VARIATION IN DEVELOPMENT PATTERNS OF LARVAL ANURANS IN TEMPORARY PONDS. I. PERSISTENT VARIATION WITHIN A HYLA GRATIOSA POPULATION.

Abstract

At the core of evolutionary theory is phenotypic variation in traits that influence survival or reproductive success, and the extent to which that variation is genetically based (Lewontin, 1974). Delineating the microevolutionary role of such persistently variable traits depends on ascertaining the genetic and non-genetic sources of variation, the environmental effects on that variation, and whether critical traits vary independently or as discrete ensembles of characters. The larval cohorts of the anurans that breed in temporary ponds are excellent subjects for examining this microevolutionary role. Three traits, growth rate, length of larval period, and size at metamorphosis, are phenotypically variable, often intercorrelated, and play critical roles in determining larval and juvenile survival. Cohorts of larvae experience a high level of mortality, due to predation, between hatching and metamorphosis (Herreid and Kinney, 1966; Licht, 1974; Kadel, 1975; Creusere and Whitford, 1976; Cecil and Just, 1979). Faster growing larvae achieve a refuge from size-limited predators at an earlier age than do more slowly growing individuals (Pritchard, 1965; Wilbur, 1972; Calef, 1973; Heyer et al., 1975; Caldwell et al., 1980) and thereby experience a lower cumulative risk of such predation. This process has been shown to cause differential survival in field enclosures among full-sib families of larvae that vary in their growth rates (Travis, 1983). Drying of a pond may restrict survival further to the most rapidly developing individuals (Bragg, 1940; Mayhew, 1965; Tevis, 1966; Shoop, 1974; Heyer et al., 1975; Wilbur, 1977a). Larvae that grow faster when young can metamorphose earlier than more slowly growing contemporaries (Wilbur and Collins, 1973; Wilbur, 1976; Collins, 1979). This relationship produces different distributions of larval period length among both full-and half-sib families that differ in their growth rates (Travis, 1980a, 1980b, 198 la). Individual growth rate is a key trait because of its relationship to the probability of survival to metamorphosis. If larval growth rates reflect parentage, then the individuals that survive to metamorphosis can represent a decidedly biased sample of the progeny of all reproducing adults. This bias can reduce the effective population size in short-lived frogs, and contribute to a potential reduction in longer lived species (Charlesworth, 1980). This bias can also produce some selective mortality every season, depending upon the degree to which the variation in growth rates is heritable (Wallace, 1981). Both effects will play a role in determining the overall level of genetic variability in these populations, and previous work (Travis, 1980a, 1981a, 1983) has suggested that such differential larval survival is a common feature of these anuran populations. But there are many other aspects to this problem. Although growth rate and development time can influence larval survival, body size at metamorphosis can influence survivorship and reproductive potential in the adult stage. Larger juveniles have a higher chance of surviving the ensuing winter than do smaller juveniles (Martof, 1956). In long-lived species, larger juveniles can have an earlier age or larger size at first reproduction (Turner, 1962; Clarke, 1974; Collins, 1975). The clutch size of a female is directly related