No abstract

We describe allelic variation at 28 gene loci in natural populations of D. willistoni. Seventy samples were studied from localities extending from Mexico and Florida, through Central America, the West Indies, and tropical South America, down to South Brazil. At least several hundred, and often several thousand, genomes were sampled for each locus. We have discovered a great deal of genetic variation. On the average, 58% loci are polymorphic in a given population. (A locus is considered polymorphic when the frequency of the most common allele is no greater than 0.95). An individual fly is heterozygous, on the average, at 18.4% loci.—Concerning the pattern of the variation, the most remarkable finding is the similarity of the configuration of allelic frequencies from locality to locality throughout the distribution of the species. Our observations support the conclusion that balancing natural selection is the major factor responsible for the considerable genetic variation observed in D. willistoni.

Abstract. We have studied by gel electrophoresis the variability of 14 structural genes in four sibling species, Drosophila willistoni, D. paulistorum, D. equinoxialis, and D. tropicalis. Samples of about 30 populations from different parts of the distribution areas of each species were examined. Genetic variants are found at every locus; 67% of the loci are polymorphic, having two or more alleles, the rarer of which has a frequency of 5% or higher.The gene frequencies are fairly uniform over the distribution area of each species, but considerably different in different species. It is estimated that individuals which belong to the different species differ on the average in somewhat more than one half of their gene loci. The morphological similarity of the four sibling species contrasts with the extensive diversity in their genetic materials.One of the basic problems of evolutionary genetics, in a sense the cardinal problem, is to ascertain what proportions of gene loci are altered in the evolutionary processes, and particularly in the process of speciation. In outbreeding sexual organisms, a species is an array of Mendelian populations, among which gene exchange can occur without impediments other than geographical separation. Different species are arrays of populations reproductively isolated from each other. A mutational or other genetic change originating in a single or in a small group of individuals of a species can spread, impelled by natural selection, to the whole species. Because of reproductive isolation such a change cannot spread from one species to others, unless it arises in these species independently. Speciation is, then, a highly significant stage of evolutionary differentiation. Fully formed species are discrete and independent units of evolution.The processes of speciation have been extensively studied and discussed.1 2 A crucially important question is how much reorganization of the gene pool occurs during the process of species divergence. It has sometimes been claimed that species differ at only a small number of loci; other evolutionists held, on the contrary, that a considerable proportion of the gene pool is altered. (It should be remarked that this question is really separate from that of the number of genes directly involved in the formation of reproductive isolating mechanisms.) Methods of investigation developed in recent years permit a fresh approach to the problems in this field.Related species have usually been studied to detect differences in their morphological and physiological traits, in ecologically significant parameters, in the 225

Experiments are reported in which genetically different strains of Drosophila willistoni compete with D. pseudoobscura. The competition was studied at three temperatures, 20°, 22°, and 25°C. The outcome of the competition depends on the genetic constitution of the competing species, but at 25° and 22°C D. willistoni flies are generally stronger competitors than D. pseudoobscura, while at 20°C D. pseudoobscura generally has a competitive advantage. There is a significant interaction between genotype and temperature; the strain RP3 is the weakest competitor of all D. willistoni strains at 22° and 25°C, but not at 20°C; the strain MIS is the best competitor at 20° and 22°C but not at 25°C.The performance of the four strains of D. willistoni was measured in two more ways. First we estimated their Darwinian fitness relative to other genotypes of the same species. Second, we measured the average population size of each strain in pure culture. There is no significant correlation between population size in pure culture and either competitive fitness or Darwinian fitness. There is, however, a strong positive correlation between Darwinian fitness and interspecific competitive fitness.It is pointed out that natural selection leads to an increase in the average Darwinian fitness of a population but not necessarily to an increase in its adaptedness to the environment. Yet the synthetic theory of evolution assumes that the genes and genotypes favored by natural selection are usually those which increase the adaptedness of their carriers to the environments where they live. The correlation between Darwinian fitness and adaptedness needs to be studied experimentally.

Fifteen second chromosomes were extracted from Drosophila willistoni flies collected in four natural populations. The adaptedness of populations homozygous for each chromosome was measured by average population size and productivity. Six 'control' populations were established with mixtures of the wild second chromosomes. The Darwinian fitness of flies homozygous for each wild second chromosome, and of flies carrying random combinations of these chromosomes, was measured relative to the fitness of flies heterozygous for a wild and a marker chromosome. The Darwinian fitness of homozygotes for each second chromosome relative to the fitness of flies carrying random combinations of the natural chromosomes was then inferred. The estimated loss of fitness on making the natural second chromosomes homozygous was substantial, ranging from 39 to 83 percent, with an average reduction in fitness of 66 percent. These results with D. willistoni are consistent with those from similar experiments with other Drosophila species, and they are compatible with a significant role for heterosis in the maintenance of genetic variability.Populations homozygous for wild chromosomes differ in their adaptedness to the experimental environment. Population size and productivity are correlated, although the correlation is far from complete. Some populations have high productivity and low population size, or vice versa. The control populations, with greater genetic variability, were superior in adaptedness to the average of the single-chromosome populations. The Darwinian fitness and the adaptedness of the genotypes in this experiment were not significantly correlated. It follows that certain measures used by population geneticists, such as genetic load and average Darwinian fitness, cannot be taken as general indices of how well adapted a population is to its environment.