Abstract
Evidence is accumulating that natural populations, not only of lower but also of higher organisms, undergo genetic changes which are sometimes surprisingly rapid. In insects which produce two or more generations per year the populations may be genetically different at different seasons; changes of a more enduring character are also known (reviews by Dobzhansky, 1951, and Andrewartha and Birch, 1954). But while the occurrence of such directly observable evolutionary changes is no longer unexpected, the causes which bring them about remain baffling even in the relatively better-studied cases (the moth Panaxia, Sheppard, 1953, the beetle Harmonia caryridis, Komnai, 1954, red fox, Butler, 1951, species of Drosophila, Dobzhansky, 1952, to cite only few instances). In this field we are still in the data-gathering stage. Yet, it is needless to labor the point that the study of even slight microevolutionary changes taking place under natural conditions in wild species is fraught with interesting possibilities for an evolutionist. Cyclic seasonal as well as year-to-year changes have been observed in Drosophila pseudoobscura on Mount San Jacinto in Southern California, and in this species as well as in D. persimilis in the Yosemite region of the Sierra Nevada of California (Dobzhansky, 1947a, 1948, 1952). Further observations were carried out in the latter region during the summer of 1954. It must be admitted that these observa-
Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
