Reproductive Isolation: Definition, Types & Examples

Typically, animals can only mate within their own species. In this lesson, you will learn about reproductive isolation, or when two species are unable to reproduce, including types and examples of isolation.
Reproductive Isolation
Picture a flood that changes the course of a river, dividing not only the landscape in half, but also a population of deer. The deer can no longer interact and, over hundreds of years, the two deer populations evolve, or change. Eventually, the river dries up and the two groups of deer are able to mingle once again. However, they are no longer able to mate because they are no longer the same species.
A species is a group of living organisms, such as animals or plants, that can interbreed or exchange genes. The above example of the deer is a case of speciation, or when one species becomes two or more, due to evolution.
Many things can separate one population into two or more groups, like shifting continents or lava flows. During their time apart, the deer experienced changes to the point of reproductive isolation. This refers to when two groups of animals live close enough to one another to interact, but are unable to interbreed with one another.
There are many reasons why two organisms cannot mate and they can be divided into the two major divisions – pre-zygotic barriers and post-zygotic barriers.
Pre-Zygotic Barriers
Pre-zygotic barriers are obstacles that are present before an egg can be fertilized. A zygote is an egg that has been fertilized by a sperm. Some examples of pre-zygotic barriers include temporal isolation, ecological isolation, behavioral isolation, and mechanical isolation.
With temporal isolation, the two species never come into contact with each other because they are not active at the same time, or they have different mating seasons. For example, the northern red-legged frog and the foothill yellow-legged frog both live in California but their mating seasons do not overlap, so these two species never get the chance to mate.
Some frog species have temporal isolation.
Some organisms prefer certain types of food, habitat or have specific mating sites. With ecological isolation, the species have the same range but do not come across one another because they are eating different foods, living in different habitats, or are mating in different areas. We can use the red-legged frog in this example, as well. Even though they live in the same region, the red-legged frog does not mate with the bullfrog because the red-legged frog breeds in fast-moving streams and bullfrogs breed in ponds.
Some animals have complex mating rituals. For example, the male porcupine urinates on the female.
With behavioral isolation, one species does not know the mating ritual of the other species or rituals are slightly different, so no mating occurs between them. There are two species of grasshoppers that will not interbreed because they have slightly different mating songs.
Some species have complex mating rituals. For example, some male birds perform dances for the female and male porcupines urinate on the females before mating. Only charming if you’re a porcupine, I guess.
With mechanical isolation, the animals actually try to mate, but are physically unable. A certain species of snail is unable to mate if the shell is not coiled in the same direction, so only right-coiled shelled snails can mate with right-coiled shelled snails; the same applies to left-coiled snails.
Post-Zygotic Barriers
In cases when post-zygotic barriers are in place, the organisms mate but no offspring are produced. Post-zygotic barriers mean the animals mated but no offspring occurred after they did so. It can also mean the offspring is a hybrid and is not viable, sterile or both.
One type of post-zygotic barrier is gametic incompatibility, where the sperm and egg are not compatible, unable to combine. A gamete is a mature sperm or egg, capable of transferring genetic code. An example of this can be seen in sea urchins, which release their sperm or eggs into the water. The gametes of the giant red urchin and the purple urchin are not compatible so, even though the sperm and egg come into contact with one another, they do not fuse together to make a baby urchin.
In gametic incompatibility the egg and sperm come into contact with each other, but they are not compatible so no offspring are produced.
Another type of barrier is zygotic mortality, where the egg and sperm have met and fused, but the zygote dies without further development. Remember, the term ‘zygote’ refers to an egg and sperm that have fused together.
The creation of an unsuccessful hybrid is also a form of post-zygotic barrier.
A hybrid is a cross between two different species. A liger is an example of a hybrid because it is a cross between a tiger and a lion. Unlike many hybrids, ligers can produce offspring.
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