Gene arrangement frequencies were determined at two stages in the life history of Drosophila pseudoobscura taken from nature. Three po ulations in the central highlands of Mexico were each sampled twice during 1976.Gene arrangement frequencies were measured in adult males and in larvae that were the offspring of females collected at the same time. The adult males were in all likelihood a representative sample of those who fathered the larvae produced by the wild females. Differences in gene arrangement frequency between these two life stages should indicate the operation of natural selection. One-third of our comparisons of common gene arrangement frequencies in males and in larvae from the next generation were statistically significant, as were one-third of our comparisons of total frequency arrays in the two life stages. We consider the components of selection that could produce such frequency changes and reason that male mating success must be the major one. Gene arrangement frequencies in the Mexican populations fluctuate within wide bounds. Selection must act to retain the polymorphism in the face of this flux in gene arrangement frequencies, and we suggest that male mating success plays an important role. One of the early triumphs of ecological genetics was the demonstration that selection in nature could be intense-in fact, one or several orders of magnitude more powerful than the founders of population genetics imagined. No experimental system played a more important role in the analysis of selection than the chromosomal polymorphism for gene arrangements in Drosophila pseudoobscura. In this species a series of inversions on the third chromosome binds large blocks of genes together as units, just as though they were alleles of a single "supergene." Natural selection was first implicated when Dobzhansky (2) showed that the frequencies of certain gene arrangements went through seasonal cycles in two of three populations on Mt. San Jacinto in California; subsequent studies showed that these cycles were repeated in years scattered over a span of 2 decades (3, 4). The frequencies of gene arrangements in the third population on Mt. San Jacinto did not cycle, but between 1939 and 1946 they underwent a directional change that Dobzhansky (5) also ascribed to natural selection. Dobzhansky and Levene (6) then showed that karyotypic frequencies in eggs laid by wild females were generally in accord with Hardy-Weinberg expectations, but that frequencies in wild males were not. They concluded that the karyotypes suffered differential mortality during the transition from fertilized egg to adult fly. That selection on the D. pseudoobscura inversions occurred in nature seemed to be settled, and it was generally taken for granted that viability differences accounted for the major part of it.Some 20 years later, a series of papers by Prout (7-10) stimulated evolutionary biologists to pay greater attention to the various components to fitness. These components determine the separate bits of selection that operate at...

The reproductive capacity of a species is one of its most important attributes, intimately related to its ability to persist in a sometimes harsh environment.Reproductive capacity is a particularly good index of fitness in organisms such as many insects that go through repeated cycles of rapid population growth. In such organisms any feature of the reproductive biology that increases reproductive rate will be favored by natural selection. Repeated mating and sperm storage are specific features which can play important roles in determining female fecundity and male mating success, and hence fitness.Both females and males of many animal species mate repeatedly, and in a sizable fraction of these species the females store sperm. Insects in particular may store sperm for periods of time approaching the lifetime of an adult female. Parker (1970) reviewed the extensive literature on repeated mating and sperm storage in insects. He concluded that these processes are adaptive and the products of a kind of selection he termed sperm competition. The existence of these phenomena in a variety of species, including dragonflies, beetles, bugs, and dipteran flies, makes them of general interest to population biologists. The same processes of repeated mating, sperm storage, and the resultant selection undoubtedly operate in other arthropods and in other phyla. Our own attention has been focused on these processes in Drosophila flies.That female Drosophila store sperm from a mating has been known a long time.Likewise, it has been common knowledge that females in the laboratory will accept additional mates, sometimes long before the sperm from the first mating are exhausted (Lefevre and Jonsson 1962). An up-to-date review of sperm transfer, sperm storage, and sperm utilization in Drosophila may be found in Fowler (1973).Multiple insemination was shown to be rather common in laboratory populations of D. pseudoobscura (Dobzhansky and Pavlovsky 1967) and D. melanogaster

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The natural selection acting on chromosomal inversions was studied in a natural population of Drosophila pseudoobscura. Females from this population were allowed to produce offspring from their matings in nature. They were then remated to males from a laboratory strain and again allowed to produce offspring. Offspring were also produced from matings of males from nature to laboratory females. Diagnosis of salivary chromosomes in these several sets of larval offspring allowed us to deduce the karyotypes of adult females and males from nature as well as the karyotypes of the offspring of these females by their matings in nature. We reason that the males collected with the females are a reasonable sample of those that mated the females and deposited the sperm they carried on capture. Chromosome frequencies in the offspring of wild females by their matings in nature were decomposed into male and female parental contributions. Changes in chromosome frequency due to male mating success were calculated by comparing chromosomal frequencies in adult males with those in the chromosomes they contributed to their offspring. These changes were sizable and provide direct evidence that male sexual selection is an important component of selection on the inversions in this natural population. We proceeded further to classify karyotypes on the basis of their frequencies and to calculate the fraction of offspring fathered by rare or common males. Rare male karyotypes as a group had a selective value nearly twice that of the common male karyotypes.

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