Olga Pavlovsky

INDETERMINATE OUTCOME OF CERTAIN EXPERIMENTS ON DROSOPHILA POPULATIONS

Repeatability of observations and experiments is taken for granted in science. If experiments on similar materials conducted under similar conditions fail to yield similar results, one suspects that some variables in the materials or in techniques have escaped detection. Yet, some natural processes happen only once; history, either on the biological or on the human level, does not repeat itself. There is, of course, nothing mysterious in this uniqueness. Evolutionary processes in nature are influenced by usually very numerous internal and environmental variables. It is unlikely that the whole constellation of variables can remain constant for long, or that it can recur in the future. The elementary components of the evolutionary process, the mutational and selectional steps, are both repeatable and reversible; evolution is however unrepeatable and irreversible. Geneticists and other experimentalists concentrate their attention chiefly on the elementary components, and are consequently reproached by those who study evolution by other means for not being able to observe evolutionary changes of any consequence. This reproach fails to take into account that historical events and trends become comprehensible only through understanding of the unspectacular everyday events which bring them about. For several years, we have studied experimental populations of Drosophila which contain mixtures of chromosomal variants (reviews in Dobzhansky, 1947

Geography of the sibling species related to Drosophila willistoni, and of the semispecies of the Drosophila paulistorum complex.

The six sibling species of Drosophila willistoni group are among the most favorable, and at the same time challenging materials for studies on the genetics of speciation processes. The species are D. willistoni Sturtevant, D. paulistorum Dobzhansky & Pavan, D. pavlovskiana Kastritsis & Dobzhansky, D. equinoxialis Dobzhansky, D. tropicalis Burla & da Cunha, and D. insularis Dobzhansky. Burla et al. (1949) found slight morphological differences that were insufficient for identification of single individuals, but Spassky (1957) noted differences in the male genitalia which do permit such identification. Unambiguous discrimination is also possible through examination of the gene arrangements in the chromosomes of larval salivary glands (Burla et al., 1949; Dobzhansky et al., 1950), and of the variant enzymes detected by electrophoresis (Ayala et al., 1970). Owing to ethological isolation, crossmating of the species occurs rarely, although under laboratory conditions some cross-inseminations can be obtained (Burla et al., 1949). Intercrosses of D. insularis females with D. tropicalis and D. willistoni males, and occasionally with D. paulistorum

INTERACTION OF THE ADAPTIVE VALUES IN POLYMORPHIC EXPERIMENTAL POPULATIONS OF DROSOPHILA PSEUDOOBSCURA

Genetic diversification is an evolutionary mechanism which enables MIendelian populations to master a wide variety of environments in the territories which they inhabit. In many species of Drosophila, some of the genetic diversification takes the form of chromosomal polymorphism. Thus, in Drosophila pseudoobscura several (up to seven) different gene arrangements occur in the third chromosomes in many natural populations. The chromosomal polymorphism can be studied not only in natural but also in artificial laboratory populations (Wright and Dobzhansky, 1946; Dobzhansky, 1949). Such studies have shown that the fitness of the structural heterozygotes which carry two third chromosomes with different gene arrangements derived from the same population is, not invariably but usually, superior to that of the corresponding structural homozygotes. The polymorphism is, therefore, balanced, and the gene arrangements are maintained in the populations with frequencies which are determined, for a given environment, by the relative adaptive values of the different karyotypes (Dobzhansky, 1948, 1949; Dobzhansky and Levene, 1948). The great interest of the chromosomal polymorphism in Drosophila is that it makes the processes of natural selection and evolutionary adaptation amenable to experimental study. The work in this

