Commentary: Adaptive speciation by reinforcement

An interesting News and Commentary piece appears in Nature Heredity
dealing with 'reinforcement'. This is a form of speciation in which
allopatry produces divergence between populations, but then the
allopatry ends (prematurely?) and reproductive isolation arises
adaptively by selection against production of hybrids.

http://www.nature.com/hdy/journal/v96/n6/full/6800826a.html#bib2

News and Commentary
Heredity (2006) 96, 422-423.

Speciation: A new role for reinforcement
C Smadja and R Butlin

Initial paragraphs:
Reinforcement is a controversial mechanism of speciation in which
reproductive isolation is strengthened in response to maladaptive
hybridisation. In a recent paper published in Nature, Hoskin et al
not only provide new empirical support for the process, in the
Australian green-eyed tree frog, but also argue for a wider impact
of reinforcement than is generally acknowledged.

Speciation commonly occurs in allopatry, as a by-product of
natural selection and genetic drift within geographically separated
populations (Figure 1a). However, an alternative hypothesis suggests
that speciation may be adaptive: natural selection can act directly
to favour prezygotic reproductive isolation between divergent taxa in
a process known as reinforcement (Dobzhansky, 1937). If hybridisation
occurs between populations that have previously diverged in allopatry,
and hybrid offspring have reduced fitness, selection will favour traits
that reduce the risk of hybridisation. Reinforcement is theoretically
plausible, and there is growing evidence for its occurrence in nature,
but its role in speciation remains uncertain. Current questions
concern the factors that promote reinforcement, how frequently
reinforcement plays a part in speciation and whether that part is
primarily to complete speciation when isolation is already substantial.
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