Genetic Drift, Gene Flow, and Types of Natural Selection

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This video explains the differences between genetic drift and gene flow, in addition to the three types of natural selection.

*** If there are any pictures used in this video, they are NOT MINE and I will not take credit for them. ***

TRANSCRIPT:
Alright guys, so today we’re gonna go over events that influence allele frequencies in a population: that is to say, genetic drift and gene flow. At the end of this video, we’ll go over the types of natural selection.
So, genetic drift is when chance events cause changes in allele frequencies. They’re unpredictable, and sometimes they can be detrimental if there ends up being a lot of a harmful allele in the population. There are two types of genetic drift: the founder effect and the bottleneck effect. The founder effect is when a small portion of individuals becomes isolated from the larger population. Let’s look at this island of birds I’ve drawn here. Perhaps a wind has caught the smallest birds, and they are not strong enough to fly against the wind. They’re blown onto another island, and… yikes! It looks like there have been a lot of green birds blown onto the island, and only one blue bird. Well, by chance, it seems that the blue allele is recessive, so over some generations, all of the birds on this new island will be green. When compared to the original population, the birds will seem very different. Over time, these two groups of birds may have adapted and diverged so much from each other that they become separate species.
The bottleneck effect is another type of genetic drift, and it’s called that because it’s like shaking marbles out of a bottle. Since the neck of the bottle is so narrow, only a few individuals are able to survive from a random and sudden change in the environment. Perhaps a fire wipes out almost all of the birds on this island. By chance, the few survivors can be any individuals, regardless of their genotypes. The fire could burn down this portion of the island, leaving many green birds as survivors, or the fire could burn down this portion, leaving many blue birds – it’s all up to chance.
Next up is gene flow – that’s basically immigration and emigration. Gene flow is the result of the movement of individuals. A good example given in the Campbell’s AP Biology textbook is that humans move freely around the world, and as mating occurs between different populations, you start to see different phenotypes appear. The contribution of one individual’s alleles to a population is therefore what gene flow is.
Time for my favorite part of this whole video, the types of natural selection! You’re gonna love this too, since it’s so easy to understand. There are three types of natural selection: directional, disruptive, and stabilizing. Let’s say we have this graph set up, and along the bottom, we place different colors of mice. Over on this side we have the light-colored mice, and going to the right, the mice get darker and darker. Here’s your normal distribution curve: most of the mice have intermediate phenotypes, and very little have extreme phenotypes. Now, natural selection can act on this population in many different ways. This first one is called directional selection, in which the whole curve shifts to one direction. That way, the whole population of mice either becomes lighter as a whole or darker. Maybe the soil that the mice walk on is darker, so it’s easier for the darker mice to hide from predators. Or, maybe the soil is a very light color, so the distribution curve shifts to the direction of the left.
The next one is called disruptive selection: the curve is disrupted, and only individuals with extreme phenotypes are favored. Maybe the soil has patches of very dark and very light dirt, and the intermediate-colored mice have no place to hide.
Finally, stabilizing selection is when the population becomes more stable in terms of phenotypes. In other words, individuals with extreme phenotypes are selected against, and only intermediate-phenotype individuals are favored. As you can see, this results in a very narrow population variation. Perhaps the soil in this example is a brown color.
Alright, thanks for watching, guys! Be sure to check out my other videos for more AP bio help, and I’ll see you in another video.