or others of that ilk trying to incorporate the concept into their creationist/ID arguments?
Very interesting stuff. What a cliffhanger.
I reckon the butler made them into different species, either that or everyone on the train did it.
I hope the big “reveal” will be done in cod-French.
Thanks! Will be reading tomorrow for sure!
This one of the things I truly enjoy about WEIT; it’s actually “teaching me a controversy” in evolutionary theory. Great post (easy to follow for a non-expert)!
Seconded. I thought at first “this is going to be beyond me”, but it was clearly explained for a layman to follow. I’m looking forward to the dénouement.
They way I think about it is that there are legitimate debates in evolutionary biology, and illegitimate ones. The evolution vs. creationism debate is, of course, an illegitimate one. Debates within evolution are (usually) legitimate; the Shapiro et. al. issue is an example of an illegitimate debate.
apology in advance, I’ve not taken a proper evolution course and speciation is only lightly mentioned in Biol. 101…
What does this mean regarding some alteration in behavior, that it would not necessarily led to sympatric speciation? perhaps I’m misremembering, but isn’t that what is used as the basis for the claim of some insect speciation, that something like a slightly different song or some alteration in plant preference could lead to a species splitting even within the same field but never again mating because they don’t “accept” or recognize each other’s songs or don’t land on the same plants for feeding/mating/egg laying?
Those things are behavioral isolating barriers; the problem is that a new individual with a song or preference that made it completely isolated from its previous conspecifics wouldn’t persist. This is why a more gradual evolution of such differences in allopatry (and that gradual evolution couldn’t occur in sympatry) is a more probable explanation. Ernst Mayr dissected this argument in his great book Animal Species and Evolution.
ok, Thanks. I’ll add that to my reading list.
A cliff hanger….
Came here to say this. Now another reason to wish for Friday to get here!
About a fifteen years ago, a “new” species of pipistrelle (Pipistrellus pipistrellus) was discovered in the UK, and called Pipistrellus pygmaeus or the soprano pipistrelle because its dominant call was at 55 kHz instead of 45 kHz. Although some minor other differences were later discovered, these had never been detected until the difference in sound was identified. Their distributions, moreover, overlap, and they seem to me to comprise a good case for sympatric speciation.
I wonder if symmetry breaking has been underestimated as a cause of sympatric speciation. Darwin emphasised that competition would be more severe between closely related groups. If a species with a wide range of some resource came under pressure to survive, then the ones favouring the middle would be in competition with both wings, whereas those wings would only be subject to competition from one direction.
Incidentally, although it is illegal in the UK to handle bats, I have occasionally needed to rescue pipistrelles from cats, or to release them from a bedroom. They really are very beautiful creatures.
Looking forward to part 2.
There’s been so much fluctuation in coastlines and climatic zones in western Europe (over relevant timescales for mammalian speciation) that current sympatry is a very weak indicator of sympatry at the time of divergence.
Re ‘symmetry breaking’ – in biology, I think it may often be a reasonable description or metaphor, but rarely or never a causal explanation (in contrast to mutation, selection, drift, and myriad details of ecology and behaviour).
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This is excellent stuff! I’d so much rather study fish than bipeds in Cameroon.
This is a great example of how science continually checks itself. Nothing is ever “case closed” even though I’m sure many scientists want to believe in the sympatric speciation of these cichlids- an “ah shucks” moment. Something you don’t find with the religionists. You cover this difference very well in Faith vs. Fact and I appreciate how this example buttresses your analysis therein.
Looking forward to tomorrow’s installment…a scientific take-down.
Wow, really great cliffhanger (and I’m not being facetious)! I’m really curious to read about how this example of sympatric speciation has been cast into doubt now.
Eagerly awaiting tomorrow’s post, along with everyone else!
Have been slightly familiar with hypotheses of sympatric speciation from a couple of angles–first, as with GB (I imagine), from learning about the amazing number of endemic cichlid species in Lakes Malawi & Tanganyika, when I was a FW aquarium hobbyist; and second, having worked for Guy Bush when he was examining the possibility among true fruit flies (Rhagoletis pomonella), some of which apparently switched food plants in the field. Alas, both involvements were some time ago.
This is a major issue for me too. I think the local radiation of the orchid genus Teagueia into thirty closely-related species is a reasonably strong example of sympatric speciation, with up to 16 species growing on a single mountain. Their phylogeny shows that they did not come from somewhere else in multiple waves; they are a monophyletic clade with no close relatives elsewhere.
This radiation is local to the mountains around my house in Ecuador.
Very interesting for this layman.
The good part of being very busy lately is that I won’t have to wait too long for part 2 🙂
Sir, you absolutely rock with your strong sense of intellectual humility.
Thank you for continuing some of the best traditions and hallmarks of science and the scientific community.