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Evolution at two levels in humans and chimpanzees

Genetic variation in natural populations: patterns and theory.

Island populations are much more prone to extinction than mainland populations. The reasons for this remain controversial. If inbreeding and loss of genetic variation are involved, then genetic variation must be lower on average in island than mainland populations. Published data on levels of genetic variation for allozymes, nuclear DNA markers, mitochondrial DNA, inversions and quantitative characters in island and mainland populations were analysed. A large and highly significant majority of island populations have less allozyme genetic variation than their mainland counterparts (165 of 202 comparisons), the average reduction being 29 per cent. The magnitude of differences was related to dispersal ability. There were related differences for all the other measures. Island endemic species showed lower genetic variation than related mainland species in 34 of 38 cases. The proportionate reduction in genetic variation was significantly greater in island endemic than in nonendemic island populations in mammals and birds, but not in insects. Genetic factors cannot be discounted as a cause of higher extinction rates of island than mainland populations.

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Do island populations have less genetic variation than mainland populations

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.

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

The evolutionary significance of genetic diversity of proteins in nature remains controversial despite the numerous protein studies conducted electrophoretically during the last two decades. Ironically, the discovery of extensive protein polymorphisms in nature (reviewed by Lewontin, 1974; Powell, 1975; Selander, 1976; Nevo 1978, 1983b; Hamrick et al., 1979; Nelson and Hedgecock, 1980), did not resolve the disagreement between the die ho torn ou s explanatory models of selection (e.g., Ayala, 1977; Milkman, 1978; Clarke, 1979; Wills, 1981) versus neutrality (Kimura, 1968; Kimura and Chta, 1971; Nei, 1975; and modifications in Kimura, 1979atb). The more general problem of the relative importance of the evolutionary forces interacting in genetic population differentiation at the molecular levels of proteins and DNA, i.e., mutation, migration, natural selection and genetic drift, remains now as enigmatic as ever.

The Evolutionary Significance of Genetic Diversity: Ecological, Demographic and Life History Correlates

Genetic Variation Among Vertebrate Species

To many early evolutionists, particularly those of the mutationist school, the existence of evolutionarily stable forms ("living fossils" or phylogenetic "relics") posed a difficult problem, and, even among contemporary biologists, the feeling that "a long-standing stability of organization seems antithetical to the concept of evolution" may persist (Volpe, 1967:140). In the early part of this century, it was popularly believed that stable or slowly evolving (bradytelic) lines lacked the genetic variation necessary for more rapid evolution as a result of having abnormally low mutation rates; as Mayr (1963) puts it, a lack of variation in time and space was interpreted as a reflection of genotypic stability. Largely because of the influence of Simpson (1944), Stebbins (1944), Schmalhausen (1949), and Dobzhansky (1951), the mutationists' interpretation of bradytely has now been all but abandoned by evolutionists in favor of the view that the principal factors controlling rates of evolution are to be found in the organism-environment relationship. Additionally, Mayr (1963 :304) has suggested that phenotypic "uniformity in space and time is due to a highly perfected 'buffering system,' in other words, to genetic homeostasis." The ecological hypothesis is well expressed in Simpson's (1944:141) conclusions that "the final and probably the most fundamental factor in the relationship is that bradytelic groups are so well adapted to a particular, continuously available environment that almost any mutation occurring in them must be disadvantageous" and that "bradytely results from the equilibrium of large breeding populations of animals specifically adapted to a continuously available environment that is relatively invariable . . . ." Yet, as Mayr (1963) notes, there is no evidence to refute (or to support) the hypothesis that bradytely results from loss of mutability. The recent development of electrophoretic techniques for demonstrating allelic variation at loci controlling enzymes and other proteins in natural populations (Hubby and Lewontin, 1966; Lewontin and Hubby, 1966) permits a test of the mutationists' thesis. The purpose of this study is to compare degrees of protein polymorphism and underlying genic heterozygosity in a classic phylogenetic "relic," the horseshoe or king crab (Limulus polyphemus), and in several representatives of horotelic lines (those evolving at standard rates) for which data are already available. Evidence of a level of genetic variation in Limulus similar to that in horotelic forms will support the modern ecological and homeostatic interpretations of bradytely. But if the mutationists' thesis is valid, Limulus should show an unusually low degree of polymorphism at its genetic loci. Limulus polyphemus is one of five living species of the Subclass Xiphosura, a group of marine arthropods usually placed with the Subclass Eurypterida (extinct giant water scorpions) in the Class Merostomata of the Subphylum Chelicerata. Following a period of adaptive radiation in the early and middle Paleozoic, the Merostomata became much less prominent after the Permian. Several Mesozoic forms are known in the Family Mesolimulidae, but by the Tertiary the fossil record is all but absent. There are presently three living genera, all belonging to the Family Limulidae, only one of which, Tachypleus, is known from the fossil record (Stormer,

Genetic variation in the horseshoe crab (limulus polyphemus), a phylogenetic "relic".

