Reproductive isolation is a collection of evolutionary mechanisms, behaviors, and physiological processes critical for speciation. It refers to the inability of two incipient species to produce viable offspring together or to avoid mating with members of the other group. Reproductive isolation can be classified into two broad categories: pre-zygotic and post-zygotic mechanisms. Pre-zygotic mechanisms act before fertilization (or before mating in the case of animals), while post-zygotic mechanisms act after it. The mechanisms of reproductive isolation can appear in species whose geographic distributions overlap (sympatric speciation) or are separate (allopatric speciation) .

Here are some of the barriers to gene flow that may contribute to speciation:

• The evolution of different mating location, mating time, or mating rituals
• Lack of attraction between individuals
• Incompatibility between the gametes of different species
• Hybrid inviability or infertility

Reproductive isolation can be caused by geographic barriers, such as mountains or rivers, or by genetic, behavioral, or physiological differences between populations. It is a process that prevents individuals of different populations from mating, surviving, or producing fertile offspring.