The Origin of Species, Side By Side

In tomorrow’s New York Times, I have an article about how new species evolve. It describes new research into how a population can split into two species. The idea that species can evolve when populations get geographically isolated is well-supported by evidence, but the idea that individuals living side by side can split apart (called sympatric speciation) has sparked more controversy. The late biologist Ernst Mayr was the lead champion of the geographic isolation mechanism, and he was always skeptical of claims of sympatric speciation. But, as he said in this 2001 interview, he was skeptical not so much of sympatric speciation itself, but of the particular cases that were made for it. That’s continued to be the sticking point for a lot of evolutionary biologists. Two new cases–one concerning palm trees, the other fish–have impressed the critics I spoke to. I just wonder what Mayr would have said.

0 thoughts on “The Origin of Species, Side By Side

  1. A lot of my friends are entomologists and I think they are, as a whole, much more enthusiastic about sympatric speciation than the people working on vertebrates or plants. Have you noticed that?

  2. I have to ask the same question here that I asked on another blog where this topic came up recently:

    Fourteen species of Galapagos finches, all clearly descended from the same ancestor-species, individuals generally restricted to their birth-island, divergence has been documented in populations of finches living on the same island, and sympatric speciation is still open to question? Why?

  3. Good morning–

    To Coturnix (#1): Yes, I can see why entomologists would be more amenable to sympatric speciation. Plant-feeding insects are so specialized on particular species of hosts that they may be prone to sympatric speciation. As I mentioned in the article, the Rhagoletis fly example was one of the best cases for sympatric speciation till now.

    To Wolfwalker (#4): The trouble with the Galapagos finches is that they live on an archipelago, not a single island. While there are signs of divergence between birds on single islands today, you can’t confidently make the jump to say that new species evolved on single islands sympatrically. A few birds of a species on one island might have wound up on a nearby island, diverged allopatrically into a new species, and then some descendants returned to the original island. Making things even trickier is the fact that some of the Galapagos islands that were there when the finches first appeared a few million years ago have since disappeared underwater. So new species could have evolved on islands that no longer exist. None of this is to say that finches absolutely couldn’t have evolved sympatrically, but allopatric speciation can’t be dismissed.

  4. Carl,

    Rhagoletis pomonella is looking more like allopatric speciation followed by reinforcement than the classic sympatric model that Guy Bush originally made it appear to be. Work from Jeff Feder’s lab indicates that a population from Mexico migrated to the United States prior to the speciation event. This population brought with it some predisposition to further differentiated from the US population, which probably helped instigate the host shift and subsequent speciation.

    The recently described speciation via hybridization in Rhagoletis (Schwarz et al 2005) could fall into the category of sympatric speciation, but it needs further examination.

  5. Carl said, “To Coturnix (#1): Yes, I can see why entomologists would be more amenable to sympatric speciation. Plant-feeding insects are so specialized on particular species of hosts that they may be prone to sympatric speciation. As I mentioned in the article, the Rhagoletis fly example was one of the best cases for sympatric speciation till now”.

    Seems to me it either happens or it doesn’t. If it CAN happen with insects (given the right set of circumstances) then it can happen for other life as well. What I would guess is that it CAN, but is obviously a lot more problematic considering the high likelihood of interbreeding when species are living amongst each other.

  6. I’m confused as to why the allopatric model has been so favored over the sympatric model. The hypothesized evolutionary history of the palm trees seems to show a fairly low threshold (relatively speaking) for side-by-side speciation to occur. Supposing that the common ancestor of both species of palm had within its genes the capability to grow both in soil and soft rocks with certain genes being switched on or off given the circumstances, it sounds like the only “isolating” shift necessary would be a change in the timing of the flower growth–possibly within the inherent plasticity of a species due to nutrients, sunlight, etc. If pollination can’t occur between the two because of timing, they’re just as isolated as if they were on different continents.

    Are allopatric and sympatric speciation really so different if we broaden our perspective on isolation? Is there more to the distinction that I’m missing?

    On another note, shame on all of us for misinterpreting Bush’s comments on teaching a critical evaluation of evolutionary theory in the science classroom. He was clearly referring to the controversy around sympatric speciation versus allopatric.

    “…the idea that individuals living side by side can split apart (called sympatric speciation) has sparked more controversy”

    We should all appreciate Bush’s scientific foresight.

  7. Interesting article. The apple maggot fly example may be another red herring, though. A 2004 paper in Trends in Ecology and Evolution by Chris Jiggins and John Bridle argues that what became the apple maggot fly was actually a preexisting incipient species of hawthorn fly from the Southern U.S. and Mexico–many of the differences between apple flies and hawthorn flies (including temperature at pupation and diapause length) are the same differences you find between northern hawthorn flies and southern hawthorn flies.

    “First, there is no evidence for reduced levels of variation within apple-specific inversions that might indicate very recent selective sweeps. Second, for an apple maggot fly, living in Texas is a bit like eating apples. The most important known adaptive differences between apple and hawthorn flies relate to diapause. The deeper diapause that is characteristic of hawthorn populations from southerly latitudes is similar to that known to be important in early-fruiting apple populations. Both involve warmer autumn days immediately following pupation compared with the northern hawthorn flies, which go into diapause when the weather is already much colder.”

    It seems likely that the addition of apples to the fly environment allowed the southern flies to move into the northern environment and take over the new niche. The two populations stayed reproductively isolated, more or less, because of differences in life cycle timing. I’m not sure how different this example is from other allopatric speciation events; although the “official” speciation may have occurred after apples were introduced, most of the groundwork was laid by allopatric separation. A lot of this depends on how you define your terms, I suppose.

    Thanks for writing such interesting stuff.


  8. Interesting article. I thought sympatric speciation only existed in insects and other host-specific organisms. At least my Floridian high school biology education tought me that much. How recent is this research? Is it new knowledge that sympatric speciation works this way?

  9. I think the logic was always pretty impeccable that sympatric speciation could happen, but the important question is how common is it vis a vis allopatric speciation, and the view on this probably won’t shift too far. Incidentally while some of Mayr’s skepticisms may be weakened regarding the flimsiness of sympatric speciation, his suspicions about the importance of genetic drift appear to be getting stronger.

  10. well, nothing much to say… Carl, I love your blog, have been reading it and learning every day or so something new and interesting. will certainly be citing you in my PhD Thesis (I stole your phrase: Natural selection is not natural perfection 🙂 – if it isn’t yours, please let me know where it does come from).
    more on the subject, this latest post dispelled some of my doubts about sympatric speciation…

  11. People make FAR to much of a deal about speciation. Its simply a CONVEINIENT point of referance. It’s not like a Quantum leap of energy from one level to the next. Its just a measure of animal behavior. Its a man made signpost. Nothing more.

  12. There are four populations of sympatric Orcinus orca in the Pacific Northwest: northern residents, southern residents, offshores, and transients. The information primarily comes from John K.B. Ford, Graeme M. Ellis, and Kenneth C. Bal comb

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