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Pattern Beneath the Chaos: The Effect of Recruitment on Genetic Patchiness in an Intertidal Limpet

Michael S. Johnson and Robert Black
Evolution
Vol. 38, No. 6 (Nov., 1984), pp. 1371-1383
DOI: 10.2307/2408642
Stable URL: http://www.jstor.org/stable/2408642
Page Count: 13
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Pattern Beneath the Chaos: The Effect of Recruitment on Genetic Patchiness in an Intertidal Limpet
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Abstract

A study of enzyme polymorphisms in the intertidal limpet Siphonaria jeanae had revealed fine-scale genetic heterogeneity for four loci at Rottnest Island, Western Australia. The inconsistency of this heterogeneity in time and space led to the proposal that it resulted from locally unique histories of recruitment. To test this proposal, the abundance, spatial distribution, and genetic composition of new recruits were monitored over two seasons of recruitment at 10 sites on Rottnest Island. Both the distribution and allelic frequencies of recruits varied over time. This temporal variation produced fine-scale genetic patchiness as a result of recruitment over the two years. This process accounted for both the amount of spatial heterogeneity at Rottnest Island and the ephemeral nature of the genetic patchiness. The genetic patchiness was not due to differential habitat selection among genotypes, as the new recruits harvested at a particular time were genetically homogeneous among sites. Furthermore, analysis of geographic variation in Siphonaria jeanae shows that it is too limited to generate the observed temporal variation through changes in source populations. The implication is that selective mortality, either in the plankton or soon after settlement, is the major determinant of allelic frequencies in recruits. This interpretation is strongest for the Np locus, which shows both a latitudinal cline and a consistent seasonal change in allelic frequencies. The temporal changes were in the opposite direction to those predicted by the seasonal patterns of currents. Instead, the allelic frequencies over both time and space were correlated with air temperature, indicating the effects of selection. Thus, the genetic changes in the recruits occur in spite of, rather than resulting from, planktonic dispersal.

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