Parapatric speciation

Parapatric speciation is a mode of speciation in which two subpopulations of a species evolve reproductive isolation from one another while continuing to exchange some genes, rather than being fully separated as in allopatric speciation. It is marked by non-random mating, unequal gene flow between the two populations, and a geographic range that may be continuous or only partly broken. Natural selection is the main driver, as different local conditions favor different traits and create a gradient of allele frequencies across the landscape that eventually produces reproductive isolation despite the ongoing exchange of genes. The grass species Agrostis tenuis demonstrates the process: populations growing on soil contaminated by copper from abandoned mines have evolved reproductive isolation from nearby populations on uncontaminated soil, largely through differences in flowering time that prevent successful cross-breeding, and a similar pattern appears in Anthoxanthum odoratum on soils contaminated with lead and zinc.