The review discusses the evidence for ecological speciation and its alternatives, focusing on the role of natural selection in the origin of species. It outlines two main mechanisms of speciation: ecological speciation, driven by divergent natural selection between environments, and mutation-order speciation, driven by the fixation of different mutations in populations adapting to similar selection pressures. The review highlights that ecological speciation is more commonly observed, with examples such as the evolution of reproductive isolation in species adapting to different environments. Mutation-order speciation is less well understood, with evidence primarily coming from cases of reproductive isolation due to intragenomic conflict. The review also addresses the challenges in identifying the genetic basis of reproductive isolation and the importance of understanding the mechanisms of natural selection in speciation. It emphasizes the need for further research to clarify the relative importance of these two mechanisms in the origin of species. The review concludes that natural selection plays a crucial role in speciation, and that understanding the genetic and ecological factors involved is essential for advancing our knowledge of evolutionary processes.The review discusses the evidence for ecological speciation and its alternatives, focusing on the role of natural selection in the origin of species. It outlines two main mechanisms of speciation: ecological speciation, driven by divergent natural selection between environments, and mutation-order speciation, driven by the fixation of different mutations in populations adapting to similar selection pressures. The review highlights that ecological speciation is more commonly observed, with examples such as the evolution of reproductive isolation in species adapting to different environments. Mutation-order speciation is less well understood, with evidence primarily coming from cases of reproductive isolation due to intragenomic conflict. The review also addresses the challenges in identifying the genetic basis of reproductive isolation and the importance of understanding the mechanisms of natural selection in speciation. It emphasizes the need for further research to clarify the relative importance of these two mechanisms in the origin of species. The review concludes that natural selection plays a crucial role in speciation, and that understanding the genetic and ecological factors involved is essential for advancing our knowledge of evolutionary processes.