Chromosomal inversion polymorphism in the southewestern range ofDrosophila subobscura distribution area

The level of chromosomal inversion polymorphism is much higher in three populations from South Morocco than in the nearby Canary and Madeira Islands. The composition of the chromosomal arrangements is also strikingly different in both areas. The similarity among the populations from the islands excludes drift or the founder principle as an explanation of these differences. A variety of reasons support the conclusion that the polymorphism in the islands is a relict polymorphism, corresponding to a less evolved stage than the one present in the mainland. The populations from South Morocco have a relatively low index of free recombination, like others from the southern range of the distribution ofD. subobscura. This contrasts with the high values found in the northern range. The populations from the North are relatively recently established ones, whereas those from the South are longer established and have become marginal only in relatively recent times. This difference in evolutionary history is sufficient to explain the differences in the index of free recombination.