Understanding Selection and Evolutionary Traits

To begin with, when we talk about selection, we're talking about the influence that traits or that, more particularly, genotypes have on reproduction by either increasing the probability of reproduction or decreasing the probability of reproduction. And consequently, the alleles that control those traits will either increase or decrease in frequency in the population. Now, if we look at a particular trait... over time and what effect it has on the population as those organisms reproduce. In any particular quantitative trait, if we look at the x-axis, and we're not specifying here what that trait is, but it could be how tall the organism is, it could be the amount of whatever something it produces, if it's a cow it could be the amount of milk it produces, it could be the amount of pigment in the flower of the organism. It could be any number of, anything that could be measured along the x-axis. And on the y-axis is the frequency of that trait in the population, how many organisms in the population have that particular phenotype. If we have directional selection, the selection is going in one direction. We have in this case, represented by the dark color, we have selection against the organisms at this end of the distribution. These organisms are not reproducing as efficiently as the organisms at this end of the distribution. And as a result, over time, over generations, the population is going to change, such that that the mean is going to shift to the right, to whatever this is. If this is the height of the organisms, they're going to get taller. We're selecting against the shorter organisms. We're selecting for the taller organisms. This is artificial selection, and it's directional selection for bristle number in Drosophila. And these are fruit flies, of course, and this is the mean abdominal bristle number. Most fruit flies are around 10 bristles. We are artificially selecting for additional bristles. We are selecting for the flies that have additional bristles up to 90 generations or so. We are selecting the fruit flies that have more bristles than the others. And we continue to increase artificially the number of bristles. So this is directional selection. We're getting more and more bristles. And this is more bristles. than the flies would normally have in nature. So we're selecting directionally. Then we stop the artificial selection at this point. We let natural selection take over. And after 90 generations, natural selection takes over. And directional selection now is going in the other direction, natural selection. So that's too many bristles. And for whatever reason, The flies with that many bristles, with 40-45 bristles, are selected against, and the average number of bristles on the flies goes down, and it will probably continue to go down until it gets back down around 10. So that's an example of directional selection. Another example is found in ground finches. And in these ground finches, the birds that have the smaller bells, feed most efficiently on small soft seeds. The birds with the larger bills feed efficiently on the large hard seeds. And these are birds from the Galapagos Islands. These are the finches that Darwin saw on his voyage, but this is a more recent study obviously in the 1970s. In this study, the size of the bills was graphed over time. And at this point in 1977 there was a period of drought and the period of drought changed the seeds that were available. And because the seeds changed as a result the finch population changed during that time to adapt to the changing conditions because there were different kinds of seeds available. And so... Birds that were better adapted to the seeds that were available after the drought became more prevalent in the population. Again, directional selection going towards the... larger sized bill. Stabilizing selection is most common. Stabilizing selection selects for the mid-range values. When organism is well adapted to its environment and the environment isn't changing, the extremes are selected against. And so here you see the darker colored brown, whatever this is, the mid-sized or mid-values, the mid-range phenotype. are best adapted and if you get too far one way or the other that's going to be selected against. The effect of the stabilizing selection is that the curve is going to be narrowed. It's going to stabilize around the best fit value. And here's an example of that. This is human birth weight and there's two graphs here superimposed on each other. This line graph is human mortality. You can see birth weight across here on the x-axis in pounds from 1 to 11 pounds. And here's the mortality on the right side on the y-axis. As you might have guessed, we have low mortality in the mid-ranges. If you're 5, 6, 7, 8, 9 pounds, you have low mortality. But very large babies... are going to have higher mortality. They're going to have a difficult birth. And of course the premature babies, the 2, 3, 4, 5 pound babies, are going to have more problems at birth, more likely to have higher mortality than those in the 6, 7, 8 pound range. The actual range of human birth weight, which is the bar graph represented by the scale on the left side. The mean birth weight is right around 7 pounds. The optimum birth weight may be about 7.5 pounds. And this is the curve of human birth weight. So the selection is against the extremes and for the mid-range values. And finally, diversifying selection selects against the mid-range values and for the extremes. And diversifying selection tends to divide the population into extreme values. In these birds, this black-bellied seedcrackers, you'll see this is the selection, this is probability of survival graphed against the mandible width, the width of the bill. There are two different size seeds in the population. So the birds are well adapted for either the small seeds, where they have the narrower bill, or the large seeds, where they have the wider bill. There aren't any seeds in the mid-range. If you have a mid-range bill, you're not very well adapted for either the small seeds or the large seeds. The birds tend to either fall into the... Narrow bill category or the wide bill category. So we have diversifying selection. If you're in the middle, you don't have much to eat. You want to eat either the small seeds or the large seeds. And so selection favors the extremes rather than the mid-values.