Johnson, Walter E. and Robert K. Selander (Department of Zoology, The University of Texas, Austin, Texas 78712) 1971. Protein variation and systematics in kangaroo rats (genus Dipodomys). Syst. Zool. 20:377-405.-Electrophoretically demonstrable genetic variation in 11 enzymes and six non-enzymatic proteins encoded by 18 loci was analyzed in 1208 specimens of 11 species of kangaroo rats (Dipodomys) from the westem United States and in a related heteromyid, the pocket-mouse Perognathus hispidus. Most of the 17 proteins studied exhibit both polymorphic and interspecific variation, but protein-I is invariable; and protein-2, protein-3, and hemoglobin are interspecifically variable although not polymorphic in any species. Except for Hb-1 (hemoglobin) and Trf-1 (transferrin), there is at each locus one allele which tends to predominate in all species of Dipodomys. The maximum number of alleles detected at a single locus in a population was three. Species of Dipodomys are, on the average, less variable genetically than other organisms for which estimates of levels of polymorphism and heterozygosity are available. An average of 7.9% of loci are polymorphic per population, and mean heterozygosity is 2.1%O. Only four species are genetically as variable as rodents of the genera Mus and Peromyscus. Genic variability is uniform within species and is not correlated among species with extent of geographic range or degree of morphological "specialization." Possible relationships between degree of genetic variability and ecological amplitude or niche width are discussed. Significant regional heterogeneity in allele frequencies was demonstrated at most polymorphic loci in species that were relatively well sampled. Because geographic variation is limited, useful estimates of frequencies of the major alleles at loci in a species can be obtained from a small number of samples taken anywhere within the range. Coefficients of genetic similarity between conspecific populations invariably exceed 0.90. For interspecific comparisons, coefficients range from 0.31 to 0.89, with a mean of 0.61. On the average, species of Dipodomys are genetically more similar than are non-sibling species of Drosophila or Anolis. The pattem of relationships among species of Dipodomys indicated by genetic analysis differs significantly from those derived by Setzer, Lidicker, and others on morphological grounds but is similar to that provided by Stock's recent analysis of karyotypic variation in the genus. [Protein variation; electrophoresis; Dipodomys.] A fundamental tenet of evolutionary theory is that race formation and speciation occur by the conversion of genetic variation within populations to variation among populations (Lewontin, 1967). For this reason, evolutionists have long sought to determine the extent of genetic variation in local populations, among geographic populations, and among closely related species (Dobzhansky, 1970; Mayr, 1970). With the recent development of electrophoretic techniques for demonstrating allelic variation at genetic loci controlling the structure of enzymes and other proteins, it has become possible to estimate several critical parameters of genetic variation in natural populations, working on the assumption that the structural genes sampled are representative of the genome (Hubby and Lewontin, 1966; Lewontin and Hubby, 1966; Harris, 1970; Selander and Johnson, 1971). The few available estimates of degree of genic polymorphism in populations indicate that there is a considerable range of variation, even among congeneric species (see review in Selander et al., 1970) and among populations of the same species (Prakash et al., 1969; Selander, Hunt, and Yang, 1969; Selander et al., 1971). Studies of geographic variation in allele frequencies and of semi-species and sibling species suggest that the amount of genomic modification

https://www.jstor.org/stable/pdfplus/2412115.pdf

Protein Variation and Systematics in Kangaroo Rats (Genus Dipodomys)

Electrophoretically demonstrable variation was analyzed in proteins encoded by 41 structural gene loci in 71 individuals of the Florida deer mouse ( Peromyscus floridanus ) representing four sample areas. Of the 39 loci that were scorable in all individuals, 15 (38 per cent) were polymorphic in one or more populations. The average number of alleles detected per polymorphic locus was 2.01 (range 2 to 3). Average individual heterozygosity was 5.3 per cent. Populations of P. floridanus are less variable genetically than sympatric populations of the old-field mouse ( P. polionotus ), but similar in this regard to populations of the latter species from t0he Florida Panhandle and from South Carolina and Georgia. Coefficients of genetic similarity for paired geographical populations of P. floridanus ranged from 0.96 to 0.98, reflecting little geographical variation in allele frequencies. A comparison of P. floridanus with P. polionotus from five regions yielded coefficients of genetic similarity varying from 0.31 to 0.33.

Biochemical Polymorphism and Systematics in the Genus Peromyscus. III. Variation in the Florida Deer Mouse (Peromyscus floridanus), a Pleistocene Relict

The early mutationists believed that single mutations could produce species, but by 1940 studies of hybrids had demonstrated that species differences involve numerous genic and chromosomal mutations, individually of small effect (18). Similarly, the "classical" model of genetic population structure, which assumed widespread homozygosity of "wild-type" alleles, was refuted by research on viability and phenotypic characters, indicating the existence of much genic polymorphism in populations (76). In 1966 the concept of a high level of polymorphism as the normal condition in populations was convincingly established by direct measurements in Drosophila (42) and humans (25). The. acquisition by evolutionary geneticists of electrophoretic and other techniques for demonstrating allelic variation at loci encoding polypeptides permits a new and extensive examination of variation in populations and of genetic differences between races and species. This review of allozymic variation in vertebrates deals with 1. genic variation in populations; 2. local genetic heterogeneity; 3. geographic variation in allele frequencies; 4. genetic similarity among populations and species; and 5. systematic applications of molecular population data.

Walter E. Johnson

Papers

Genetic Variation Among Vertebrate Species

Genetic variation in the horseshoe crab (limulus polyphemus), a phylogenetic "relic".

Protein Variation and Systematics in Kangaroo Rats (Genus Dipodomys)

Biochemical Polymorphism and Systematics in the Genus Peromyscus. III. Variation in the Florida Deer Mouse (Peromyscus floridanus), a Pleistocene Relict

Genetic variation *4053 among vertebrate species