Distances between populations ofDrosophila subobscura, based on chromosome arrangement frequencies.

Distances between populations ofDrosophila subobscura, based on differences in the frequencies of chromosomal arrangements have been estimated using data from about 65 populations. The distances have been calculated using the formula:[Formula: see text] wherer is the number of loci or chromosomes (in the case of chromosomal polymorphism) considered,p 1jk the frequency of the allele or chromosomal arrangementk in the locus or chromosomej in the first population, andp zjk the corrsponding value in the second population.The main conclusion drawn from this analysis is that historical as well as adaptive factors are important in explaining the geographical distribution of chromosomal arrangements inD. subobscura. In general, isolated populations maintain primitive features in their chromosomal polymorphism. This is reflected in a tendency to similarity between these populations. Also, a very sharp effect of geographical barriers is detected in the distribution of the chromosomal arrangements.Two main factors are considered responsible for the strong effect that isolation has on geographical distribution of chromosome arrangements. These factors are the non-recurrence of rearrangements and the difficulty of establishing in one population the supergenes originated in another area, because of lack of coadaptation with the gene pool of the recipient population.