/pdf/evolution-at-two-levels-in-humans-and-chimpanzees-4j80qi0jr2.pdf

Evolution at two levels in humans and chimpanzees

Gene Flow in Natural Populations

To investigate the time course of speciation, we gathered literature data on 119 pairs of closely related Drosophila species with known genetic distances, mating discrimination, strength of hybrid sterility and inviability, and geographic ranges. Because genetic distance is correlated with divergence time, these data provide a cross-section of taxa at different stages of speciation. Mating discrimination and the sterility or inviability of hybrids increase gradually with time. Hybrid sterility and inviability evolve at similar rates. Among allopatric species, mating discrim- ination and postzygotic isolation evolve at comparable rates, but among sympatric species strong mating discrimination appears well before severe sterility or inviability. This suggests that pre- zygotic reproductive isolation may be reinforced when allopatric taxa become sympatric. Analysis of the evolution of postzygotic isolation shows that recently diverged taxa usually produce sterile or inviable male but not female hybrids. Moreover, there is a large temporal gap between the evolution of male-limited and female hybrid sterility or inviability. This gap, which is predicted by recent theories about the genetics of speciation, explains the overwhelming pre- ponderance of hybridizations yielding male-limited hybrid sterility or inviability (Haldane's rule).

http://max2.ese.u-psud.fr/epc/conservation/PDFs/HIPE/Coyne1989.pdf

Patterns of speciation in drosophila.

A. method for estimating the average level of gene flow in a subdivided population, as measured by the average number of migrants exchanged between local populations, Nm, is presented. The results from a computer simulation model show that the logarithm of Nm is approximately linearly related to the logarithm of the average frequency of private alleles, p(1), in a sample of alleles from the population. It is shown that this result is relatively insensitive to changes in parameters of the model other than Nm and the number of individuals sampled per population. The dependence of the value of p(1) on the numbers of individuals sampled provides a rough way to correct for differences in sample size. This method was applied to data from 16 species, showing that estimated values of Nm range from much greater than 1 to less than 0.1. These results confirm the qualitative analysis of Slatkin (1981). This method was also applied to subsamples from a population to show how to measure the extent of isolation of local populations.

Rare alleles as indicators of gene flow

Genetic variation in natural populations: patterns and theory.

We have studied genetic variation at 30-32 loci coding for enzymes in natural populations of five species of Drosophila. The average proportion of heterozygous loci per individual is 17.7 ± 0.4%. The average proportion of polymorphic loci per population is 69.2 ± 2.6% or 49.8 ± 2.2%, depending on what criterion of polymorphism is used. The following generalizations are advanced: (1) The amount of genetic polymorphism varies considerably from locus to locus. (2) At a given locus, populations of the same species are very similar in the amount and pattern of genetic variation. (3) However, at some loci large differences sometimes occur between local populations of the same species. (4) The amount of variation at a given locus is approximately the same in all five species. (5) When different species are compared, the pattern of the variation is either essentially identical or totally different at a majority of loci. We have tested the hypothesis that protein polymorphisms are selectively neutral by examining four predictions derived from the hypothesis. Our results are at variance with every one of the predictions. We have measured the amount of genetic differentiation, D, between taxa of various degrees of evolutionary divergence. The average value of D is 0.033 for local populations, 0.228 for subspecies, 0.226 for semispecies, 0.538 for sibling species, and 1.214 for morphologically distinguishable species. Our results indicate that a substantial degree of genetic differentiation (22.8 allelic substitutions for every 100 loci) occurs between allopatric populations that have diverged to the point where they might become different species if they were to become sympatric. However, very little additional genetic change is required for the development of complete reproductive isolation. After the speciation process is completed, species continue to diverge genetically from each other.

https://www.genetics.org/content/genetics/77/2/343.full.pdf

Genetic variation in natural populations of five drosophila species and the hypothesis of the selective neutrality of protein polymorphisms

The distribution of geinetic variability in deep-sea organisms is virtually unknown, although estimates of genetic variability in the deep-sea fauna are directly relevant to some hypotheses concerning adaptive strategies in stable and unstable environments. One hypothesis, advanced to account for the great species diversity observed in the deep-sea, argues that deep-sea populations are highly specialized and possess little genetic variability (Grassle, 1972). Another hypothesis, also predicting that deep-sea populations should have little genetic variability, has been advanced by Bretsky and Lorenz (1969, 1970) to account for the waves of extinctions registered in the paleontological record. This hypothesis proposes that organisms living in stable environments should be genetically depauperate. Since the deep-sea provides one of the most stable environments in the world, both hypotheses can be tested by examining the amount of genetic variation in deep-sea organisms. In the course of a broad survey of genetic variability in diverse marine environments we have studied the genetic variability of four species of deep-sea asteroids from the San Diego Trough at a depth of 1,244 meters. Allelic variation at 24 individual loci coding for enzymes and other soluble proteins has been studied using the techniques of starch gel electrophoresis.

