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

Evolution at two levels in humans and chimpanzees

J+=j

We believe that punctuational change dominates the history of life: evolution is concentrated in very rapid events of speciation (geologically instantaneous, even if tolerably continuous in ecological time). Most species, during their geological history, either do not change in any appreciable way, or else they fluctuate mildly in morphology, with no apparent direction. Phyletic gradualism is very rare and too slow, in any case, to produce the major events of evolution. Evolutionary trends are not the product of slow, directional transformation within lineages; they represent the differential success of certain species within a clade—speciation may be random with respect to the direction of a trend (Wright's rule).As an a priori bias, phyletic gradualism has precluded any fair assessment of evolutionary tempos and modes. It could not be refuted by empirical catalogues constructed in its light because it excluded contrary information as the artificial result of an imperfect fossil record. With the model of punctuated equilibria, an unbiased distribution of evolutionary tempos can be established by treating stasis as data and by recording the pattern of change for all species in an assemblage. This distribution of tempos can lead to strong inferences about modes. If, as we predict, the punctuational tempo is prevalent, then speciation—not phyletic evolution—must be the dominant mode of evolution.We argue that virtually none of the examples brought forward to refute our model can stand as support for phyletic gradualism; many are so weak and ambiguous that they only reflect the persistent bias for gradualism still deeply embedded in paleontological thought. Of the few stronger cases, we concentrate on Gingerich's data for Hyopsodus and argue that it provides an excellent example of species selection under our model. We then review the data of several studies that have supported our model since we published it five years ago. The record of human evolution seems to provide a particularly good example: no gradualism has been detected within any hominid taxon, and many are long-ranging; the trend to larger brains arises from differential success of essentially static taxa. The data of molecular genetics support our assumption that large genetic changes often accompany the process of speciation.Phyletic gradualism was an a priori assertion from the start—it was never “seen” in the rocks; it expressed the cultural and political biases of 19th century liberalism. Huxley advised Darwin to eschew it as an “unnecessary difficulty.” We think that it has now become an empirical fallacy. A punctuational view of change may have wide validity at all levels of evolutionary processes. At the very least, it deserves consideration as an alternate way of interpreting the history of life.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0094837300005224

Punctuated equilibria; the tempo and mode of evolution reconsidered

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.

We describe allelic variation at 28 gene loci in natural populations of D. willistoni. Seventy samples were studied from localities extending from Mexico and Florida, through Central America, the West Indies, and tropical South America, down to South Brazil. At least several hundred, and often several thousand, genomes were sampled for each locus. We have discovered a great deal of genetic variation. On the average, 58% loci are polymorphic in a given population. (A locus is considered polymorphic when the frequency of the most common allele is no greater than 0.95). An individual fly is heterozygous, on the average, at 18.4% loci.—Concerning the pattern of the variation, the most remarkable finding is the similarity of the configuration of allelic frequencies from locality to locality throughout the distribution of the species. Our observations support the conclusion that balancing natural selection is the major factor responsible for the considerable genetic variation observed in D. willistoni.

/pdf/enzyme-variability-in-the-drosophila-willistoni-group-iv-1lt5ngx0z9.pdf

Enzyme variability in the drosophila willistoni group. iv. genic variation in natural populations of drosophila willistoni

Evolution consists of changes in the genetic constitution (gene pool) of populations. The process of evolution may be seen in two ways-which have been called anagenesis and cladogenesis (Rensch, 1960). Changes occurring within a given phylogenetic line as time proceeds, are anagenetic evolution. They result in increased adaptation to the environment, and often reflect changes of the physical or biotic conditions of the environments. Cladogenesis occurs when a phylogenetic lineage splits into two or more independently evolving lineages. The great diversity of the living world is the result of cladogenetic evolution, which results in adaptation to a greater variety of niches, or ways of life. Among cladogenetic processes, the most decisive one is speciation, the process by which one species splits into two or more species. Species are groups of populations reproductively isolated from any other such groups. Species are independent evolutionary units. Adaptive changes occurring in an individual or population may be extended to all members of the species by natural selection; however, they cannot be passed on to different species. One of the most important questions in evolutionary genetics is the amount of genetic differentiation occurring in the speciation process. Speciation may occur by a variety of processes. In sexually reproducing organisms speciation most generally occurs according to the model of "geographic speciation." Two main stages may be recognized in this process. First, allopatric populations of the same species become genetically differentiated, mostly as a consequence of their adaptation to different environments. This genetic differentiation can only occur if the populations are geographically separated for some time, and there is no or very little migration between them. The second stage takes place when genetically differentiated populations come into geographic contact. If the gene pools of two populations are sufficiently different, progenies from interpopulational crosses are likely to have less fitness than progenies from intrapopulational crosses. Mating preferences are affected by genes. Alleles that decrease the probability of mating with individuals of a different population will, then, be selected against, while alleles increasing the probability of intrapopulational mating will be favored by natural selection. Eventually, the process may result in two reproductively isolated populations, and thus two different species. The question, "how much genetic differentiation occurs in the process of speciation," must be unfolded into two separate questions concerning the two stages of the process. The first question concerns the genetic differentiation occurring between allopatric populations that are likely to give rise to different species if and when they become, at least partially, sympatric. The second question refers to the amount of genetic differentiation taking place after sympatry, when the populations become reproductively isolated. 1 This is paper number XII in a series: "Enzyme variability in the Drosophila willistoni group." 2 Supported by NSF grant GB32895. 'Present address: Dept. of Biology, Brock University, St. Catherines, Ontario, Canada. 'Address: Department of Zoology, University of Indiana, Bloomington, Indiana.

Genetic differentiation during the speciation process in drosophila.

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

Eight populations of American lobsters, Homarus americanus, were surveyed for genetic variation at 44 loci encoding electrophoretically detectable proteins. Rather low levels of genetic variability...

Biochemical Genetics of Lobsters: Genetic Variation and the Structure of American Lobster (Homarus americanus) Populations

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

Martin L. Tracey

Papers

Enzyme variability in the drosophila willistoni group. iv. genic variation in natural populations of drosophila willistoni

Genetic differentiation during the speciation process in drosophila.

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

Biochemical Genetics of Lobsters: Genetic Variation and the Structure of American Lobster (Homarus americanus) Populations

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