Hybridization, Introgression, and the Nature of Species Boundaries

TLDR

Species are diagnosably distinct populations whose boundaries, maintained by intrinsic barriers, are often semipermeable and vary across space, time, and the genome—a pattern first noted in hybrid zones and now clarified by molecular tools, though debate remains about the extent of genome divergence during speciation. The study investigates which phenotypes, genes, or genome regions remain differentiated despite hybridization and introgression, thereby defining the species boundary. Species boundaries are semipermeable, with gene exchange varying by genome region, and recent DNA sequence data confirm this pattern.

Abstract

Species can be defined as populations that are diagnosably distinct, reproductively isolated, cohesive, or exclusive groups of organisms. Boundaries between species in sympatry are maintained by intrinsic barriers to gene exchange; these boundaries may not be uniform in space, in time, or across the genome. Here, we explore the nature of the species boundary, defined as the phenotypes/genes/genome regions that remain differentiated in the face of potential hybridization and introgression. We emphasize that species boundaries are semipermeable, with permeability (gene exchange) being a function of genome region. The early evidence for semipermeable species boundaries came from data on differential introgression in hybrid zones. This "genic view" of species was common in the hybrid zone literature even when few molecular markers were available to characterize genome-wide patterns of variation. Now, molecular tools allow detailed characterization of differentiation between diverging lineages and patterns of variation across natural hybrid zones, but the questions being asked by evolutionary biologists have remained much the same. Recent data (from DNA sequences and genotypes) reinforce earlier conclusions about the semipermeable nature of most species boundaries. However, debate persists over the nature and extent of genome divergence that accompanies speciation.