MOLECULAR VARIATION IN INSULAR ENDEMIC DROSOPHILA SPECIES OF THE MACARONESIAN ARCHIPELAGOS.

Abstract

As Carson (1970, 1973) has pointed out, islands, especially oceanic ones, constitute an advantageous arena for the study of biological material in that they have simpler conditions than those characteristic of continental biotas which frequently are too geographically, historically and ecologically complex to permit accurate inferences concerning the process of the origin of species. In spite of this, the fundamental research on the systematics and population genetics of the Drosophila species has been concentrated in the past on forms which have continental or even world-wide distributions, with the notable exception of the Hawaiian Drosophilidae. The Drosophila fauna from the Macaronesian Archipelagos of Madeira and the Canaries contrast markedly with the Hawaiian fauna. While the greatest species radiation of the genus can be found in the former (Hardy, 1973), only two endemic species are known in the latter, D. madeirensis (Monclus, pers. comm.), in the island of Madeira and D. guanche (Monclus, 1976), in some of the Canary Islands. From geological data it can be deduced that the species radiation of the Hawaiian Drosophila, at least in the major islands of the archipelago, has taken place only in the last 5-6 million years (MacDonald and Abbot, 1970). In contrast, the ancestral line which gave rise to the Macaronesian species may have colonized the islands many millions of years ago as revealed by the association of both species with the relict tertiary flora of the islands and by the similarity of their chromosomes to the presumed primitive karyotype of the genus (Prevosti, 1976 and pers. comm.). Another difference is that the species from Hawaii usually show only partial sexual isolation, even if their morphologies are very different (Craddock, 1973). However, D. guanche and D. madeirensis, in spite of being morphologically indistinguishable, are considered to be allopatric sibling species and are completely isolated reproductively, with no hybrids yet obtained in the laboratory (Monclu's, pers. comm.). A great amount of data is now available which has shed some light on the genetic changes which accompanied the adaptation and speciation processes in the Hawaiian Archipelago. We know that most Hawaiian species have enzymatic polymorphism levels similar to those of the continents, in spite of their extremely restricted distributions (Steiner et al., 1976; Steiner, 1979a, 1979b). Moreover, populations of closely related species can display extraordinary interand intrapopulation genetic variability (Carson and Johnson, 1975), as opposed to a greater similarity found between populations of continental species (for review see Ayala, 1975). Yet species of some subgroups show little variability in the degree of enzymatic polymorphism, even when a clear morphological and behavioral divergence exists between them (Sene and Carson, 1977; Craddock and Johnson, 1979). Almost all of these characteristics of the Hawaiian endemic species can be explained by the flush-founder speciation hypothesis of Carson (1970, 1973, 1975), although this does not seem to be a general process of speciation in oceanic islands. The different characteristics mentioned above for D. guanche and D. madeirensis are a hint that they have originated differently from the Hawaiian species. Since it is not known how this process has taken place, we have decided to examine it in more detail. Thus, we have studied the