Mechanical Isolation: Examples And Definition

Mechanical isolation is a form of reproductive isolation that prevents two different species from interbreeding with one another. Along with gametic isolation, temporal isolation, ecological isolation, and behavioral isolation, which limit which species can breed with one another, thus preventing different species from merging into one species.

Mechanical isolation functions as one of the processes necessary for speciation.

What Is Reproductive Isolation?

Reproductive isolation is an evolutionary mechanism that promotes speciation, the formation of distinct, new species on the evolutionary tree of life. Reproductive isolation promotes speciation by preventing the interbreeding of two different species. This reproductive isolation benefits a species since interbreeding between two species is usually not in the best interest of a species. This is because any offspring that might occur from two different species are likely to be sterile, and thus cannot pass the species’ DNA onto the next generation.

There are different methods of reproductive isolation, such as physical/geographical boundaries and different mating behavior. However, mechanical isolation functions by having the reproductive parts of two different species be incompatible with one another – being the wrong size or shape.

“All nature’s creatures join to express nature’s purpose. Somewhere in their mounting and mating, rutting and butting, is the very secret of nature itself.” — Graham Swift

Mechanical Isolation

Two species are mechanically isolated from one another when their reproductive organs are too different from one another to allow interbreeding between the two species. This prevents the species from successfully breeding and creating a hybrid that will most likely be sterile. Even if two species are fairly closely related and the members of the species succeed in courting one another, they will not be able to copulate due to their different reproductive organs.

Mechanical isolation represents a physical barrier to breeding, in contrast to the other forms of reproductive isolation. In the classification of reproductive barriers, there are prezygotic and postzygotic barriers. A prezygotic barrier is any form of isolation that prevents mating/copulation between two organisms (it occurs before the zygote is fertilized). Postzygotic barriers occur after the formation of a zygote, and they are biological processes that act to reduce the viability of the zygote or reduce the reproductive capacity of the offspring (by making them sterile).

Mechanical isolation is a form of prezygotic barrier, as it prevents the two different species from mating in the first place.

While mechanical isolation is most prominent and visible in animals, it also occurs within species of plants. In plants, mechanical isolation is associated with the co-evolution of two separate species, as they will end up attracting different types of pollinators to ensure that the transportation of pollen to other species won’t happen.

As a point of comparison, consider how behavioral isolation is another form of biological isolation that prevents two species from mating – yet works through a different method. Behavioral isolation is where two different species are unable to mate because they have different mating rituals. Note that while behavioral isolation is still a barrier to reproduction it is a behavioral barrier, while mechanical isolation is a physical barrier.

Examples of Mechanical Isolation

Photo: By Jacobmacmillan – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=55088475

Bushbabies, also known as galagos, are found on the continent of Africa. Bushbabies are found throughout the continent and there are over 20 different species of the animal located there. Even though some species of bushbaby have an overlapping habitat, they cannot interbreed with one another. This is because the structure of the genitalia in bushbabies varies from species to species, so a male bushbaby of one species cannot successfully copulate with a female bushbaby of a different species. The two species of bushbaby won’t even be able to produce sterile offspring due to mechanical isolation.

Most species of snails also express mechanical isolation. If a female of one species of snail tries to mate with the male of another species, their reproductive organs won’t match with one another and the two species won’t be able to mate. The sexual organs of a snail are within the shell of the snail and if one species has a short, disc-like shell it won’t be able to mate with a snail that has a tall, cone-shaped shell.

Photo: By Trougnouf – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=69725430

Mechanical isolation also exists in plants. One of the most notable examples of mechanical isolation in plants is the isolation that exists between white sage and black sage. While the two species of sage share a geographical range, the two species can’t interbreed because they rely on different pollinators. Black sage is pollinated almost exclusively by honeybees, while carpenter bees pollinate white sage. Black sage controls what insects pollinate it by having pressure-sensitive petals. Carpenter bees are substantially larger than honey bees and the increased weight of the carpenter bees means that when they land on black sage the petals close before they can reach the reproductive area of the plant. On the flip side, the honeybees that pollinate black sage can’t reach the reproductive regions of white sage because they are too small.

Other Forms Of Reproductive Isolation

“Nature has invented reproduction as a mechanism for life to move forward. As a life force that passes right through us and makes us a link in the evolution of life.” — Louis Schwartzberg

Other types of reproductive isolation include temporal isolation, habitat isolation, and behavioral isolation. Temporal isolation is where two similar species are active at different times, whether they are active at different times of the day or if their breeding season occurs at different times of the year. Habitat isolation is when two species have a different habitat range that prevents them from interbreeding. As previously mentioned, behavioral isolation is when two different species have different courtship rituals or mating practices that prevent them from being receptive to the advances of another species.

To Sum Up:

Mechanical isolation is a form of reproductive barrier that prevents two different species from mating with each other. It functions by having the genitalia of the two species by incompatible with one another, ensuring that there is no reproduction (even the creation of a sterile offspring) and that the two species continue to evolve separately.