THE ROLE OF SEXUAL SELECTION AS AN ISOLATING MECHANISM IN THREE SPECIES OF POECILIID FISHES

The role of sexual isolating mechanisms in maintaining the int-egrity of ecoIogically coincident yet specifically distinct populations of sympatric species does not seem to have been very fully investigated. In part, this may be due to the actual rarity of such complete ecological coincidence among species mixtures under natural conditions. It is well known that, among a very considerable proportion of sympatric terrestrial species, ecological differentiation, if only in response to minorfeatures of the common environment, is sufficiently marked so that its importance as an isolating mechanism in speciation cannot be neglected. In part, the paucity of observations of this kind is undoubtedly due to the relatively small number of species mixtures of terrestrial organisms which are sufficiently well understood to make possible any certain conclusion as to the presence or absence of such ecological isolating mechanisms, or any estimate of their relative importance. Observations upon mixtures of sympatric species the members of which. were known to be essentially identical in food habits and environmental preferences would obviously be of great value in a study of the role of physiological and sexual isolating mechanisms in the absence of ecological factors. On the whole, populations of aquatic organisms seem to present more promising material for investigations of this sort than do terrestrial ones. Such truly mixed populations are perhaps most evident among freshwater organisms which inhabit streams and restricted lake environments. It seems likely, for instance, that certain members of the so-called "species swarms" of Cichlidae found in some African lakes, such as those studied by Bertram, Borley, and Trewavas (1942) and others, may properly fall within this category. In other cases, although the ecological coincidence is less perfect, there is still a large and important environmental area in which intimate species mixtures occur without any loss of distinctness on the part of either population. The geographical overlap of the viviparous Poeciliid fishes Xiphophorus hellerii and Platypoecilus maculatus in Mexican streams is well known and has been extensively studied by Gordon and his coworkers (1947). The present report is concerned with a single aspect of a study now in progress of the ecological relations within a mixture of three sympatric species of PoeciIiid fishes which show an extraordinary degree of habitat coincidence in certain of the fresh waters of Trinidad. These three species are Lebistes reticulatus Peters, Micropoecilia parae Eigenmann, and Poecilia vivipara Bloch and Schneider. All three are closely related taxonomically, being included by Hubbs (1926) in a single tribe, the Poeciliini of the subfamily Poeciliinae. Mixed schools of these three species of fish are found in many Trinidad waters, and they exhibit almost identical feeding habits and prefer closely similar stream environments. Such populations appear to offer unusually good opportunities for a study of the operation of sexual and physiological isolating mechanisms in speciation quite apart from the influence of ecological isolation.