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Speciation II: Prezygotic Barriers

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  1. 0:06 Types of Speciation
  2. 3:46 Spatial Isolation
  3. 4:36 Temporal Isolation
  4. 5:34 Mechanical Isolation
  5. 5:48 Gametic Isolation
  6. 6:07 Behavioral Isolation
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Taught by

Greg Chin

We may take for granted why animals choose to mate with other animals of similar appearance, but it's not that simple. There are actually biological barriers to reproduction that can prevent even seemingly closely related species from reproducing. This lesson looks at one such category of hindrances, prezygotic barriers, which make fertilization impossible.

Types of Speciation

So, let's delve into this question of whether or not flying hamsters constitute a different species from your run-of-the-mill hamster. We learned that speciation occurs when the gene pool of a population becomes split. The barriers to gene flow could be geographic or biological in nature. However, speciation is often caused by multiple factors, so let's break down the possible mechanisms for speciation a little differently.

Mechanisms that cause population isolation can be divided into pre- and postzygotic barriers
Prezygotic barrier

Another way to think about how the gene pool could become split is the timing of the barrier to reproduction relative to egg fertilization. Recall that the cell that forms when a sperm cell fertilizes an egg cell is called a zygote. Therefore, scientists can also divide the mechanisms of population isolation into pre- and postzygotic reproductive barriers. That is (in the case of a prezygotic reproductive barrier), the barrier could prevent fertilization itself and thus occur before a zygote can be formed.

There are two other ways populations of organisms could become different species. Some barriers affect the offspring that result from a mating between members of the two populations. Recall that hybrid refers to something derived from a mixed origin. Since these offspring are a mixture of the two types of organisms, they are called hybrid offspring. Mechanisms that reduce the viability or reproductive capacity of hybrid offspring are called postzygotic reproductive barriers.

If hybrid offspring have a lower viability, speciation can occur. Viability is, basically, the ability to avoid dying. For instance, if an organism is sickly due to genetic defects, it has a lower viability. An organism with a super, awesome mutation, like say fire breath, has a relative higher viability than its non-fire-breathing counterparts because predators are probably going to eat things that don't spit fire first! If the reproductive capacity of hybrid offspring is lower, hybrid organisms would have trouble passing on their genetic material to future generations. In this way, even if the hybrid organism itself is perfectly fine, if it makes little or no contribution to future gene pools, speciation can still occur.

Well, it looks like we need to get ourselves some regular hamsters to see if they can mate with our flying ones, so let's head off to the pet shop. Okay, so it looks like the local pet shop is out of hamsters. Well, while we wait for the next shipment of hamsters to arrive, let's do more research to see what kinds of prezygotic barriers could have split the two types of hamsters into separate species. It looks like there are five major types of prezygotic barriers to reproduction: spatial isolation, temporal isolation, mechanical isolation, gametic isolation and behavioral isolation.

Spatial Isolation

Species that live in different areas may not be able to mate; this is known as spatial isolation
Spatial isolation

Probably, the most obvious way that two species may not be able to mate anymore is that they live in different areas. This is known as spatial isolation. For instance, right now we're having trouble finding regular hamsters, and we have to wait to get them flown into the area. In the wild, geographical separation of different populations of organisms could result in speciation. However, speciation does not always occur when populations become separated. For instance, if our flying hamsters are able to mate with regular hamsters when given the opportunity, then spatial isolation is not a prezygotic barrier to the hamsters mating.

Temporal Isolation

The next way the hamsters could be blocked from mating is temporal isolation. If two populations mate at different times of the year, reproduction would be blocked temporally. In our example, the hamsters might normally live in the same habitat, but if they mate at different times of the year, obviously that'd be a problem. If this is the case, this is where using a controlled laboratory environment becomes a huge advantage to solve a pretty difficult problem. By controlling lab conditions, like the ratio of light and dark the experimental hamsters experience during the day, we might be able to fool both types of hamsters into mating at the same time. That would allow us to determine if the two possibly different species of hamsters are isolated from each other temporally.

Mechanical Isolation

Another prezygotic barrier that could result in speciation is mechanical isolation. This means that the size or shape of the reproductive organs prevent successful mating.

Gametic Isolation

A fourth way reproduction could be blocked prezygotically is gametic isolation. If the sperm of one type of hamster cannot find, attach or fuse with the egg of the other hamster, a zygote would never be produced.

Behavioral Isolation

Finally, different species often simply reject or don't even recognize individuals of other species as possible mates. If one type of hamster fails to recognize, or simply rejects, the other type of hamster, behavioral isolation could have separated these two populations of hamsters into different species.

Spatial, temporal and behavioral considerations of other organisms can also affect speciation. For example, two populations of plants could diverge because of the preference of their respective pollinators for one species of plant over the other, preventing cross-pollination. Once we can conduct some mating experiments with some non-flying hamsters, we'll be able to determine if any of these prezygotic barriers to reproduction have caused flying hamsters to diverge from non-flying ones.

Lesson Summary

In summary, a zygote is the cell that forms when a sperm cell fertilizes an egg cell. A prezygotic reproductive barrier is a mechanism that prevents fertilization from occurring. A postzygotic reproductive barrier is a mechanism that reduces the viability or reproductive capacity of hybrid offspring.

Spatial isolation is a type of prezygotic barrier in which different species are reproductively isolated by location. Temporal isolation is a type of prezygotic barrier in which different species are reproductively isolated by different mating seasons. Mechanical isolation is a type of prezygotic barrier in which different species are reproductively isolated due to incompatible reproductive organs. Gametic isolation is a type of prezygotic barrier in which different species are reproductively isolated because the sperm of one species cannot find, attach or fuse with the egg of the other species. Behavioral isolation is a type of prezygotic barrier in which species are reproductively isolated because individuals in one species reject individuals of another species as possible mates.

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