Genetic differentiation in speciose versus depauperate phylads: evidence from the California minnows

Avise, J.; Ayala, F.

We have examined electrophoretic variation in proteins encoded by 24 gene loci in natural populations of nine genera of minnows (family Cyprinidae) endemic to waters of California. The mean proportion of polymorphic loci per population is 12.5%, and the mean frequency of heterozygotes per locus is 3.78 ¦ 0.80%. These levels of genetic variation are within the low part of the range characteristic of vertebrate species. Average genetic distance, D, for all pairwise comparisons among species is 0.57, i.e., about 57 allelic substitutions, on the average, are estimated to have occurred for every 100 loci in the separate evolution of any two species. At least four genera (Hesperoleucus, Lavinia, Mylopharodon, and Ptychocheilus) are genetically very similar, and have probably evolved from a relatively recent common ancestor. The other genera are less similar; levels of genetic differentiation among them may be fairly representative for the very species-diverse North American minnows. We have also calculated the mean genetic distance among 10 of the 11 known species of the genus Lepomis; this is D = 0.63. The North American minnows and Lepomis are of approximately equal evolutionary age, although the minnows are highly speciose (about 250 species), while Lepomis is relatively depauperate (11 species). To compare the amount of genetic differentiation in a speciose group and a depauperate group, we have considered two alternative models: (1) genetic differentiation is a function of time, unrelated to the number of cladogenetic events; (2) genetic differentiation is proportional to the number of cladogenetic events in the group. According to model 1, the values of D are approximately equal in speciose and depauperate phylads of comparable age. However, according to model 2, the value of D is substantially greater in a speciose than in a depauperate phylad. Our findings of about equal average amount of genetic differentiation in the speciose minnows and in the depauperate Lepomis, support the notion that time since divergence from a common ancestor is more important than the number of intermediate cladogenetic events in determining the level of genetic divergence between species. Apparently, the development of reproductive isolating mechanisms per se does not involve change at a substantial proportion of structural genes.

Genetic differentiation in speciose versus depauperate phylads: evidence from the California minnows

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