Any physical, behavioural or structural barriers that prevent individuals from being able to mate together.

Pre-zygotic barriers: barriers before fertilization

Prevent Mating

Ecological: Two species breed in the same area and during the same time, but not in the same place. One certain frog breeds In quiet streams, while the other breeds in rain pools.

Temporal: Species breed at different times or seasons, of the year. Ex. Red and Black Sea urchins release gametes at different times of the year.

Behavioural: Some individuals might not find certain behavioural characteristics of another individual attractive, such as mating rituals, birdsong,

Prevent Fertilization

Mechanical: When the sex organs are not compatible, or when pollen of a plant cannot occur due to certain structures. ex. Bird beaks have to be long, or short enough to pollinate a flower.

Gametic: Males and females do copulate but sperm do not survive inside of females, and gametes do not unite.

Post-zygotic: After fertilization

Zygote Mortality: No fertilized zygotes or embryos develop to maturity. Don’t get born

Hybrid Inviability: Gametes unite but genes are not compatible and result in weak offspring. Are born but do not survive for long.

Hybrid Sterility: Offspring does get born and is healthy, but cannot reproduce.