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Genetic Drift

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Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Genetic drift results from random changes the genes and allele frequency of a group of organisms. Genetic drift is usually found in small populations because smaller random events have a bigger effect. A specific example of genetic drift is the founder effect which occurs when some organisms migrate into a new area but randomly don't match the parental population's genetic make up.

When Darwin first came up with this theory of how evolution works with the mechanism known as natural selection, he didn't realize that there was another force that was having big impacts on evolution and that's something called genetic drift. Genetic drift is kind of annoying to us humans because we love the we love it when there's reason for something to happen. If I trip I think, it was because a black cat crossed my path two days ago. No I tripped because I'm clumsy or there just happened to be a stick where I stepped. It's not because of the black cat.

Well, turns out that you can have random changes in the genetic make up of a population. If you started off with half of the population one way, half another and it just so happens that maybe a couple of the people say it's blondes versus brunettes. a couple of the blondes die off just randomly not because of hair color but because of random events. Well, that changes it from 50-50 to maybe 48-52 percent. This usually happens in small populations and that's because the small population there's less of a buffer against these random chances. If you're in a car accident in China and you happen to die from that car accident and you take with you your genetic uniqueness. In China's population you represent a very tiny tiny fraction. Whereas if you're talking about an island with a population of five people and you die, there goes 20 percent of the population.

So, the founder effect is a subset of genetic drift and this is when a new population a new group of a particular species happens to get founded or started by a group who does not represent the average distribution or genetic frequencies of the parental population. So examples of this include the Amish.

Now the Amish are always fun to look at and study because they have a really cool song all about them and they have this trait called polydactilism. Polydactilism means that you have more than five fingers. Five fingers actually is a recessive trait but five fingers obviously is the much more commentary. However, it just turned out that amongst the people who founded the Amish community here in the United States in Pennsylvania, it happened that one person had polydactilism, that's not why he joined the Amish. It just was one of his genetic components.

Well, in the German population the number of people with polydactilism is pretty low, percentage wise based now with the modern Amish, there's a fairly high frequency of polydactilism compared to the original starting population that the Amish came from. Kind of more violent but the same general idea is something called "a genetic bottle neck." And that's when you have a population and some disaster or something happens that wipes out most of the population and those who survive wound up not being representative of the starting original population's genetic frequencies. And a very interesting example of this is on the island of Kosrae in the I believe it's one of the Islands of Micronesia. They had a typhoon that wiped out almost everybody left only 20 survivors. Now amongst those 20 survivors there happenned to be somebody who wasn't color blind but they were a carrier for complete color blindness. Well, time has passed and the survivors got together and got together and now the popu- the island is repopulated but now interestingly enough, 30 percent of their population are carriers for color blindness and actually, five percent, one out of every 20 individuals is completely color blind. That's an example of how you have a fairly sizable population. All of a sudden it got squished, and that created a weirdness and our smaller subset population founded off or created a new po- replacement population that didn't match the original.

That's genetic drift.