Genetic variability levels among widespread species

The comparative study of congeners is a potentially important approach in understanding adaptations of colonizing species and could reveal special genetic features of colonists, although many potential difficulties in this aim will be apparent from the previous two chapters. At the phenotypic level, there is certainly a parallel between the ecological phenotypes of marginal populations and the characteristics of colonizing species, by comparison with congeneric noncolonist species (Chapter 3). Here, the question is the degree to which this phenotypic conclusion can be applied at the genotypic level.

Commencing with the eight cosmopolitan Drosophila species that occur in the six biogeographical zones, Carson (1965) has classified them according to degrees of chromosomal morphism:

1. Polymorphism essentially ubiquitous; a few rare local aberrations – ananassae, busckii, hydei, immigrans, and melanogaster.

2. Polymorphism frequent and extensive in some populations but reduced in others – funebris.

3. Monomorphism throughout – repleta, simulans.

Considering the major subgenera of the genus Drosophila (Table 1.2), subgenus Drosophila (hydei, immigrans, funebris, repleta) is represented in all three categories, whereas the sibling species D. melanogaster and D. simulans are in different categories. Hence, there is no association with taxonomic divergence in contrast with resource-utilization patterns of three sympatric species of the Melbourne area (Section 3.4) and those of a number of species from a market in Leeds, England (Atkinson and Shorrocks, 1977).