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The European species Drosophilasubobscura has colonized, in a very short time, large areas in South America (Chile and Argentina) and North America (The Pacific Coast, from Vancouver to Southern California), where it has been established as a very common species. Its capacity of dispersal was analyzed and results compared with the few data available from other authors. The estimates of the mean distance travelled during one activity period (159.20 meters) are considerably higher than those obtained by other authors. The Brownian motion model seems most probable because the values of oDF and oBM are very much alike. The effects on the estimates of some environmental factors, such as wind, are also discussed.

An analysis of the effects of inbreeding on the genetic structure of a colonizing population of Drosophila subobscura has been carried out. Species of Drosophila, particularly D. subobscura, may have lethal alleles associated with chromosomal inversions and our aim was to assess the extent to which the genome is balanced in this way. The frequencies of chromosomal inversions were compared between a large population and a set of 72 lines that were maintained by brother-sister mating for 10 generations. Fisher's matrix method was used to calculate the expected homozygosity in these inbred lines for 5 allozyme loci (Aph, Hk-1, Lap, Odh, and Pept-1) used as markers of large chromosomal segments. Furthermore, the expected rates of fixation corresponding to these allozyme loci were also calculated. The results show that the amount of homozygosis observed did not differ significantly from expectations (with the corresponding loss of lines as a consequence of the reduction in viability). However, two deviations from strict neutrality were observed: there was a heterozygote excess at the Lap locus, and the frequency of the O5 inversion (always associated with a lethal gene in colonizing populations) was higher than expected.

In this work, the process of colonization of North and South America by the species Drosophila subobscura has been studied by analyzing the variability of lethal genes. The genetic structures of a Palearctic natural central population (Bordils, Spain) and a colonizer population from America (Gilroy, California) have been compared. The frequencies of lethal chromosomes and their allelism are 29.007% and 0.0069 in the first population and 14.414% and 0.0526 in the American population. A founder effect is detected after the computation of some population parameters (N , h, he and the lethal load). Furthermore, the allelism of lethal chromosomes has revealed a strong association between a lethal gene and the O inversion both in Gilroy and in the population of Puerto Montt (Chile). The interpopulation allelism shows that the O arrangement from the USA and Chile is the same, confirming that the colonizing processes of North and South America are correlated. The O arrangement can also be useful as a genetic marker to trace the origin of the colonization. The frequency of the O arrangement in the original population of the colonization could be used to estimate the number of colonizers. This population is still unknown, but taking the extreme values of the frequency of the O inversion in natural Palearctic populations (1-15%), the number of colonizers could vary between 9 and 149 individuals.