The molecular basis of speciation: from patterns to processes, rules to mechanisms.

The empirical study of speciation has brought us closer to unlocking the origins of life's vast diversity. By examining recently formed species, a number of general patterns, or rules, become apparent. Among fixed differences between species, sexual genes and traits are one of the most rapidly evolving and novel functional classes, and premating isolation often develops earlier than postmating isolation. Among interspecific hybrids, sterility evolves faster than inviability, the X-chromosome has a greater effect on incompatibilities than autosomes, and hybrid dysfunction affects the heterogametic sex more frequently than the homogametic sex (Haldane's rule). Haldane's rule, in particular, has played a major role in reviving interest in the genetics of speciation. However, the large genetic and reproductive differences between taxa and the multi-factorial nature of each rule have made it difficult to ascribe general mechanisms. Here, we review the extensive progress made since Darwin on understanding the origin of species. We revisit the rules of speciation, regarding them as landmarks as species evolve through time. We contrast these 'rules' of speciation to 'mechanisms' of speciation representing primary causal factors ranging across various levels of organization-from genic to chromosomal to organismal. To explain the rules, we propose a new 'hierarchical faster-sex' theory: the rapid evolution of sex and reproduction-related (SRR) genes (faster-SRR evolution), in combination with the preferential involvement of the X-chromosome (hemizygous X-effects) and sexually selected male traits (faster-male evolution). This unified theory explains a comprehensive set of speciation rules at both the prezyotic and postzygotic levels and also serves as a cohesive alternative to dominance, composite, and recent genomic conflict interpretations of Haldane's rule.