Science kicks off the week of Darwin's 200th with a special section devoted to the latest on speciation [$-a], the literal origin of species. It includes a new review by Dolph Schluter, discussing the role of natural selection speciation [$-a], which suggests a new way to think about selection creating reproductive isolation.
Schluter contrasts ecological speciation, in which reproductive isolation arises in the course of adaptation to different environments, "mutation-order" speciation - isolation arising by the accumulation of different genetic and morphological changes in the course of adaptation to the same (or the same kind of) environment. That is, natural selection can cause a population to split into two species if different parts of population are "solving" different ecological problems, or if they arrive at different "answers" to the same problem.
The mutation-order scenario makes sense, though it's new to me. As an example, Schluter cites a recent study in Mimulus in which a mutation of the mitochondrial DNA in one population creates sterile males in hybridization with other populations [$-a]. He proposes that much mutation-order speciation occurs because of conflict between different levels of natural selection, as when "selfish genes" create reproductive incompatibilities in the course of spreading through a host population. This is a departure from what biologists usually consider speciation by natural selection, but Schluter makes an interesting point.
A.L. Case, J.H. Willis (2008). Hybrid male sterility in Mimulus (Phrymaceae) is associated with a geographically restricted mitochondrial rearrangement Evolution, 62 (5), 1026-39 DOI: 10.1111/j.1558-5646.2008.00360.x
D. Schluter (2009). Evidence for ecological speciation and its alternative Science, 323 (5915), 737-41 DOI: 10.1126/science.1160006
A. Sugden, C. Ash, B. Hanson, L. Zahn (2009). Happy birthday, Mr. Darwin Science, 323 (5915) DOI: 10.1126/science.323.5915.727