THE GENETICS OF CENTRAL AND MARGINAL POPULATIONS OF DROSOPHILA SERRATA. II. 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 Wright's shifting-balance theory of evolution. A selection experiment was conducted in an attempt to push different geographic populations to different fitness peaks as a correlated response to selection for an additively controlled character (desiccation resistance). Four populations of Drosophila serrata, sampled from central and marginal areas of its distribution along a 3000-km stretch of Australia's east coast, underwent selection for desiccation resistance for 14 generations. After selection had ceased, control lines from each of the populations were crossed 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 three 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 field increased. This relationship was lost in the selected lines. Therefore, selection for desiccation resistance influenced the level of hybrid breakdown in a fitness trait, although selection may need to be maintained for longer than 14 generations if a new relationship between hybrid breakdown and distance is to be formed.