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slide 1
Over time, the squirrels on each side of the Grand Canyon evolved independently, and we might now identify them as different species. In order for speciation like this to take place, gene flow between populations must be blocked. Geographic isolation occurs when gene flow is blocked because two populations are physically separated. Geographic isolation caused the origin of a new species of squirrel at the Grand Canyon. This type of speciation is called allopatric speciation. "Allopatric" comes from roots meaning "different homeland."
slide 2
Whether or not a geographic barrier will isolate two populations depends on the organisms involved. The Grand Canyon was a barrier for small ground animals, like squirrels. However, we find the same birds on both sides of the canyon. And humans can move from one side to the other. Examples of geographic barriers include the formation of a mountain range, the movement of a glacier, and the division of a large lake into several smaller lakes.
slide 3
Speciation can also occur if genetic changes cause two populations to become unable to mate with each other, even though they aren't geographically isolated. This is called reproductive isolation. When new species arise due to reproductive isolation, it's called sympatric speciation. "Sympatric" comes from roots meaning "same homland."
Reproductive isolation can occur if reproduction is blocked before the formation of a zygote, or fertilized egg. This type of barrier is called a prezygotic barrier. Reproduction can also be blocked after fertilization by a postzygotic barrier. Prezygotic barriers keep organisms from mating with each other and forming viable zygotes.
slide 4
Let's look at a few prezygotic barriers. For two organisms to mate, they must be in the same location within a particular habitat. They must mate at the same time of the day or season of the year, and have the same mating rituals. They also must have compatible anatomical parts. Finally, the egg and sperm must be able to fuse to form a viable zygote. If any of these conditions aren't met, the organisms can't mate and produce offspring.
slide 5
Postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult. Let's consider the case of a hybrid. A hybrid is the product of breeding by organisms of different species. Most hybrid zygotes that form do not completely develop, and those that do are generally not very healthy. If a hybrid does grow into a viable adult, it's often not fertile.
An example of a hybrid is a mule, the offspring of a horse and a donkey. The mule is healthy, but it's unable to produce any offspring. If a hybrid is fertile, generally its offspring are frail and sterile. Now that we've introduced the barriers that lead to speciation, we'll look at how each type of speciation occurs.
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