THE EVOLUTION AND ECOLOGY OF CLOSELY RELATED SPECIES LIVING IN THE SAME AREA

Abstract

It would seem that, provided it is accepted that populations of plant species are regulated or controlled by a reaction to density, two or more species may only persist together in a habitat if their populations are subject to different controlling factors. The presence of two or more systematically closely allied species living in the same habitat must imply therefore that in the course of their evolutionary divergence these species have become subject to different controlling factors. Differences in site requirement for seedling establishment, differences in susceptibility to parasites or predators, differences in germination time or requirement for breaking dormancy seem to offer the kinds of differences between closely related species which might permit cohabitation and which might relatively easily be acquired during the course of species differentiation. The frequency with which closely related species are found together in the same habitat is then a function of (a) the opportunities which have been available for the ancestral populations to become subdivided into independently evolving populations, (b) the opportunities for migration together after evolutionary divergence, (c) the chance that the meeting populations have acquired sufficient differences in reproductive behavior that they can co-exist without merging, (d) the likelihood that the populations after divergence are still tolerant of the major physical hazards of the area in which they meet, and (e) the chance that the differences which they have acquired during divergence are such that the species populations become independently controlled. A cycle involving isolation, differentiation and divergence followed by migration together (and perhaps further sympatric speciation by hybridization and allopoly-ploidy) offers the opportunity for closely allied species to meet. The outcome of the meeting[long dash]either cohabitation of the species or a more or less rapid ousting of one by another[long dash]must then depend on the nature of the differences acquired during speciation. Those differences which result in the reproductive isolation of the species and those which place the species under different controlling factors may be both strengthened and modified after the meeting of the species. Much of the interest which has been concentrated on the evolution of species has been concerned with the mechanisms by which reproductive isolation is achieved and fixed. This is, however, only a part of the process by which systematic diversity is developed. The continued persistence of the genetically isolated populations, except in geographical isolation from each other, must depend on a divergence in biological properties of the species such that they fill complementary and not mutually exclusive roles in vegetation. A major aim of this essay is to focus attention on the sorts of differences in the biology of higher plants which make them ecologically complementary. || ABSTRACT AUTHORS: Author