Proc Natl Acad Sci U S A. 69(8):2100-2104. (Aug 1972)
Microgeographical Variation in Allozyme Frequencies in Avena barbata
Hamrick JL, Allard RW.
Department of Genetics, University of California, Davis, Calif. 95616.


Microgeographical distribution of alleles of five enzyme loci and one morphological variant has been determined within populations of A. barbata occupying sites that are transitional between different vegetational zones. The results show that the spatial distribution of alleles is nonrandom and highly correlated with habitat in patterns that parallel the distribution of the same alleles in major climatic zones in California. It is concluded that the observed patterns of variation are best explained by Neo-Darwinian evolutionary models, in which selection plays a predominant role.

In a survey of genetic variation in Avena barbata in California, Clegg and Allard (1) found that allelic frequencies at five enzyme loci and at two loci governing morphological variants are distributed in nonrandom patterns closely associated with environment. In the semi-arid warm summer region of California all populations of this species are monomorphic for a specific combination of alleles, whereas most populations in the generally more mesic Mediterranean cool-summer region are extensively polymorphic for these loci. Moreover, allelic frequencies in the most mesic sites within the Mediterranean region showed the greatest departure from the allelic frequencies that characterize the arid zone. These authors concluded that selection favoring a single genotype in arid habitats, and the contrasting genotype in mesic habitats, is the primary force responsible for the observed patterns of monomorphism and polymorphism in California.

In the topographically diverse Mediterranean cool-summer region of California, change from mesic to xeric habitats often takes place over very short distances. This paper reports a study of allelic frequencies within populations of the wild oat, A. barbata, that span transitions from mesic to xeric environments. Sharp microdifferentiation in allelic frequencies correlated with environment were found, providing support for the conclusion that selection is the predominant force determining the nonrandom patterns of genetic variation in A. barbata in California.

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