Which type of reproductive isolation occurs when there are differences in the size or shape of reproductive parts that may prevent interbreeding?

How Prezygotic Isolation Leads to New Species

The Five Mechanisms that Encourage Evolution

Updated on August 30, 2019

In order for different species to diverge from common ancestors and drive evolution, reproductive isolation must occur. There are several types of reproductive isolation that lead to speciation. One of the most common methods is prezygotic isolation which takes place before fertilization occurs between gametes and prevents different species from sexually reproducing. Basically, if individuals cannot reproduce, they're considered to be different species and diverge on the tree of life.

There are several types of prezygotic isolation that range from incompatibility of gametes to behaviors that result in incompatibility, and even a type of isolation that physically inhibits individuals from breeding.

Mechanical Isolation

Mechanical isolation—the incompatibility of sexual organs—is probably the simplest way to stop individuals from reproducing with each other. Whether it's the shape of the reproductive organs, the location, or differences in size that prohibit individuals from coupling, when the sexual organs don't fit together, mating is not likely to occur.

In plants, mechanical isolation works a bit differently. Since size and shape are irrelevant to plant reproduction, mechanical isolation usually results from the use of a different pollinator for the plants. For example, a plant that's structured for bee pollination will not be compatible with flowers that rely on hummingbirds to spread their pollen. While this is still a result of differing shapes, it's not the shape of the actual gametes that matters, but rather, the incompatibility of the flower shape and the pollinator.

Temporal Isolation

Different species tend to have different breeding seasons. The timing of female fertility cycles can result in temporal isolation. Similar species may be physically compatible, yet may still not reproduce due to their mating seasons occurring at different times of the year. If the females of one species are fertile during a given month, but the males are not able to reproduce at that time of the year, that can lead to reproductive isolation between the two species.

Sometimes, mating seasons of very similar species overlap somewhat. This is especially true if the species live in different areas leaving no chance for hybridization. However, it's been shown that similar species that live in the same area generally don't have phases of overlapping mating, even if they do when in divergent environments. Most likely, this is an adaptation nature designed to reduce competition for resources and mates.

Behavioral Isolation

Another type of prezygotic isolation between species has to do with the behaviors of the individuals, and, in particular, the behaviors around mating time. Even if two populations of different species are both mechanically and temporally compatible, their actual mating ritual behavior could be enough to keep the species in reproductive isolation from one another.

Mating rituals, along with other necessary mating behaviors—such as mating calls and dances—are very necessary for males and females of the same species to indicate it's time to reproduce. If the mating ritual is rejected or not recognized, then not mating will occur and the species will be reproductively isolated from each other.

For instance, the blue-footed booby bird has a very elaborate mating dance the males must perform to woo the female. The female will either accept or reject the advances of the male, however, other bird species that don't have the same mating dance will be fully ignored by the female—meaning they have no chance of reproducing with a female blue-footed booby.

Habitat Isolation

Even very closely related species have preferences regarding where they live and where they reproduce. Sometimes, these preferred locations for reproductive events are incompatible between species, which leads to what's known as habitat isolation. Obviously, if individuals of two different species live nowhere near each other, there will be no opportunity to reproduce. This type of reproductive isolation leads to even further speciation.

However, even different species that live in the same locale may not be compatible due to their preferred place of reproduction. There are some birds that prefer a certain type of tree, or even different parts of the same tree, to lay their eggs and make their nests. If similar species of birds are in the area, they will choose different locations and will not interbreed. This keeps the species separate and unable to reproduce with one another.​

Gametic Isolation

Gametic isolation ensures that only sperm of the same species can penetrate the egg of that species and no others. During sexual reproduction, the female egg is fused with the male sperm and, together, they create a zygote. If the sperm and egg are not compatible, fertilization cannot occur. Due to some chemical signals released by an egg, the sperm may not even be attracted to it. Another factor that prevents fusion is a sperm that can't penetrate an egg due to its own chemical make-up. Either of these reasons is sufficient to frustrate fusion and prevent the formation of a zygote.

This type of reproductive isolation is especially important for species that reproduce externally in water. For instance, the females of most fish species simply release their eggs into the water of their preferred breeding locale. Male fish of that species then come along and release their sperm over the eggs to fertilize them. However, since this takes place in a liquid environment, some of the sperm get swept away by water molecules and dispersed. Were there no gametic isolation mechanisms in place, any sperm could fuse with any egg, which would result in hybrids of whatever species happened to be mating in the water there at the time.

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