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What prevents species that are closely related from mating with each other? Sometimes it’s just a matter of having the right equipment – mechanical isolation is one evolutionary mechanism that prevents different species from interbreeding.

Mechanical Isolation

Having the right tool for the job is essential. When you need a hammer, a wrench just won’t do. The same is true for animals that mate through sexual reproduction. Without compatible sex organs, the individuals of different species (even closely related ones) will not be able to mate and produce offspring.

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These biological features that prevent different species from interbreeding are called reproductive barriers. There are two types: prezygotic and postzygotic. Prezygotic barriers prevent mating from even happening. If mating does occur, postzygotic barriers reduce the chances that an offspring will survive before being born.One prezygotic reproductive barrier is mechanical isolation. This evolutionary mechanism is when different species are isolated by their mechanics – the genitalia of the males and females may have different sizes, shapes, or locations. While mating may still be attempted, evolution has led to genitals that are extremely complex and unique to each species, and this can prevent breeding among even closely related species.

Examples of Mechanical Isolation

Mechanical isolation is very common in plants. For example, flowering plants that do not have the correct shape for a pollinator will not receive a pollen transfer, and will therefore not be fertilized. In this case, the shape is the barrier.

But keep in mind that mechanical isolation is not always so clear-cut. Think of a Great Dane and a Chihuahua. While you may think mating would be possible because they are very closely related, the size difference is so great between these two animals that mating would not be very likely. So, in this case, size is the barrier.In another example, two different species of snail are prevented from mating because the spiral shape of their shells prevents them from properly aligning their genitalia.

Mating may very well be attempted, but, in this case, breeding is prevented because of the different location of the species’ sex organs.

Lesson Summary

While mating may be attempted between closely related species, mechanical isolation prevents breeding from actually occurring. Even a very small difference in size, shape, or location of sex organs can have a very profound effect on the outcome of such an attempt.

Learning Outcomes

Once you are done with this lesson you should be able to:

  • Explain prezygotic and postzygotic reproductive barriers
  • Describe mechanical isolation and give examples
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