Introduction

An organism's life history is its lifetime pattern of growth, differentiation, storage of reserves and reproduction (Begon et al., 1990: 473). Life-history traits therefore include size at birth and maturity and the investment in reproduction at different ages and sizes (Roff, 1992; Stearns, 1992).

The temperatures under which ectothermic organisms evolve can sometimes have the same qualitative effects on life-history traits as temperatures experienced during development. The evolutionary effect comprises differences between genotypes arising from selection for several generations at different temperatures, and are observed when the different genetic lines are then reared under identical environmental conditions. By contrast, the developmental effect is the result of phenotypic plasticity and can even be observed among members of a clone reared at different temperatures. An example of such similar responses is the common decrease in adult size in Drosophila following evolution or development at increased temperatures (pp. 265–92; Partridge et al., 1994). Whether this similarity arises because the same selection pressures cause evolution of both size and the developmental response (or reaction norm; Stearns, 1992) of size to different temperatures, is not known. Developmental responses of body size to temperature are reviewed and discussed in the present chapter, while both developmental and evolutionary responses are discussed by Partridge & French (pp. 265–92) with special reference to studies on Drosophila.