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

Abstract

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.