Is speciation an unrelenting march to reproductive isolation?

Speciation is often portrayed as an "incomplete" or "incipient" process if two groups of organisms, technically distinguishable either by morphology or genetics, can exchange genes. The ultimate outcome of diversification, given this perspective, is complete reproductive isolation. But an increasing amount of evidence suggests that speciation is rarely complete and inter-fertility between different taxonomically accepted species is consistently maintained. In this issue of Molecular Ecology, Linan et al. (2021) provide results that bridge evolutionary processes from populations to phylogenies that indicate suites of closely related tree species in the Mascarene Islands actively exchange genes, evolving as a nested set of syngameons with a hierarchical pattern of interfertility. The deep insight into diversification provided by this study is particularly powerful because of the genomic scale of the data and the complete taxonomic sampling of an island clade evolving in situ. The prevalence of syngameon dynamics in a broad range of organisms indicates that we should adopt a fluid and comprehensive approach to defining evolutionary units for conservation and research. We should move beyond focusing on single endangered species in evolutionary and ecological isolation from other species but consider the entire network of potentially interfertile species and the potential for future adaptation and innovation, particularly in a human dominated world.