Prezygotic and postzygotic isolation are two separate ways in which species are disallowed from mating with one another and producing a fertile offspring. Prezygotic isolation refers to the prevention of the fertilization of eggs while postzygotic isolation refers to the prevention of the production of fertile offspring. There are multiple different ways species are isolated before the egg can even be fertilized. Habitat isolation is one of the most significant forms of prezygotic isolation. If two species do not live in the same habitat, there is no chance that an egg of one species will be fertilized by another. A similar prezygotic measure is mating season isolation. A species that mates in the spring will have no chance of mating with a species that mates in the winter. Mating season isolation ties in closely with behavioral isolation. The time of year that a species mates is certainly a behavior, but behavior is not limited to mating seasons. If a species has certain pre-mating rituals, another species may not be familiar with this ritual, making it very difficult for the two species to agree to copulate. Still, two species might have similar enough behaviors and similar enough mating times that there may be a chance that the two could mate. In this event, between certain species, there is something known as mechanical isolation. Mechanical isolation refers to the physical inability of two separate species to mate. For example, spiders have penises shaped specifically for spiders of the same species, similar to a key and a lock. It would be physically impossible for a spider of one species to mate with another.
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