Evolution

Splitting on the Edges

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Science  26 Mar 2010:
Vol. 327, Issue 5973, pp. 1555
DOI: 10.1126/science.327.5973.1555-c
CREDIT: MACHEL MALAY

Explanations of the species richness on coral reefs have often inferred diversification processes from large-scale patterns. In contrast, quantitative phylogenetic studies, based on thorough taxonomic and spatial sampling, can document the speciation events responsible for regional diversity patterns. Applying this approach to the abundant and colorful Calcinus hermit crabs, Malay and Paulay have constructed mitochondrial and nuclear gene phylogenies for 56 species (9 undescribed) and mapped their distributions. Differences in color patterns follow species boundaries and evolve rapidly, indicating a likely role in species recognition. Most speciation in the genus has occurred peripherally, in remote areas. All of the younger species pairs are narrowly allopatric, and molecular clock analyses imply that sister species require at least 2 million years (and usually much longer) to develop secondary sympatry. There are a few major ecological shifts between sister species, but environmental preferences are conserved across most speciation events (niche conservatism). These hermit crabs' strong preference for oceanic areas and substantial ability for dispersal have led to diversity peaks in the Mariana Islands and in southeast Polynesia rather than in the Indo-Malayan triangle (the locus of maximum marine biodiversity). The example of Calcinus serves as a reminder that different groups, having their individual histories and ecologies, should not be expected to show identical biogeographic patterns.

Evolution 64, 634 (2010).

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