Introduction | Task | Process | Evaluation |Conclusion | References

In nature, barriers can break large populations into smaller ones preventing them from breeding and producing a living offspring. Two types of barriers exist. The first is called prezygotic barriers. These barriers prevent organisms from coming together and breeding. Examples include habitat, temporal, behavioral, mechanical, and gametic isolation. The second type of barrier is called posyzygotic barriers. In this situation, species come together and breed but an offspring is either not produced or is sterile. Examples of this type of isolation includes reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown.

Prezygotic Barriers
(A barrier that prevents organisms from coming together and breeding.)


Habitat Isolation- Two species living within the same area may encounter each other rarely, if even at all. For example two species of garter snakes occur in the same area but live within different parts of the area. One species lives within the water while the other lives upon the land. Since they occupy a different area, they seldom come into contact with each other therefore, they don't mate.

* Photos taken from Splash
Erik's page


Temporal Isolation- Two species living within the same area breed at different times of the day, season, or year. For example a brown trout and rainbow trout breed in different seasons. Brown trout breed in the fall while rainbow trout breed in the spring. Since they breed at different times of the year, both species remains reproductively isolated.

* Photos taken from San Francisco State University
Fish Stock


Behavioral Isolation - Two species living within the same area demonstrate particular behaviors to attract a mate. For example male fireflies of different species signal females by blinking their lights in a characteristic pattern. Females discriminate/choose the different signals and flash back to males of their species. They maintain their species integrity by responding to unique calls/signals specific to their species.

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  Mechanical Isolation- Anatomical incompatibility (physically not able to breed) may prevent sperm transfer when closely related species attempt to mate. One example from animals is that of Drosophila pseudoobscura females and Drosophila melanogaster males. Copulation/breeding between these individuals can result in injury or even death to the offspring (Dobzhansky et al., 1977).

* Photos taken from
Cornell University


Gametic Isolation- Gametes of different species that meet rarely fuse to form a zygote. There can be a few possibilities for this isolation. The first possibility may be that sperm of one species may not be able to withstand the internal environment of the females reproductive body. Another possibility could be that specific molecules on the coat around the egg can not adhere to complementary molecules on sperm cells of the same species. In either situation gamete cells (sperm and egg) are prevented from coming together to complete fertilization.



* Photos taken from Natra Care

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