Understanding speciation is a fundamental biological problem. It is believed that many species originated through allopatric divergence in geographically isolated populations of the same ancestral species (1-3). In contrast, the possibility of sympatric speciation has often been dismissed, partly because of theoretical difficulties (2,3). Most previous models analyzing sympatric speciation concentrated on particular aspects of the problem while neglecting others (4-10). We present a model which integrates a novel combination of different features and shows that sympatric speciation is a likely outcome of competition for resources. We use explicit multilocus genetics to describe sexual reproduction in an individual-based model, and we consider the evolution of assortative mating depending either on the ecological character affecting intraspecific resource competition or on a selectively neutral marker trait. In both cases, evolution of assortative mating often leads to reproductive isolation between ecologically diverging subpopulations. When assortative mating depends on a marker trait, and is therefore not directly linked to resource competition, speciation occurs when genetic drift breaks the lineage equilibrium between marker and ecological trait. Our theory conforms well with mounting empirical evidence for the sympatric origin of many species (10-18).
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