Genetic Homeostasis and Developmental Rate in Tribolium.

ENETIC analysis of a variety of traits in populations of several species has revealed a similar pattern for many traits which are important components of fitness. In general such characters have intermediate optima, and prolonged artificial selection for extreme expression leads to a reduction in fitness. LERNER (1954, 1958) has postulated that one possible mechanism for natural selection in favor of phenotypic intermediates is superiority of heterozygotes with respect to fitness. Thus, although the value of a metric character may be determined by additively acting alleles, the pleiotropic effects of these genes on fitness are overdominant in nature. It has been shown that this situation will lead to maintenance of genetic variability in a population (ROBERTSON 1956). The recent studies of LERNER and Ho (1961), which demonstrated that the outcome of interspecific competition trials with flour beetles is influenced by genetic as well as environmental factors, has made it of increased interest to attempt to determine the importance of various components of competitive ability. Developmental rate, which is an important component of fitness in Drosophila, according to HIRAIZUMI (1961), was chosen to be used in the present investigation. An extensive review of pertinent literature on the genetics of developmental rate was made by DAWSON (1964a). Selection for rate of development has been used several times as a tool for studying other problems: HUNTER (1959) examined correlated responses in DDT resistance in Drosophila melanogaster; CLARKE, SMITH and SONDHI (1961 ) used it for a study of asymmetrical response to selection in D. subobscura; MARIEN (1958) investigated the variable outcome of replicate selection experiments with D. pseudoobscura; PROUT ( 1962) studied the effects of stabilizing and disruptive selection in D. melanogaster. HOLLINGSWORTH and SMITH (1955) combined selection with inbreeding to study the correlation between slow development and infertility. Other relevant studies include those of BONNIER (1961a, b, and BONNIER, JONSSON and RAMEL 1959) in which it was demonstrated that in D. melanogaster heterozygous individuals exhibit a faster rate of development and a better competing ability than homozygotes. SMITH and SMITH (1954) and HOLLINGSWORTH