SEXUAL SELECTION, GEOTAXIS, AND CHROMOSOMAL POLYMORPHISM IN EXPERIMENTAL POPULATIONS OF DROSOPHILA PSEUDOOBSCURA

Hirsch and his students (Hirsch and Erlenmeyer-Kimling, 1961; ErlenmeyerKimling, et al., 1962; and other publications) have shown that Drosophila melanogaster responds readily to selection for positive and for negative geotactic behavior. So does D. pseudoobscura (Dobzhansky and Spassky, 1962). In both species, the geotactic behavior is under polygenic control. Also in both species, the genetic basis of the geotaxis displays the phenomenon called by Lerner (19 S4) genetic homeostasis; when the selection is relaxed, the selected strains relapse partly towards the original, preselection states. The loss of the selection gains is even more rapid if the direction of the selection is reversed. Using the apparatus devised by Hirsch (a classification maze), the geotactic behavior becomes a trait lending itself readily and conveniently to selection and other genetic experiments. After the exploratory experiments referred to above, Dobzhanskyand Spassky have initiated (in 1962) a study of the effects of selection for positive, or for negative, geotaxis in pairs of populations of very unequal size (one population 10 times larger than the other) , which in every generation exchange a fixed number of migrants. It is hoped that the diversifying (disruptive) selection, combined with migration between the experimental populations of Drosophila, may to some extent serve as a model of certain genetic processes which take place also in human populations having certain types of social structures. Be that as it may, these experiments have yielded some completely unanticipated adventitious results

A further study of fitness of chromosomally polymorphic and monomorphic populations of Drosophila pseudoobscura

THE present report is a sequel to that of Beardmore, Dobzhansky and Pavlovsky (1960), who made an exploratory study of the fitness of chromosomally polymorphic and monomorphic experimental populations of Drosophila pseudoobscura. The populations of this previous study were kept for about 3 years in wood-and-glass cages, each with 15 cups containing Drosophila culture medium. A cup of fresh medium was introduced, and one with used-up medium removed, on alternate days. The populations each contained some 1000-4000 adult flies, and at least ten times as many eggs and 1arv. The polymorphic populations had flies with AR and with CH gene arrangements in their third chromosomes; the homokaryotypes AR/AR and CH/CH, and the heterokaryotype AR/CH, were represented with frequencies approaching those demanded by the Hardy-Weinberg rule. The monomorphic populations had only AR, or only CH, chromosomes. The study has shown that the polymorphic populations produced more flies per cup of culture medium than did the monomorphic ones. Individual flies in the polymorphic populations were neither consistently larger nor heavier than in the monomorphic ones; however, since the former produced more flies, they produced also a greater fly biomass. Furthermore, the variances of the numbers of the flies produced per cup, and the variances of individual weights and of sizes were lower in the polymorphic than in the monomorphic populations. The conclusion seems warranted that, under the conditions of the experiments, the polymorphic populations exploit more efficiently the resources of their environment than do the monomorphic populations. The environment offered to Drosophila in the experimental population cages is a highly competitive one. The food given is sufficient to sustain the development to the adult stage of fewer than io per cent, though probably of more than i per cent, of the eggs deposited. However, the adult flies probably do not starve; a part of the surface of the yeasted food medium is almost always free of feeding or ovipositing flies. In the experiments to be described below the situation is different. In these experiments we have used a variant

Repeated Mating and Sperm Mixing in Drosophila pseudoobscura

The mating habits of Drosophila pseudoobscura have been studied by exposing females of known chromosomal constitution to males whose chromosomes were also known. Although the females repel all males for some hours after a copulation, they often mate again within 5 to 15 days. A batch of eggs deposited by a female may be fertilized by the sperms of more than a single male.

DEPENDENCE OF THE ADAPTIVE VALUES OF CERTAIN GENOTYPES IN DROSOPHILA PSEUDOOBSCURA ON THE COMPOSITION OF THE GENE POOL

In a previous publication (Levene, Pavlovsky, Dobzhansky, 1954) we have reported experiments on laboratory populations of Drosophila pseudoobscura in which three different gene arrangements were present among the third chromosomes. The adaptive values of six karyotypes (three inversion homozygotes and three heterozygotes) were estimated from the observed changes in the relative frequencies of the gene arrangements in the experimental populations. The results indicated that the adaptive values of at least some of the karyotypes were not constant; they depended upon the presence or absence of certain other karyotypes in the same population. Thus, individuals homozygous for the CH gene arrangement in third chromosome (CH/ CH) had a fitness apparently superior to AR homozygotes (AR/AR) in populations in which ST third chromosomes were also present, but CH/CH were inferior to AR/AR in fitness when ST chromosomes were absent. ST homozygotes (ST/ST) were superior to ST/CH heterozygotes in populations which also had AR chromosomes, but ST/CH were superior to ST/ST in the absence of AR. Lewontin (1955) reported some elegant experiments in which constant numbers of larvae of certain genotypes of Drosophila melanogaster were introduced

