Wednesday, August 10, 2011

Sympatric speciation

The generation of a new species from an existing population requires, at some point, reproductive isolation. Why? The sharing of genes between different parts of the one population (gene flow) ensures that any new mutations that occur will be spread throughout the entire population. Since new species arise in part as new mutations occur, then there needs to be a way to keep those new mutations in one part of a population, the part that will become the new species. The simplest way to do this is to physically separate populations, for example by isolating part of the population on an island. That's called allopatric speciation. It is generally thought that sympatric speciation, the generation of a new species without any physical isolation, is much rarer than allopatric speciation. After all, any new mutation should be spread through a population and thereby prevent the generation of a new species. (The major exception to this is polyploidization in plants, which results in immediate reproductive isolation.)

When I was at Evolution 2011 this summer, I heard a talk by Alex Papadopulos on sympatric speciation on Lord Howe island. Here's how I described that work at the time:
Alex Papadopulos talked about sympatric speciation on Lord Howe Island. Lord Howe is a lone island in the pacific 600 km east of Australia. He's found endemic sister species of flowering plants that must have speciated on the island itself. Granted, he estimates the frequency of Lord Howe species that originated by sympatric speciation to be only 4.5-8%, but still, I'm fascinated to see any example of sympatric speciation. I should add for those in the know that the examples he discussed were not simply polyploids (the most frequent and presumably simplest form of sympatric speciation).
Yesterday, PNAS published his full paper. From the abstract:
Speciation with initially strong gene flow is thought to be extremely rare, and few convincing empirical examples have been published. However, using phylogenetic, karyological, and ecological data for the flora of a minute oceanic island (Lord Howe Island, LHI), we demonstrate that speciation with gene flow may, in fact, be frequent in some instances and could account for one in five of the endemic plant species of LHI. We present 11 potential instances of species divergence with gene flow, including an in situ radiation of five species of Coprosma (Rubiaceae, the coffee family). These results, together with the speciation of Howea palms on LHI, challenge current views on the origin of species diversity.
Check it out:
Papadopulos et al. 2011. Speciation with gene flow on Lord Howe Island. PNAS 108:13188-13193.

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