Showing papers by "John C. Avise published in 1975"

Adaptive differentiation with little genic change between two native california minnows

Abstract: Recent studies of protein variation using electrophoretic techniques have provided estimates of two parameters of paramount importance in evolutionary theory: (1) the amount of genic variability in natural populations, and (2) the amount of genic differentiation between populations. Two generalizations have emerged from these studies: (1) the amount of genic variation in sexually reproducing, outcrossing species is far higher than had been anticipated according to some models of population structure (reviews in Selander and Kaufman, 1973; Lewontin, 1974), and (2) the amount of genic differentiation between local populations of a species is usually small relative to that between populations of different species (review in Avise, 1974). This second observation is relevant to the data presented in this paper. A variety of statistics have been devised to measure genetic similarity (or its converse, genetic differentiation) between populations. However, different methods generally give similar numerical values when applied to the same data. As a rule measures of genetic similarity range from 1 ( complete similarity, i.e., the same alleles and in the same frequencies exist in the two populations compared) to 0 (= complete differentiation, i.e., the two populations share no alleles). If a number of gene loci are studied, the values of genetic similarity are averaged over loci. We have surveyed the literature, and found 651 pairwise comparisons between closely related species, each comparison involving 14 or more gene loci (Fig. 1). A total of 615, or 94%, of all pairwise comparisons give genetic similarities no greater than 0.90. Typically the similarities in a group of congeneric species range from 0.30 to 0.80, while the mean usually lies between 0.50 and 0.60. (The lower histogram in Fig. 1 has its mode between 0.10 and 0.20 because it includes a large number of comparisons between the relatively distantly related species of the Drosophila obscura and affinis subgroups.) This low degree of genic similarity is also observed between species that by other criteria appear very similar, such as sibling species (Johnson and Selander, 1971; Ayala et al., 1974), or hybridizing species (Avise and Smith, 1974). Typical results are shown in Figure 2. In contrast, conspecific populations have similarity values very rarely smaller than 0.80, and generally greater than 0.90. These results suggest that speciation may usually be accompanied by substantial genic differentiation. We report here results that, to a certain extent, disagree with those just summarized. We have studied allozyme variation at 24 loci in two presumed species of California minnows, Hesperoleucus symmetricus and Lavinia exilicauda. Most populations of the two species cannot be distinguished at 23 of the loci, while at a single locus the two species are fixed for alternate alleles. Within the Pajaro River system in Central California, where the two species are sympatric, they are polymorphic for alleles at that locus, and the allele frequencies are associated with environmental variables.

Genetic change and rates of cladogenesis.

Abstract: Models are introduced which predict ratios of mean levels of genetic divergence in species-rich versus species-poor phylads under two competing assumptions: (1) genetic differentiation is a function of time, unrelated to the number of cladogenetic events and (2) genetic differentiation is proportional to the number of speciation events in the group. The models are simple, general, and biologically real, but not precise. They lead to qualitatively distinct predictions about levels of genetic divergence depending upon the relationship between rates of speciation and amount of genetic change. When genetic distance between species is a function of time, mean genetic distances in speciose and depauperate phylads of equal evolutionary age are very similar. On the contrary, when genetic distance is a function of the number of speciations in the history of a phylad, the ratio of mean genetic distances separating species in speciose versus depauperate phylads is greater than one, and increases rapidly as the frequency of speciations in one group relative to the other increases. The models may be tested with data from natural populations to assess (1) possible correlations between rates of anagenesis and cladogenesis and (2) the amount of genetic differentiation accompanying the speciation process. The data collected in electrophoretic surveys and other kinds of studies can be used to test the predictions of the models. For this purpose genetic distances need to be measured in speciose and depauperate phylads of equal evolutionary age. The limited information presently available agrees better with the model predicting that genetic change is primarily a function of time, and is not correlated with rates of speciation. Further testing of the models is, however, required before firm conclusions can be drawn.