ADAPTIVE VERSUS INCIDENTAL EXPLANATIONS FOR THE OCCURRENCE OF PROTANDRY IN A BUTTERFLY, LEPTIDEA SINAPIS L.

Abstract

In many butterflies males emerge before females (Petersen, 1892; Demoll, 1908; Berge and Rebel, 1910; Ford, 1945; Petersen, 1947; Forster, 1954; Newman, 1968). This phenomenon, here referred to as protandry, is also known from other insects (Petersen, 1892; Demoll, 1908; Petersen, 1947). At least two general classes of hypotheses can be invoked to explain the occurrence of protandry. First, the time difference per se in eclosion times may be the result of selective pressures on males and females to emerge at different times in relation to each other. This class of explanations is here called adaptive. Thus, for example, Wiklund and Fagerstr6m (1977) argue that sexual selection between males to maximize the number of matings leads to protandry. It has also been argued that sexual selection between females to minimize the risk of death prior to mating also leads to protandry (Ford,, 1945; Bowden, 1979; Wiklund and Fagerstrom, 1979). If eclosions are similarly distributed among males and females the optimal difference in eclosion time is similar for male and female strategies (Wiklund and Fagerstr6m, unpubl.). Another hypothesis claims that selection for outbreeding can explain the incidence of protandry in butterflies (Petersen, 1892) thus presupposing that males are better suited for dispersal than females. Second, the difference in eclosion times may be a byproduct of selection for other life history For example, in many butterflies males are smaller than females (which may be adaptive per se) and thus as an byproduct of the size difference between the sexes. We call this class of explanations for the occurrence of protandry incidental. Recently Lewontin (1977, 1978a, 1978b) and Gould and Lewontin (1979) have stressed the importance of seriously considering non-adaptive or explanations for various traits. The aim of this paper is to report the results of experiments aimed at distinguishing between and incidental explanations for the occurrence of protandry in a butterfly, the Wood White Leptidea sinapis L. (Lepidoptera: Pieridae). (We are aware of the fact that the term is in part ambiguous. For example a trait can have a positive adaptive value without having been for [cf. Williams, 1966; Futuyma, 1979]. We therefore want to state explicitly that in this paper is used synonymously with selected for. ) The basis for our attempt to distinguish between these two classes of explanations is the occurrence of two developmental phenotypes in L. sinapis. By using different phenotypes (in this case two generations) of the same species we gain the advantage of comparing organisms with the same physiological constraints or Bauplan (cf. Gould and Lewontin, 1979) thus avoiding the problems of comparing different species that are always to some extent trapped in their particular evolutionary pasts (cf. Solbreck, 1978). Pertinent points in the biology of L. sinapis in Sweden follow. The insect overwinters in the pupal stage. In spring males emerge from the pupae a couple of days before females (Wiklund, 1977). Follow-