 Reproductive isolation

Reproductive isolation consists of barriers that reduce gene flow through sexual reproduction. Distinguish premating isolation from barriers after mating. Barriers before fertilization also include incompatible gametes; barriers after fertilization can reduce offspring survival or fertility. ^[ref]

An assay should state which step it measures. Observing fewer cross-group matings does not measure offspring fertility; observing fertilization does not demonstrate that the offspring will reproduce. These distinctions make a partial barrier informative without declaring completed speciation.