THE POPULATION STRUCTURE OF AN ASEXUAL VERTEBRATE, POECILIOPSIS 2 MONACHA-LUCIDA (PISCES: POECILIIDAE).

A prediction of contemporary population genetics is that the amount of genotypic variation in populations of an asexually reproducing species should be very much less than that of comparable sexual species. There are two reasons why this should be true. First, mutations that arise in distinct asexual lineages cannot be recombined and, as a consequence, only a small subset of the possible genotypic permutations are ever realized in an asexual population (Muller, 1932). Second, the conditions that allow genetic polymorphisms to persist in asexual populations are more restrictive than those in sexual populations. Specifically, selection cannot maintain clonal diversity in an asexual population in a spatially homogeneous environment even if the selection coefficients vary through time. In contrast, a sexual population can sustain a polymorphism provided the geometric mean of the heterozygote fitnesses is greater than that of either homozygote as the environment fluctuates from generation to generation. Where there is spatial heterogeneity, the situation is more complex: generally, a polyclonal structure could be maintained in an asexual population, but the existence and nature of the polymorphism depend upon migration between environmental "patches" (see Felsenstein, 1976, for a review and specific citations), and the necessary conditions are more restrictive than those required to sustain polymorphisms in a sexual population. Therefore, a polyclonal structure in an asexual population in a homogeneous environment must necessarily be a dynamic process representing some balance between new clones entering the population (mutation, immigration, etc.) and extinctions of old clones. Thus. population size takes on additional importance in genetical studies of asexual populations because this parameter affects both the rate at which neVw genotypes enter the population via mutation and the rate of random extinction. A related idea is that sexual species should evolve more rapidly than asexual species. This idea can be traced at least to Fisher (1930) and Muller (1932) and is a manifestation of Fisher's fundamental theorem of natural selection; viz., for equal amounts of selection, evolution will be more rapid in the population with the greater genetic variation (i.e., the sexual species). Thus, evolutionary potential is commonly a focal point in discussions of parthenogenetic organisms. The rationale of this study is to use Poeciliopsis 2 monacha-lucida, a parthenogenetic fish, as a test species to explore the empirical content of some of these theoretical results. Specifically, data are reported that pertain to the genetic and demographic structure of a complex of interconnected demes that occur in the headwaters of the Rio Fuerte in Sonora, Mexico. Resource limitations precluded intensive, concurrent analysis of all pertinent aspects of the problem; therefore, this report focuses on interdeme variation, population size and the existence of migration between demes. The method we employ to detect clonal variation is that of tissue transplantation. Although this is a rather old procedure, and quite unsophisticated by contemporary immunological standards, it is reliable, inexpensive and very sensitive to genetic differences in that subset of the vertebrate genome concerned with H-antigens (histocompatibility antigens). The