Variation, Covariation and Genotype-Environment Interaction in the Life-History of Natural Populations of 'Tribolium castaneum'

Abstract

A life-history includes "the entire sequence of changes through which an organism passes in its development from conception to death" (Lande 1982b). Life-history traits, as major determinants of fitness, should be subject to the influence of natural selection and undergo evolutionary change. Thus a distinction must be made between genotypic variation in the life-history and environmentally induced phenotypic variation. In this thesis, two separate but complementary experiments address specific areas concerning life-history evolution in natural populations. The model organism used in these laboratory studies was the "rust-red flour beetle" 'Tribolium castaneum' (Herbst), a well known pest of stored grain and cereal products. In the Nested experiment (Chapter 3), the methods of quantitative genetics were used to give a statistical description of the genetic sources of variation, covariation and genotype-environment interaction for several life-history traits measured on female beetles derived from two geographically distinct natural populations of 'T. castaneum'. These results are used to make inferences regarding the potential of populations to respond to natural selection, as well as the past selective regimes experienced by the populations. Since evolutionary change in response to natural selection requires that phenotypic variation be heritable, as well as that fitness varies according to the phenotype (Endler 1986), heritabilities were estimated for the individual life-history traits within each population. Information about the relationships between the different life-history characters, expressed as genetic correlations, were used to test for the presence of "trade-off's" and more specifically "reproductive costs". These genetic correlations also were used to test for "antagonistic pleiotropy" and its potential role in maintaining additive genetic variation within natural populations. In addition, the experiment was conducted under two constant laboratory temperatures (33° and 37°C) to investigate the importance of genotype-environment interaction in the evolution of life-histories. Thus the Nested experiment evaluated the potential role of genotype-environment interaction in maintaining additive genetic variation, and the stability of the quantitative genetic parameters under different environments. Comparisons between the populations assessed the magnitude of intraspecific genetic variation in life-history traits.