Earlier today, Jerry mentioned to me that he noticed my earlier blog posts on the meeting, and thought I wasn’t being critical enough. So I think that means I’m supposed to let my inner beast out for this one. (Nah, actually, it’s because I’m in note-taking transcription mode while listening to these talks. I have to digest them for a bit before I can do any synthesis.)
What is the biogeography of speciation? Can one species split into two while splitting into two? Allopatric speciation: no gene exchange; Parapatric: limited exchange; Sympatric: free gene exchange. Allopatric is sort of the dogma of evolutionary biology. Everybody assumes gene flow and biogeography are the same thing, but they really aren’t.
Nobody contests whether allopatric speciation happens, the question is simply how often it happens. Species concept Coyne uses: groups of interbreeding populations that show substantial reproductive isolation from other forms.
Why is there a controversy about biogeography? Darwin’s concept was largely sympatric. The existence of species in the same area implies that they arose in the same area (clearly not necessarily true). The environment is regarded as important. There haven’t been enough opportunities for allopatric speciation — not that many barriers in the history of the world. Speciation is relatively difficulty with gene flow. Biologists opinions about geography have been conditioned by their own histories.
Can we estimate the frequencies of these different kinds of speciation? We have so many indubitable examples of allopatric speciation. Conditions are present everywhere.
Parapatric speciation: conditions are fairly easy, but data from nature is sparse and hard to get. Need evidence of clinal differentiation or evidence that allopatry never happened (which would be very hard to do). One example given: cave salamanders, two species, that abut a surface species, enabling a path for gene flow. Can’t entirely rule out the possibility of an allopatric speciation event in their history, however.
It’s easier to find evidence for the most controversial pattern, sympatric speciation. Theoretically supportable, and there are also experiments that demonstrate in the lab (with unlikely requirements, such as the complete lethality of intermediates).
- Complete or substantial sympatry
- True sister taxa not based on hybridization
- Substantial reproductive isolation
- History of taxa must make allopatry unlikely
Criteria for verifying sympatric speciation:
- Polyploidy. Up to 70% of angiosperm species have a polyploidization event somewhere in their ancestry. May still require allopatry to keep hybrids from being diluted out.
- Homoploid hybrid speciation.
- Parasitic indigobirds imprint on the song of the father, so laying eggs in different host yields individuals that only breed with individuals with similar stepparents.
- Palms on Lord Howe Island: very small island, so necessarily sympatric. Reproductive isolation by flowering time depending on soil type offers an alternative explanation, though: it reduces gene flow
Coyne doesn’t regard this as true sympatric speciation because there was some kind of trickery that set up a reproductive barrier.
What about cases that satisfy the case of gene flow while speciation occurs? Under these stringent criteria, Coyne thinks 5 cases satisfy. The best cases are cichlids in crater lakes in Cameroon and Nicaragua. Littorina, banded molluscs that live in different tidal zones. Rhagoletis, the apple maggot fly, may not be the best case; they eclose at different times depending on the fruit on which they are laid, which represents a reproductive barrier.
- There is no doubt that allopatric and peripatrica speciation occur
- Parapatric speciation may occur, but evidence is hard to come by
- Sympatric speciation is theoretically feasible, but…
- The few studies that suggest sympatic speciation occurs suggest that it only occurs rarely