Speciation Not Allopatric?
 

"ScienceDaily (Jan. 1, 2009) — New evidence uncovered by oceanographers challenges one of the most long-standing theories about how species evolve in the oceans.

"Most scientists believe that allopatric speciation, where different species arise from an ancestral species only after breeding populations have become physically isolated from each other, is the dominant mode of speciation both on land and in the sea. The key to this theory is the existence of some kind of physical barrier that operates to restrict interbreeding (gene flow) between populations so that, given enough time, such populations diverge until they're considered separate species.

"Research by Dr Philip Sexton formerly of the National Oceanography Centre, Southampton (now at the Scripps Institution of Oceanography, San Diego) and Dr Richard Norris (also of Scripps) suggests, however, that this mode of diversification may not be as prevalent for oceanic creatures as it is for land dwellers and somewhat controversially, they assert that the above model of speciation may actually be very rare in the world's oceans."

More at http://www.sciencedaily.com/releases...1231175357.htm
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Re: Speciation Not Allopatric?
 

From the Science Daily article:

These observations suggest that species’ distributions are more controlled by habitat availability rather than by an inability to disperse.


Mayr commented that there's evidence that sympatric speciation happens in insects that specialize on specific host plants, and at least some of the freshwater fish. He said it was actually demonstrated for some of them through females having a simultaneous preference for (1) a certain habitat and (2) males with a preference for that same habitat. The simultaneous aquisition of mate preference and habitat preference seems to be what drives it in those cases. That happens in some cichlids for example, but not others. I wonder how often that occurs in the ocean.

ref: Ernst Mayr, "What Evolution Is," Basic Books, 2001 (pp. 180-181)