Chromosome 3 rearrangements were studied in five Drosophila pseudoobscura populations from the high plateau of the Colombian Andes. As in previous studies, the Santa Cruz and Tree Line rearrangements were predominant in these populations, but for the first time other rearrangements such as the Olympic, Cuernavaca and a rearrangement similar to the endemic Mexican Amecameca rearrangement were also discovered. Researchers in the early 1960’s showed that Colombian D. pseudoobscura populations were not in accordance with Carson’s theory. They found a special heterotic system in this geographically isolated population. Our current results do not support these findings but instead favor Carson’s theory, because in practically all the populations studied the homokaryotype excess was close to fixation. These new results indicate that some stabilizing selective pressures in favor of the homokaryotypes (especially Santa Cruz) have appeared in the Colombian plateau during the last 10-12 years. These new changes may be related to deforestation and habitat destruction by human beings and/or climatic changes motivated by the El Niño phenomenon. Genetic heterogeneity between populations was not significant and there was no isolation-by-distance between them, findings which are contrary to those observed in some North American populations and which show that the Colombian populations now have rigid genetic systems. The frequency of the Santa Cruz rearrangement was also found to have increased in some populations over the last few years.

Key words: Drosophila pseudoobscura, Colombia, Chromosomal polymorphism, Selection in favor of homokaryotypes, Changes in rearrangement frequencies.