GENE FLOW DISTANCES IN NATURAL POPULATIONS OF TETRAOPES TETRAOPHTHALMUS.

Abstract

Estimates of the rate or scale of flow are required for evaluating the various processes likely to maintain spatial variation in frequencies in any given species. A measure of flow must be incorporated into models of geographic differentiation under spatially varying selection pressures, most models of speciation, and models of inter-demic selection accompanying population differentiation. One convenient measure of the geographic scale of flow is the gene flow 1, described by several authors (i.e., Slatkin, 1973; Endler, 1979). This distance is defined as the square root of the mean squared distance from birth to reproduction and is proportional to the radius of the genetic neighborhood as described by Wright (Wright, 1946; Endler, 1979). Measures of flow in natural populations are usually obtained by studies of dispersal, be they studies of dispersal of the whole organism (i.e., animal and seed dispersal studies) or gametes (i.e., pollen dispersal studies). Endler (1979) points out that to properly evaluate flow from dispersal studies one must measure several life-history characteristics of the organism during the dispersal period. In studies of sexually reproducing animals that reproduce more or less continually over the dispersal period these parameters include age-specific survivorship and fecundity schedules as well as temporal patterns in the mating frequencies of males. Endler (1979) showed that dispersal studies of Drosophila that did not consider lifehistory patterns could over-estimate flow by 40% or more. This paper reports on dispersal and lifehistory studies of natural populations of the milkweed beetle Tetraopes tetraophthalmus. Previous studies (McCauley et al., 1981; Lawrence, 1982) have shown that dispersal is quite limited in this insect and that populations could be structured on a rather fine geographic scale. In neither study, however, was dispersal directly related to flow. Given that mating by both sexes and egg laying by females continues over the entire six week flight period of this univoltine insect, a more detailed analysis of movement and life-history as suggested by Endler (1979) is required to properly evaluate the scale of flow in this species. Movement patterns of reproducing adults and information on survivorship and fertility patterns during the reproductive/dispersal period are combined here to provide estimates of the flow distance, 1, in two populations with contrasting habitat structure. The implications of these results for micro-evolutionary processes in this species are discussed.