restricted access I.18 Ecological Speciation: Natural Selection and the Formation of New Species
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I.18 Ecological Speciation: Natural Selection and the Formation of New Species Patrik Nosil and Howard Rundle OUTLINE 1. Ecological speciation: What it is and how to test for it 2. Forms of divergent selection 3. Forms of reproductive isolation 4. Genetic mechanisms linking selection and reproductive isolation 5. Geography of ecological speciation 6. Generality of ecological speciation 7. Remaining questions in the study of ecological speciation Understanding how new species arise is a central goal of evolutionary biology. Recent years have seen renewed interest in the classic idea that adaptive evolution within species and the origin of new species are intimately linked. More specifically, barriers to genetic exchange between populations (termed reproductive isolation) are the hallmark of species, and evolutionary biologists have been asking whether ecologically based divergent natural selection , the process that is responsible for adaptive divergence between populations, may cause such reproductive barriers to evolve. Convincing examples of this process, termed ecological speciation, are accumulating in the literature , and comparative approaches suggest that it may be a widespread phenomenon taxonomically. Attention is now being given to understanding details of the process and uncovering generalities in its operation. Three main components of ecological speciation can be recognized: a source of ecologically based divergent selection, a form of reproductive isolation, and a genetic mechanism linking the two. Current research is focused on understanding these components during the various stages of ecological speciation from initiation to completion. GLOSSARY ecologically based divergent selection. Selection arising from environmental differences and/or ecological interactions (e.g., competition) that acts in contrasting directions on two populations (e.g., large body size confers high survival in one environment and low survival in the other) or favors opposite extremes of a trait within a single population (i.e., disruptive selection) linkage disequilibrium. A statistical association between alleles at one locus and alleles at a different locus, the consequence of which is that selection on one locus (e.g., a locus affecting an ecological trait such as color pattern) causes a correlated evolutionary response at the other locus (e.g., a locus affecting mating preference) pleiotropy. Multiple phenotypic effects of a gene (e.g., a gene affecting color pattern also affects mating preferences) postmating isolation. Barriers to gene flow that act after mating (e.g., intermediate trait values of hybrids that make them poor competitors for resources, reducing their fitness) premating isolation. Barriers to gene flow that act before mating (e.g., divergent mate preferences that prevent copulation between individuals from different populations) reproductive isolation. A reduction or lack of genetic exchange (gene flow) between taxa sympatric speciation. A geographic mode of speciation whereby a single population splits into two species in the absence of any geographic separation, often via disruptive selection 1. ECOLOGICAL SPECIATION: WHAT IT IS AND HOW TO TEST FOR IT The idea that the macroevolutionary phenomenon of speciation is the result of the microevolutionary process of adaptation dates back at least to Charles Darwin . However, it was not until the popularization of the biological species concept in the middle of the last century, whereby speciation was defined as the process by which barriers to genetic exchange evolve between populations, that the study of Darwin’s ‘‘mystery of mysteries,’’ the origin of species, became empirically tractable. The past two decades have witnessed an explosion of speciation research, with much attention being given to understanding the role of divergent selection in speciation. As defined by Dolph Schluter and others, ecological speciation is the process in which barriers to genetic exchange evolve between populations as a result of ecologically based divergent natural selection. Selection is ecological when it arises from differences in the environment or from interactions between populations over resource acquisition. Ecologically based selection can thus arise, for example, from an individual’s quest to obtain food and other nutrients, attract pollinators, or avoid predators. It can also arise from an individual ’s interaction with other organisms in its attempt to achieve these goals (e.g., resource competition, predation ). Selection is divergent when it acts in contrasting directions in the two populations. Included here is the special case of disruptive selection on a single population , in which selection favors opposite extremes of the same trait. During ecological speciation, populations experience divergent selection between environments or niches and thus differentiate in ecologically important traits. If these traits, or ones that are genetically correlated with them, affect reproductive isolation, then speciation occurs as a consequence. Ecological speciation is distinguished from other models of speciation...