Yesterday I went to a seminar at the University of Maryland by James Mallet (University College, London), who studies speciation in butterflies. I had read some of Dr. Mallet’s papers on species definitions and speciation, so this was a great opportunity to listen to him in person.
Dr. Mallet's seminar was about the speciation processes in Heliconius butterflies of South America and whether or not these butterflies speciate in allopatry or sympatry. These butterflies display Müllerian mimicry and there are many polymorphic races differing in wing color patterns. The races also hybridize and create hybrid zones. The predatory birds are more likely to go after the hybrids with rare color forms, because they are less likely to recognize them as toxic (frequency dependent selection). This results in the hybrid zones being narrow (~10-30 km wide). Furthermore, the hybrid zones shift spatially. In other words, one race seems to displace another.
Heliconius melpomene picture from James Mallet's page.
The classical explanation of the presence of such polymorphic species is that during the last ice ages the South American rainforests were assumed to have been fragmented and the species to have evolved in allopatry in the remaining forest refugia before the gaps were reforested. But according to Dr. Mallet, recent pollen analyses indicate that the rainforests were never fragmented. He instead thinks that the current species distribution patterns were due to parapatric mechanisms in a continuous forest. In parapatric speciation, the new species evolve in contiguous populations initially with slight genetic differences.
All of this was presented in one hour. It was indeed a fast-paced, information-packed seminar. I enjoyed it. I was hoping to meet Dr. Mallet personally after the seminar, but I had to leave immediately due to a meeting I had elsewhere.
Dr. Mallet started and ended his talk with a picture of the Bank of England's £10 Charles Darwin note. Based on his interpretation of some statements in the Origin of Species, Dr. Mallet thinks Darwin may have favored a sympatric speciation process. If I can track down the relevant pages from the Origin, I will post them here.