Speciation is not a precise division: overlap between closely related species can occur in areas called hybrid zones. Organisms may mate to produce _____________________with other similar organisms. The fitness of these hybrid offspring can affect the two species' evolutionary path. In hybrid _____________________, low hybrid fitness can_____________________ speciation. In hybrid _____________________, high hybrid fitness can delay or _____________________speciation. Scientists propose two models for the rate of speciation: the _____________________model illustrates how a species can change slowly over time. The _____________________model demonstrates how change can occur quickly from a parent generation to a new species.