THE GENETICS OF CENTRAL AND MARGINAL POPULATIONS OF DROSOPHILA-SERRATA .2. HYBRID BREAKDOWN IN FITNESS COMPONENTS AS A CORRELATED RESPONSE TO SELECTION FOR DESICCATION RESISTANCE

Multiple-peak epistasis is one of the four premises that underlie Wrig ht's shifting-balance theory of evolution. A selection experiment was conducted in an attempt to push different geographic populations to di fferent fitness peaks as a correlated response to selection for an add itively controlled character (desiccation resistance). Four population s of Drosophila serrata, sampled from central and marginal areas of it s distribution along a 3000-km stretch of Australia's east coast, unde rwent selection for desiccation resistance for 14 generations. After s election had ceased, control lines from each of the populations were c rossed to determine the amount of hybrid breakdown that existed before selection and selected lines were crossed to determine the amount of hybrid breakdown after selection. Hybrid breakdown was measured in thr ee fitness traits: developmental time, viability, and fecundity. When the individual crosses were examined, virtually no evidence was found for hybrid breakdown between these populations. However, the level of hybrid breakdown in development time in the control lines increased as the distance between the populations in the held increased. This rela tionship was lost in the selected lines. Therefore, selection for desi ccation resistance influenced the level of hybrid breakdown in a fitne ss trait, although selection may need to be maintained for longer than 14 generations if a new relationship between hybrid breakdown and dis tance is to be formed.