Genetics of Natural Populations. XII. Experimental Reproduction of Some of the Changes Caused by Natural Selection in Certain Populations of Drosophila Pseudoobscura
Genetics, 31 (2):
125-156 (March 1946)Abstract
RAPID changes in the genetic composition occlir in populations of Dro-
sophila pseudoobscura which inhabit certain localities on Mount San Ja-
cinto, California (DOBZHANSKY
1943). These changes are cyclic and connected
with the succession of the year's seasons. The genetic variable involved is
the gene arrangement in the third chromosome. Three gene arrangements are
common among the third chromosomes of the San Jacinto populations. One
of them, called Standard (abbreviated ST), is most frequent in the populations
in winter and in early spring, reaches its lowest frequency in early summer,
and increases in frequency during middle and late summer. The second,
Chiricahua (abbreviated CH), shows a cycle opposite to that of ST. The third,
Arrowhead (AR), tends to follow a path resembling that of CH but with less
regularity. Inversion homozygotes and heterozygotes occur in the populations
with frequencies which are close to those which are expected if the carriers of
the different gene arrangements mate a t random.
Analysis of the data has led to the working hypothesis according to which
the changes in the relative frequencies of the gene arrangements are induced
by natural selection in response to the seasonal alterations in the environment.
The gene arrangements may be in themselves adaptively neutral (that is,
free from position effects), but they contain different gene complexes which
make their carriers adapted to different seasonal environments (DOBZHANSKY
1943). Since the changes in the composition of the populations are consider-
able and rapid, the chromosomal types concerned must be subject to intense
selection pressures. The selective advantages and disadvantages that must
be postulated are, indeed, high enough to justify an attempt to detect them
in laboratory experiments. The present article reports the results of experi-
ments designed to test the validity of the above hypothesis.