Isozyme Variation in Colonizing Plants

Abstract

The colonization of new environments is an integral feature of the biology of most plant species. However, a relatively small number of plants, commonly referred to as colonizers, weeds, or invading species, possess attributes that enable them to establish populations continuously in areas or habitats that they have not previously occupied. In comparison with other plant life forms, and considering their number, colonizing species have received disproportionate attention from biologists. This interest is probably because some are of economic importance in agriculture and, because, from an evolutionary perspective, others provide excellent experimental systems for microevolutionary studies. The rapid life cycles of many colonizers and their ability to invade diverse environments, often in a short period, provide population biologists with a series of evolutionary experiments that are not available in most other plant groups. Prior to the advent of enzyme electrophoretic techniques for measuring the levels of genetic variation in plant populations, mucli of the focus on colonizing species involved the identification of ecological attributes responsible for colonizing success. Generalizations arising from the symposium on "The Genetics of Col(!)nizing Species" (Baker and Stebbins. 1965) stimulated comparative work on a variety of plant species to determine the adaptive significance of variation in traits such as dormancy, rate of development, phenotypic plasticity, fecundity, reproductive effort, and seed dispersal (reviewed in Baker, 1974; Harper, 1977; Grime, 1979; Jain, 1979). More recently, studies of colonizing species have compared levels of genetic diversity in related species and examined the consequences of colonizing episodes on genetic variation and opportunities for evolutionary response in novel environments. Although it is generally recognized that colonizing species employ a diverse array of ecogenetic strategies for invading unoccupied territory, several recurrent patterns have emerged from electrophoretic studies conducted during the past decade. Polyploidy, uniparental reproductive systems, depauperate levels of genetic variation, marked interpopulation differentiation, and a high degree of multi-locus association have commonly been reportted in colonizing species (reviewed in Brown and Marshall, 1981; Rice and Jain, 1985; Barrett and Richardson, 1986). In this chapter we review electrophomtic evidence from isozyme surveys to evaluate whether generalizations can be made as to tlie processes influencing population genetic structure and evolutionary response in colonizing plants. We also assess the problems associated with determining the relative importance of liistorical factors, reproductive traits, and environmental heterogeneity in affecting population genetic structure. Because many successful colonizers are of polyploid origin, part of oUF review deals with isozyme variation in polyploid colonizers and considers how polyploidy might contribute to the evolution of colonizing ability. The colonization of new environments is an integral feature of the biology of most plant species. However, a relatively small number of plants, commonly referred to as colonizers, weeds, or invading species, possess attributes that enable them to establish populations continuously in areas or habitats that they have not previously occupied. In comparison with other plant life forms, and considering their number, colonizing species have received disproportionate attention from biologists. This interest is probably because some are of economic importance in agriculture and, because, from an evolutionary perspective, others provide excellent experimental systems for microevolutionary studies. The rapid life cycles of many colonizers and their ability to invade diverse environments, often in a short period, provide population biologists with a series of evolutionary experiments that are not available in most other plant groups. Prior to the advent of enzyme electrophoretic techniques for measuring the levels of genetic variation in plant populations, much of the focus on colonizing species involved the identification of ecological attributes responsible for colonizing success. Generalizations arising from the symposium on "The Genetics of Colonizing Species" (Baker and Stebbins, 1965) stimulated comparative work on a variety of plant species to determine the adaptive significance of variation in traits such as dormancy, rate of development, phenotypic plasticity, fecundity, reproductive effort, and seed dispersal (reviewed in Baker, 1974; Harper, 1977; Grime, 1979; Jain, 1979). More recently, studies of colonizing species have compared levels of genetic diversity in related species and examined the consequences of colonizing episodes on genetic variation and opportunities for evolutionary response in novel environments. Although it is generally recognized that colonizing species employ a diverse array of ecogenetic strategies for invading unoccupied territory, several recurrent patterns have emerged from electrophoretic studies conducted during the past decade. Polyploidy, uniparental reproductive systems, depauperate levels of genetic variation, marked interpopulation differentiation, and a high degree of multi-locus association have commonly been reported in colonizing species (reviewed in Brown and Marshall, 1981; Rice and Jain, 1985; Barrett and Richardson, 1986). In this chapter we review electrophoretic evidence from isozyme surveys to evaluate whether generalizations can be made as to the processes influencing population genetic structure and evolutionary response in colonizing plants. We also assess the problems associated with determining the relative importance of historical factors, reproductive traits, and environmental heterogeneity in affecting population genetic structure. Because many successful colonizers are of polyploid origin, part of our review deals with isozyme variation in polyploid colonizers and considers how polyploidy might contribute to the evolution of colonizing ability.