Deep-sea asteroids: high genetic variability in a stable environment.

Sibling species are species morphologically so similar as to be practically indistinguishable from each other by inspection of their structural features. The Drosophila willistoni group consists of at least twelve species, six of which are siblings. Two of the sibling species, D. pavlovskiana Dobzhansky and Kastritis, and D. insularis Dobzhansky, are narrow endemics; the former in Guyana and the latter on some islands of the Lesser Antilles. The other four species, D. willistoni Sturtevant, D. equinoxialis Dobzhansky, D. paulistorum Dobzhansky and Pavan, and D. tropicalis Burla and de Canba, have wide and largely overlapping geographic distributions. The four species are sympatric through Central America and the northern half of continental South America; two or three of them are sympatric in the southern half of Mexico, the larger Caribbean islands, Peru, Bolivia, and the southern half of Brazil (Spassky et al., 1971). We have studied for several years genetic variation in the sibling species of the D. willistoni group, using techniques of gel electrophoresis and enzyme assay. D. equinoxialis consists of two subspecies, D. e. equinoxialis and D. e. caribbensis (Ayala, 1973). In this paper we report results of the study of reproductive isolation between these two subspecies, and we compare genetic variation within and between the subspecies. Ayala and Powell (1972) have

Genetic and reproductive differentiation of the subspecies, drosophila equinoxialis caribbensis.

The Phoronida are a coelomate phylum consisting of only two genera and about 12–15 described species. Phoronids probably represent the common ancestral stock of all lophophorates, and may be the most primitive living deuterostomes. Using the techniques of starch gel electrophoresis, we have studied genetic variation at 39 loci in 120 individuals of Phoronopsis viridis collected in Bodega Harbor, Bodega Bay, California. Allelic variation was found at 27 (69.2%) loci. If a locus is considered polymorphic when the frequency of the most common allele is no greater than 0.99, the proportion of polymorphic loci in the total sample is 48.7%. The average number of alleles per locus is 2.23. The expected frequency of heterozygous loci per individual on the assumption of Hardy-Weinberg equilibrium is 9.4%. There is evidence of inbreeding; the mean value of F, Wright's fixation index, is 0.21±0.02. Genetic variability in P. viridis is intermediate among marine invertebrates. The tropical clam, Tridacna maxima, has on the average 20.2% heterozygous loci per individual. At the other extreme, a brachiopod from Antarctica, Liothyrella notorcadensis, has an average of 3.9% heterozygous loci per individual. Among marine invertebrates, there seems to be a gradient of decreasing genetic variability from low to high latitudes, which may reflect their different adaptive strategies.

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Genetic variability in a temperate intertidal Phoronid, Phoronopsis viridis

We have obtained monthly samples of two species, Drosophila pseudoobscura and Drosophila persimilis, in a natural population from Napa County, California. In each species, about 300 genes have been assayed by electrophoresis for each of seven enzyme loci in each monthly sample from March 1972 to June 1975. Using statistical methods developed for the purpose, we have examined whether the allele frequencies at different loci vary in a correlated fashion. The methods used do not detect natural selection when it is deterministic (e.g., overdominance or directional selection), but only when alleles at different loci vary simultaneously in response to the same environmental variations. Moreover, only relatively large fitness differences (of the order of 15%) are detectable. We have found strong evidence of correlated allele frequency variation in 13-20% of the cases examined. We interpret this as evidence that natural selection plays a major role in the evolution of protein polymorphisms in nature.

/pdf/natural-selection-vs-random-drift-evidence-from-temporal-1weqyd8dlo.pdf

Lorraine G. Barr

Papers

Genetic variation in natural populations of five drosophila species and the hypothesis of the selective neutrality of protein polymorphisms

Deep-sea asteroids: high genetic variability in a stable environment.

Genetic and reproductive differentiation of the subspecies, drosophila equinoxialis caribbensis.

Genetic variability in a temperate intertidal Phoronid, Phoronopsis viridis

Natural selection vs. random drift: evidence from temporal variation in allele frequencies in nature.