We also sequenced two neutral mtDNA genes, COIII and ND3, in the same individuals. To determine the frequency of the two Mclr allelic classes across the dark-co1ored lava and neighboring light-colored granite, we sequenced the Mclr gene in 175 individuals from a 35-km transect in the Pinacate lava region. Previous work has demonstrated that two Mclr alleles, D and d, differ by four amino acids, and are responsible for the color polymorphism: DD and Dd genotypes are melanic whereas dd genotypes are light colored. This color polymorphism is a presumed adaptation to avoid predation. Pocket mice inhabiting volcanic lava have dark coats with unbanded, uniformly melanic hairs, whereas mice from nearby light-colored granitic rocks have light coats with banded hairs. Here, we compare phenotypic variation in color, allele frequencies of a pigmentation gene (the melanocortin-1 receptor, Mc1r), and patterns of neutral mitochondrial DNA (mtDNA) variation in rock pocket mice (Chaetodipus intermedius) across a habitat gradient in southern Arizona.
Patterns of geographic variation in phenotype or genotype may provide evidence for natural selection.
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