Widespread Homogeneity

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Science  07 Feb 2003:
Vol. 299, Issue 5608, pp. 783-785
DOI: 10.1126/science.299.5608.783e

The relationship between the geographical range of a genus of organisms and the rate of speciation within it is disputed. The prevalent view has been that wide distribution might favor speciation through the occupation of a more diverse range of habitat and a greater likelihood of the emergence of dispersal barriers. But a negative relationship is also plausible: A wider distribution might be facilitated by biological characters, such as good dispersibility or wide ecological tolerance, that would tend to depress speciation.

Using fossil and extant molluscs, Jablonski and Roy find evidence in support of a negative relationship between speciation rate and distribution. Cretaceous gastropods from the Atlantic Coastal Plain of what is now eastern North America show a significant inverse relationship between species production rates over 35 million years and geographical range; 13 clades of living bivalves and gastropods distributed globally show no relation between species richness and range. This result suggests that the factors that lead to wide distribution also tend to dampen speciation rates, at least in marine molluscs. — AMS

Proc. R. Soc. London Ser. B10.1098/rspb.2002.2243 (2003).

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