Selection In Natural Populations Research Articles

Differential Climatic Selection in Natural Population of Land Snail Cepaea nemoralis

DURING a study of the energetics of a population of the polymorphic land snail Cepaea nemoralis L. on the sand dunes at Braunton Burrows, North Devon, numbers of dead and dying snails were collected during the daylight hours in the summer. They showed similar symptoms, being unable to retract the foot which had become dried, discoloured and unresponsive to touch stimuli. Some animals were unable to retract the optic tentacles and in some, the genitalia protruded. Snails found in this condition, although alive (as evidenced by sensitivity of the mantle area) early in the day, were usually dead by evening.

GEOGRAPHIC VARIATION AND ENVIRONMENTAL SELECTION IN GASTEROSTEUS ACULEATUS L. IN THE PACIFIC NORTHWEST, AMERICA.

Ecological geneticists are giving considerable attention to the study of selection in natural populations, its intensity, and what it is capable of doing. Such studies have begun to tell us something about the variation found in natural populations, and how they may adjust to the demands of the prevailing environment. However, these studies have largely been confined to the invertebrates (review in Ford, 1964; Lewontin, 1967; Spiess, 1968; Sheppard, 1969). There is a need for comparative material, and the threespine stickleback (Gasterosteus aculeatus L.) among vertebrates seems to be unusually favorable. Freshwater populations of G. aculeatus are exceedingly variable, and Miller and Hubbs (1969) argued that nearly all the variation is a consequence of introgression between leiurus (freshwater populations) and trackurus (marine populations). But it has become increasingly clear that introgression cannot account for much of the variation in ieiurus (Hagen, 1967; McPhail, 1969; Hagen and McPhail, 1970; Moodie, in press). G. aculeatus is well suited to studies in ecological genetics for its ecology and breeding behavior are easily observed and investigated in the wild. It is unusually abundant, withstands handling and marking very well, and is easily kept and reared in captivity. Its small size facilitates rearing and experimental work in the laboratory, and generation time is relatively short, such that about two generations can be obtained a year. A genetical basis for the more variable traits has been established (Hagen, 1967; McPhail, 1969), and the genetics of several of these (lateral plate number, plate morphs, and gill rakers) is reasonably well known (Hagen, MS; McPhail, unpub. data). Moreover, there is an extensive literature on reproduction, physiology, and life history providing the necessary background (for references see Heuts, 1947; Iersel, 1953; Greenbank and Nelson, 1959; Mullen and Vlugt, 1964; Baggerman, 1966; Assem, 1967). This report is a preliminary investigation of geographic variation in permanent freshwater populations of G. aculeatus, excluding the marine trachurus. The study seeks to establish correlations of phenotype with environmental variables, where they exist, applying the techniques so profitably exploited by ecological geneticists. The importance of introgression as a cause of the variation in leiurus is considered since the data are also relevant to this question.

Isozyme Variation under Selection in Zea mays

THE extent of polymorphism in genetically determined isozyme variants (allozyme variation) in natural populations raises the issue of its adaptive significance. It was originally thought that this variation is neutral with respect to fitness1, but recent evidence supports the assertion that its maintenance involves a selective component2. If an allelic substitution yields an enzyme with altered properties3, maintenance by selection in natural populations may result in a distinct geographic distribution of allele frequencies. Such a pattern has been described4 in the case of a serum esterase polymorphism in the freshwater fish Catostomus clarkii.

Selection in natural populations

Selection in natural populations

Selection in Natural Populations. 6. Variation Genetics, and More Graphs for Estimation

Selection in Natural Populations. 6. Variation Genetics, and More Graphs for Estimation

Selection in Natural Populations. III. Measurement and Estimation

Selection in Natural Populations. III. Measurement and Estimation

Selection in natural populations. IV. British housemice (Mus musculus).

In a number of populations of semi-commensal housemice in England and Wales, the older individuals are on the average larger and more variable in molar width than the younger individuals of the same populations. Several environmental variables have been shown to be unimportant with respect to these within-population differences, and it therefore seems likely that the differences in this age-invariant character are due to directional and destabilizing natural selection.

Selection in natural populations

Selection in natural populations

MEASUREMENT OF NATURAL SELECTION IN NATURAL POPULATIONS.

VAN VALEN1 has proposed a measure of natural selection intensity as an extension of Haldane's2 measure of intensity I and applied this to data on fossil horses. Van Valen's measure, at least in the example given, is actually a special case of Haldane's I. Van Valen wrote: “The intensity Ip of natural (or artificial) selection by means of differential viability, relative to statistical parameter or set of parameters p, may be defined as the minimum reduction of the original population that would yield a population with specified values of the parameter or set of parameters”. For a single normally distributed character this corresponds to the truncation area of a normal curve necessary to shift the mean and standard deviation to the new observed values under directional selection. Haldane's measure of intensity is defined as: where s0 is the survival percentage at the optimum phenotype or genotype and S is the overall survival percentage. Haldane has shown that (1) can be written: where ƒ1(x0) and ƒ2(x0) are the probability densities of the characters at the optimum value x0 before and after selection, respectively. Furthermore, if ƒ1(x) and ƒ2(x) are normal densities (the notation of Rao3 is used throughout)

SELECTION IN NATURAL POPULATIONS: HUMAN FINGERPRINTS.

IN order to estimate the normal intensities of natural selection on quantitative characters, characters must be chosen for reasons other than the likelihood of natural selection operating on them1. This communication deals with one suitable character.

Selection in Natural Populations: Merychippus Primus, a Fossil Horse

Selection in Natural Populations: Merychippus Primus, a Fossil Horse

Chromosomes and Natural Selection

Gene mutations can be lethal in their effects, they can be mildly disruptive and diminish individual fitness, they can be neutral or even slightly favourable. The same is true for chromosomal aberrations. Consequently, natural selection in populations can be set up by alterations in chromosomal structure just as it is by gene changes which affect the hereditary material of a species. There are, however, important differences in the manner by which selective effects are produced following the various kinds of spontaneous change.The present paper deals primarily with chromosomal changes but there are two ways in which selection may act on them. First, there is the immediate effect of an aberration upon the fitness of an individual who carries it and which leads to persistence or extinction of that particular aberration. Secondly, there is the less direct effect of an aberration upon the frequency of any gene which may be responsible for inducing it. Study of the immediate effects is comparable to the analysis of the behaviour of mutant genes in populations by classical methods (Fisher 1930, Wright 1931, Haldane 1932). Study of the second, less direct, process is comparable to the analysis of the behaviour of mutagenic genes (mutators).

Genetics of natural populations. XIX. Origin of heterosis through natural selection in populations of Drosophila pseudoobscura.

Genetics of natural populations. XIX. Origin of heterosis through natural selection in populations of Drosophila pseudoobscura.