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Evolution of Populations

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.

The evolution of populations is defined as the changes populations undergo when organisms change over time as predicted by Darwin's Theory of Evolution. Over time, organisms which are most fit for their environment survive while unfit organisms die, changing the genetics of a species until that species is well adapted for its environment. These changes are often caused by natural selection or genetic drift.

In evolution a lot of times, they'll talk about what's going on at the population level. Now to understand what a population is you have to just think that it's a group of members of a particular species they're all integrating with each other. So you can talk about here in California, you can talk about squirrels but in San Francisco there's squirrels in Golden Gate Park or there maybe squirrels in the Pacitio. Those two groups of squirrels they're the exact same species but they're different populations because unless they hop on a bus and take a ride from one park to the next they're not going to be interbreeding with each other. So when you have a small population, you can sometimes have a process known as genetic drift occur. And that's when you have genetic changes within the small population not due to some kind of selection pressure but due to just random changes, couple of the squirrels die because they get run over by a car, that removes their genes from their gene pool.

Now if you do have natural selection working on a population there are some particular patter that are often seen in these genetic changes within the population. One of them is something known as, directional selection now if we graphed the gene pool or the distribution of size for example within a population, you'll typically see this kind of distribution and this is called a bell curve. Where you have very few members or percentage of the population r small, a fair number r medium size and then a small number R large. Now directional selection is when one end of the spectrum or the other is favored. For example, if you're talking about Elephant Seals and the males, with male Elephant Seals when they're in their mating season the males will compete with each other.

Smashing at each other and generally the larger you are the greater chance you have of winning and thus claiming a lot of the beach and therefore territory for the females to come and mate with you. So the advantage is on the far end over here in the size distribution. So over time we'd see this shift to one direction so after a number of generations have gone by, the average size of the male Elephant Seal will have shifted in one direction, that's why it's called directional selection. Stabilizing selection instead of it giving the advantage to one extreme or the other, instead the extremes are selected against and the middle is favored. Will be an example of that, well for quite sometime human birth weight demonstrated stabilizing selection.

If you were born and you only weighed a couple of pounds your chances of survival were extremely low. And so low birth weight babies died I'm sorry about that. Meanwhile if you were of 16 pounds you couldn't escape mummy you just sit there going, and you and or mummy would die. So that would remove the other extreme and so that's why human birth weight for centuries was fairly stable at around 6 to 8 pounds. Now however with cesarean section we can have some larger weight babies being born and with the advances and incubators and other things for a prematurely born children we're also removing some of the selective pressure here. So we may see a broadening of this but stabilizing selection in general tends to create a narrower bell curve like that.

Disruptive selection is kind of those 2 ideas merged, where instead of favoring one extreme or the other or favoring the middle instead you disfavor the middle and give the advantage to either end and so you'll tend to see what sometimes is a bimodal curve where you'll have a fair number who are large or whatever this extreme is and a fair number who are smaller or whatever this extreme is. Well being an example of that, well in a particular kind of fish species when it's time for mating the male fish will claim a region of the stream bed as their territory and the big male fishes will try to drive off all other male fish. And the bigger you are the greater area you are able to claim because that's how the male fish decides who is dominant. The big ones will drive off the smaller males and they will go elsewhere and then the females will come to this area to lay their eggs and the big male fish will then do it's courtship dance with the females, she will sprays out her eggs he will spray off his sperm and thus fertilize the eggs.

Well you would predict directional selection towards having large males if that's the case, but in reality what happens is that scientists started discovering yes there were a lot of large males. But there's also a lot of really small males, and they said how could this be? So they watched the fish a lot more carefully and what they discovered is that the big male fish will ignore juvenile i.e. the equivalent of younger than teenager fish, he won't attack them because they're not interested in reproduction and so it'll be a waste of his energy to attack juvenile fish. Well it turns out that there are some males who are adults but they're about the size of juveniles and they'll sit there and they'll say hey don't worry about me I'm not here to do anything.

They'd hide behinds rocks and then they'd wait for the big male to drive off all the other males. Then he'd start his dance when the female fish came along and right at the end of the dance, right as she sprays out her eggs, the little guy would zip in between spray off his sperm and zip off which would completely disrupt the concentration of the big guy and very often he won't even get a chance to spray off much of his sperm. Thus in the eggs they'll get fertilized by little guys sperm. So the sneaky little fish, he was getting his genes passed on the next generation, the big male fish he can get his genes passed on to the next generation, the medium size fish was too big to pull the ninja trick and he is too small to pull off the I'm going to drive off my competitors trick and so they get selected against. So that would be an example of disruptive selection.

Now if the disruptive selection is not just on one gender but on the entire population both male and female this may eventually lead to a branching or splitting off of one population from the other and ultimately into two new species. Now a fourth kind of pattern that is often as something called frequency dependent evolution or frequency dependent selection and that's where you have a particular phenotype like being tall or being red haired or whatever, that's an advantage only when it's rare, commonly or sometimes it's only advantage when it's common it depends on the circumstance. An example of this is amongst a particular breed of fish, again we're talking about fish and with those fish the vast majority of fish are gray. Why? Because that protects them against predatory birds because when a bird is flying over and they look down they see rocks and rocks are gray. So if the fish looks like a rock the bird doesn't catch them, there's a few mutant fish that are red.

Now if they're rare one disadvantage of being gray is that it's hard for the females to find you. If you're a red male fish however the female fish can find you and so yeah you're a at greater risk from the birds but the female fish can find you. And all you have to so is be found by the females a few times. And so when you're a rare red fish then the females go I can see you and that red male has lots of offspring which leads to lots of red fish. That's when it becomes a disadvantage to be red because once you have lots of red fish the birds flying over will go look at that and they start seeing all of these red fish and they start eating them. Which makes being red less common and then it becomes a disadvantage again. So there's some of the patterns of evolutionary change that you see in populations.