The paper discusses the origin of species through sympatric speciation, a process where new species arise from the same ancestral population without geographic isolation. The authors present a model showing that sympatric speciation is likely due to competition for resources. They use an individual-based model with explicit multilocus genetics to describe sexual reproduction and consider the evolution of assortative mating based on ecological or neutral marker traits. In both cases, assortative mating often leads to reproductive isolation between ecologically diverging subpopulations. When assortative mating depends on a neutral marker trait, speciation occurs when genetic drift breaks the linkage equilibrium between the marker and ecological traits. The theory aligns with empirical evidence for the sympatric origin of many species. The study also explores evolutionary branching, a phenomenon where directional selection leads to a split into two coexisting phenotypic clusters. This occurs when the curvature of the carrying capacity at its maximum is less than that of the competition function. In sexual populations, evolutionary branching occurs when mating is not random, as random mating prevents the split due to recombination. Assortative mating based on ecological traits or neutral marker traits can lead to speciation, with the latter requiring linkage disequilibrium between marker and ecological traits. The paper highlights the importance of ecological interactions in speciation, showing that competition for unimodal resources can initiate sympatric speciation even with assortative mating based on neutral marker traits. The results are robust against various model changes, including varying numbers of loci, mutation rates, and competition functions. The study suggests that ecological factors are crucial for speciation, providing an integrative framework for understanding the origins of many sympatric species, including cichlids, sticklebacks, snails, and anolis lizards. The findings indicate that stochastic fluctuations in finite populations can lead to sympatric speciation, especially in recently colonized habitats where competition is not strong. The paper concludes that ecologically driven speciation plays a prominent role in sympatric speciation.The paper discusses the origin of species through sympatric speciation, a process where new species arise from the same ancestral population without geographic isolation. The authors present a model showing that sympatric speciation is likely due to competition for resources. They use an individual-based model with explicit multilocus genetics to describe sexual reproduction and consider the evolution of assortative mating based on ecological or neutral marker traits. In both cases, assortative mating often leads to reproductive isolation between ecologically diverging subpopulations. When assortative mating depends on a neutral marker trait, speciation occurs when genetic drift breaks the linkage equilibrium between the marker and ecological traits. The theory aligns with empirical evidence for the sympatric origin of many species. The study also explores evolutionary branching, a phenomenon where directional selection leads to a split into two coexisting phenotypic clusters. This occurs when the curvature of the carrying capacity at its maximum is less than that of the competition function. In sexual populations, evolutionary branching occurs when mating is not random, as random mating prevents the split due to recombination. Assortative mating based on ecological traits or neutral marker traits can lead to speciation, with the latter requiring linkage disequilibrium between marker and ecological traits. The paper highlights the importance of ecological interactions in speciation, showing that competition for unimodal resources can initiate sympatric speciation even with assortative mating based on neutral marker traits. The results are robust against various model changes, including varying numbers of loci, mutation rates, and competition functions. The study suggests that ecological factors are crucial for speciation, providing an integrative framework for understanding the origins of many sympatric species, including cichlids, sticklebacks, snails, and anolis lizards. The findings indicate that stochastic fluctuations in finite populations can lead to sympatric speciation, especially in recently colonized habitats where competition is not strong. The paper concludes that ecologically driven speciation plays a prominent role in sympatric speciation.