Reproductive isolation refers to a set of mechanisms that prevent animals of the same group from breeding. These are reproductive barriers that do not allow the species to mate and produce offspring.
Did You Know?
Reproductive isolation that occurs before fertilization is known as prezygotic isolation. However, when it occurs after fertilization and prevents the fertilized egg from turning into a fertile offspring, it is known as post-zygotic isolation.
Reproductive isolation refers to a set of conditions, that can be psychological, ecological, genetic or behavioral, which do not allow animals of closely-related species to unite and mate. In reproductive isolation, there are strong reproductive barriers that keep the related species separate. Two species belonging to the same family are unable to breed and produce offspring successfully due to various differences, which form an important aspect of reproductive isolation.
Types of Reproductive Isolation with Examples
- Here, the breeding or mating calls act as powerful reproductive barriers. If the closely-related species are unaware of their mating rituals (which are often bizarre), inbreeding does not occur. In behavioral isolation, despite being close to each other there is no sexual attraction between the male and female species. This happens because either of the species is unaware of special signals or the ritual that has to be performed before mating.
- Male hooded seals inflate their nasal cavities like a bubble gum to attract their female counterparts. The female seal will mate the one with the most visually appealing nasal balloon. So unless the male blows up his nasal, the female won’t come near its prospective mate.
- Similarly, light production in male fireflies is necessary to draw female attention. Depending upon the flash pattern produced by the male, the female will then respond and light up to signal the mate.
In mechanical isolation, the reproductive barrier arises due to incompatibility between the sex organs of male and female species. The organs do not fit correctly (probably due to variation in shape or size) or cannot be aligned due to their unique body structure, thereby preventing inbreeding. Mechanical isolation is commonly noticed among snails with their shell coiling in the opposite direction.
- Sexual receptivity in male and female species is possible only if their breeding season is the same. In temporal isolation, mating seasons of the closely-related species do not match.
- In some animal species breeding seasons differ and arrive at different times of the year. Also, some species are sexually active in the morning while their counterparts prefer nighttime for mating. These different times of the day for remaining sexually active are also one of the isolating mechanisms of temporal isolation.
- Some species of fruit flies such as Drosophila persimilis are active in the morning, whereas its close relative Drosophila pseudoobscura prefer mating in the afternoon.
- In ecological isolation, the male and female species prefer a different habitat for mating. The difference in mating sites forms the reproductive barrier in ecological isolation. As their places of breeding do not coincide, the species prefer not to mate.
- Rana aurora, one of the species of frogs found in northern California prefer rapidly moving water streams for inbreeding. Whereas, the Rana catesbeiana species prefer the still, slow-moving waters of permanent ponds.
The reproductive barrier occurs due to incompatibility between the sperm and the egg. As we all know, the interaction between a female egg and a male sperm that produces a zygote (fertilized egg) is necessary to produce offspring. However, the sperm is not drawn towards the egg due to genetic or chemical incompatibility. Gametic isolation helps prevent explosion of population.
Many species of female fish simply drop their eggs into the water. Also, the sperms of male fish of similar species are deposited into the water. The mechanism of isolation prevents formation of hybrid species and ensures that the eggs and the sperms of only the same species combine. Gametic isolation is essentially a hybridization barrier to control the population of similar species.
In this type of reproductive isolation, the closely related species mate and even the female eggs get fertilized. However, the zygote formed is immature, hence suffers an early death. So even if the inbreeding of the species takes place, no offspring is produced due to the immature zygotes.
- An offspring that is produced from two different species is essentially called a hybrid. The term ‘inviable’ means something that is incapable of surviving. In hybrid inviability, the zygote formed from combining of the sperm and egg of two different species is incapable of sustaining its own life.
- The fertilized egg formed of different species does not mature beyond its early stages of embryonic development. The mating of a female cat with a male dog or rabbit may produce a fertilized egg, but it will die shortly due to genetic differences.
In hybrid sterility, different species breed and produce an offspring, which does survive to become an adult. However, the adult suffers from very low fertility, thereby proving to be incapable of giving birth to an offspring. For instance, mating between a horse and a donkey gives rise to a species, commonly referred to as mule, which is found to be sterile.
Due to reproductive barriers, closely related species may remain separated for centuries and evolve to become completely different species. This is a perfect example of speciation wherein the two species cannot mate because they are now distinct species with absolutely no similarity.