TRANSITIONAL POPULATIONS OF DROSOPHILA PAULISTORUM

Dobzhansky and Spassky (1959) and Dobzhansky, Ehrman, Pavlovsky, and Spassky (1964) found that Drosopha paulistorum is a superspecies composed of several semispecies. The semispecies show so strong an ethological isolation from each other that in some places they coexist sympatrically without interbreeding. Moreover, among the hybrids between the semispecies produced in the laboratory, the females are fertile while males are completely sterile. Originally, six semispecies were recognized-the Centroamerican, Orinocan, Amazonian, Andean-Brazilian, Transitional, and Guianan. More recently, the Guianan was raised to a full-fledged species, Drosophila pavlovskiana (Kastritsis and Dobzhansky, 1966). The Transitional was so named because certain strains from Colombia were found to cross and to produce fertile hybrids with some Centroamerican as well as some Andean-Brazilian strains. Cytological studies of Kastritsis (1966, 1967), as well as our own genetic data, led us however to conclude that some of the strains originally classified as Transitional were in reality extreme northern representatives of the Andean-Brazilian semispecies. Meanwhile, the accession of freshly collected strains from western and northern Colombia and northwestern Venezuela forces us to redefine the Transitional race (or semispecies), as will be described in the report that follows. The Transitional so redefined includes only a part of the strains formerly included under that name. A wholly unexpected development has been a change in the behavior of one of the strains; this strain was originally classified a member of the Orinocan semispecies, but several years later it began to produce sterile male hybrids with Orinocan strains (Dobzhansky and Pavlovsky, 1967). Most recently, an additional semispecies, called Interior, has been discovered (unpublished). As seen in the light of the information now available, the superspecies Drosophila paulistorum is a compound of five semispecies (Centroamerican, Orinocan, Amazonian, Andean-Brazilian, Interior), and a sixth population complex, too heterogeneous to be regarded a single semispecies, the Transitional.

GENETICS OF NATURAL POPULATIONS. XXXVIII. CONTINUITY AND CHANGE IN POPULATIONS OF DROSOPHILA PSEUDOOBSCURA IN WESTERN UNITED STATES

Several reports have been published describing the genetic changes which took place in populations of Drosophila pseudoobscura, mainly in California and the adjacent states, during approximately a quarter of a century, since 1940 (Dobzhansky, 1947, 1952, 1956, 1958, 1963; Dobzhansky et al., 1964; Epling and Lower, 1957). The changes involve the relative frequencies in the populations of certain polymorphs, namely inversions of blocks of genes in the third chromosomes. A population of D. pseudoobscura can be described in terms of the incidence in it of these chromosomal variants, somewhat as a human population may be characterized by the relative frequencies of individuals with various blood groups. In Drosophila, the frequencies of the chromosomes at some localities change, however, from month to month. Such short-term changes are usually cyclic, and follow the succession of the years seasons. More long-term changes occur from year to year; whether such changes may also be cyclic is not known. Like the seasonal changes, the yearly ones usually involve only the relative frequencies of the same set of polymorphs; occasionally, however, a karyotype which was formerly absent or so rare that it was not recorded, appears and increases in frequency. As a baseline in the present study we take the records of the chromosomal constitution of the series of samples of D. pseudoobscura taken at about 100 localities in the western United States in the late nineteen thirties and the early nine-

PARTIALLY SUCCESSFUL ATTEMPT TO ENHANCE REPRODUCTIVE ISOLATION BETWEEN SEMISPECIES OF DROSOPHILA PAULISTORUM

A female of Drosophila paulistorum which was captured on March 19, 1958, south of Villavicencio in the Llanos of Colombia, gave rise to a laboratory strain with a most interesting behavior. Tested within a few months from the time of its foundation, the strain gave fertile hybrids of both sexes with the then available strains of the Orinocan semispecies of Drosophila paulistorum. The Llanos strain was accordingly classified as a member of the Orinocan semispecies. However in 1963 and thereafter, the Llanos strain consistently gave fertile female but sterile male hybrids with all other Orinocan strains. It can no longer be classed as Orinocan. And yet, having acquired a hybrid sterility in crosses with Orinocan, the Llanos strain did not become ethologically isolated from the Orinocan semispecies. In October 1966, experiments were started the purpose of which was to induce ethological isolation between Llanos and Orinocan strains. Artificial selection was applied for 131 generations. The selection favored the offspring of homogamic matings (i.e., Llanos X Llanos and nonLlanos X non-Llanos), and discriminated