The concept of evolution supposes that the goal of a species is survival. One evolutionary strategy involves reproductive isolation so that a species can thrive in its environment. The easiest way to keep similar species from interbreeding is to geographically separate them. If the two can't come into contact with one another, there is no risk of cross-breeding.
It's a small world, though, and geographical barriers can't always prevent interbreeding. That's where reproductive barriers come in. A reproductive barrier is a biological feature of organisms that prevents their populations from breeding, even in close quarters. These barriers work by isolating the gene pools of a species either before or after the formation of zygotes.
Prezygotic barriers prevent mating and fertilization between species. These barriers include:
- Temporal Isolation: Right place, wrong time.
Two species live in the same neighborhood, but they breed during different seasons, times of day, or even years. Plants are pollinated in different months, animals breed in different seasons.
- Habitat Isolation: Wrong place, right time.
The species may breed at the same times, and may even share the same local area. They do not, however, share habitats. Some animals live in trees, others in valleys; some swim in rivers, others grow at the tops of trees.
- Behavioral Isolation: Right place, right time, wrong outfit.
Little to no sexual attraction between the sexes of different species will also prohibit interbreeding. Whether by visual cues, odor, or mating rituals, the male and females of a species have ways of attracting each other. A fruitfly zipping elaborately through a swarm isn't going to attract a moth that's looking for a particular scent. The species are behaviorally non-compatible.
- Mechanical Isolation: Right place, right time, wrong equipment.
The sexual organs of different species can make interbreeding impossible. "Insert tab A into slot B" doesn't work if your partner has a tab D. Similarly, many plants have flower structures that can only be pollinated by specific animals, preventing cross-pollination.
- Gametic Isolation: Right place, right time, wrong ingredients.
When two members of a species love each other very much, they put their gametes together to make a zygote. Males and females of separate species may fall in love--maybe even copulate--but it won't result in fertilization. Their gametes are incompatible. This is the ultimate form of birth control, but do not have sex with horses. Seriously, don't.
Postzygotic barriers prevent mating after interspecies fertilization has occured. If the genes of the two parents are not compatible, there is hybrid inviability. In other words, the offspring do not survive. On the rare occasions that they withstand production, they are born without the development or strength to survive.
Another postzygotic barrier is hybrid sterility, in which the hybrids survive, but are unable to reproduce with the parent species, keeping the original gene pool clean. Lastly, hybrid breakdown occurs when fertile hybrids are produced, but their offspring are sterile.
Isn't nature fabulous?
I get by with a little help from old biology notes.