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.
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 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 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.
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.
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.
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.
The term ‘hybrid‘ refers to the offspring between two different species and ‘inviability‘ refers to an organism that does not thrive. In hybrid inviability, mating occurs and a hybrid is produced, but it does not develop properly and typically dies early.
An example of hybrid inviability: when certain species of irises are crossed, the offspring die before a seed can be formed.
In cases of hybrid breakdown, the hybrid can reproduce but later generations are sterile, not viable, or both.
An example of hybrid breakdown can be seen in cotton plants. Cotton farmers have tried to cross different types of cotton in order to get a good cotton plant. Although the first hybrid crop is fine, later generations are less viable.
Reproductive isolation is why different species of animals and plants are not able to interbreed with one another.
Speciation, or when one species splits into two or more species, is to be blamed for many cases of reproductive isolation within similar types of organisms.Both pre-zygotic barriers and post-zygotic barriers can keep species from interbreeding. Pre-zygotic barriers prevent animals from mating at all, while post-zygotic barriers apply to cases when organisms do mate and their cells do join, but the development of offspring is impeded.Pre-zygotic barriers include:
- Temporal isolation, when species do not come into contact with one another due to timing
- Ecological isolation, when species do not come into contact with one another due to habitat or lifestyle differences
- Behavioral isolation, when species’ mating rituals are different enough from one another to prevent interest in copulation
- Mechanical isolation, when physical differences between two animals prevent mating
Post-zygotic barriers include:
- Gametic incompatibility, when the sperm and eggs between two species are not able to fuse due to biological differences
- Zygotic mortality, when the egg and sperm do fuse, but the resulting zygote is not viable and dies
- Hybrid inviability, when the zygote does form, but does not do so completely and usually dies early in its life cycle
- Hybrid sterility, when the zygote does grow to become a hybrid, or the offspring of two different species of organisms, but the hybrid itself is unable to breed
- Hybrid breakdown, when the hybrid is able to breed, but future generations are not viable, sterile, or both
Barriers to Reproduction
Reproductive isolation can be caused by pre-zygotic and post-zygotic barriers.
- Pre-zygotic barriers: barriers that prevent animals from mating.
These include temporal isolation, ecological isolation, behavioral isolation, and mechanical isolation.
- Post-zygotic barriers: barriers that come into play after two species have mated. These include genetic incompatibility, zygotic mortality, hybrid inviability, hybrid sterility, and hybrid breakdown.
After you are finished, you should be able to:
- Discuss the factors that can contribute to reproductive isolation
- Describe the types of pre-zygotic and post-zygotic barriers to reproduction
- Identify examples of reproductive isolation