It could be any number of, anything that could be measured along the x-axis. And on the y-axis is the frequency of that trait in the population, how many organisms in the population have that particular phenotype. If we have directional selection, the selection is going in one direction. We have in this case, represented by the dark color, we have selection against the organisms at this end of the distribution.

These organisms are not reproducing as efficiently as the organisms at this end of the distribution. And as a result, over time, over generations, the population is going to change, such that that the mean is going to shift to the right, to whatever this is. If this is the height of the organisms, they're going to get taller. We're selecting against the shorter organisms.

We're selecting for the taller organisms. This is artificial selection, and it's directional selection for bristle number in Drosophila. And these are fruit flies, of course, and this is the mean abdominal bristle number. Most fruit flies are around 10 bristles. We are artificially selecting for additional bristles.

We are selecting for the flies that have additional bristles up to 90 generations or so. We are selecting the fruit flies that have more bristles than the others. And we continue to increase artificially the number of bristles. So this is directional selection.

We're getting more and more bristles. And this is more bristles. than the flies would normally have in nature.

So we're selecting directionally. Then we stop the artificial selection at this point. We let natural selection take over. And after 90 generations, natural selection takes over.

And directional selection now is going in the other direction, natural selection. So that's too many bristles. And for whatever reason, The flies with that many bristles, with 40-45 bristles, are selected against, and the average number of bristles on the flies goes down, and it will probably continue to go down until it gets back down around 10. So that's an example of directional selection. Another example is found in ground finches. And in these ground finches, the birds that have the smaller bells, feed most efficiently on small soft seeds.

The birds with the larger bills feed efficiently on the large hard seeds. And these are birds from the Galapagos Islands. These are the finches that Darwin saw on his voyage, but this is a more recent study obviously in the 1970s. In this study, the size of the bills was graphed over time. And at this point in 1977 there was a period of drought and the period of drought changed the seeds that were available.

And because the seeds changed as a result the finch population changed during that time to adapt to the changing conditions because there were different kinds of seeds available. And so... Birds that were better adapted to the seeds that were available after the drought became more prevalent in the population.

Again, directional selection going towards the... larger sized bill. Stabilizing selection is most common.

Stabilizing selection selects for the mid-range values. When organism is well adapted to its environment and the environment isn't changing, the extremes are selected against. And so here you see the darker colored brown, whatever this is, the mid-sized or mid-values, the mid-range phenotype. are best adapted and if you get too far one way or the other that's going to be selected against.

The effect of the stabilizing selection is that the curve is going to be narrowed. It's going to stabilize around the best fit value. And here's an example of that.

This is human birth weight and there's two graphs here superimposed on each other. This line graph is human mortality. You can see birth weight across here on the x-axis in pounds from 1 to 11 pounds. And here's the mortality on the right side on the y-axis.

As you might have guessed, we have low mortality in the mid-ranges. If you're 5, 6, 7, 8, 9 pounds, you have low mortality. But very large babies... are going to have higher mortality. They're going to have a difficult birth.

And of course the premature babies, the 2, 3, 4, 5 pound babies, are going to have more problems at birth, more likely to have higher mortality than those in the 6, 7, 8 pound range. The actual range of human birth weight, which is the bar graph represented by the scale on the left side. The mean birth weight is right around 7 pounds. The optimum birth weight may be about 7.5 pounds. And this is the curve of human birth weight.

So the selection is against the extremes and for the mid-range values. And finally, diversifying selection selects against the mid-range values and for the extremes. And diversifying selection tends to divide the population into extreme values.

In these birds, this black-bellied seedcrackers, you'll see this is the selection, this is probability of survival graphed against the mandible width, the width of the bill. There are two different size seeds in the population. So the birds are well adapted for either the small seeds, where they have the narrower bill, or the large seeds, where they have the wider bill.

There aren't any seeds in the mid-range. If you have a mid-range bill, you're not very well adapted for either the small seeds or the large seeds. The birds tend to either fall into the... Narrow bill category or the wide bill category. So we have diversifying selection.

If you're in the middle, you don't have much to eat. You want to eat either the small seeds or the large seeds. And so selection favors the extremes rather than the mid-values.

Transcript for:Understanding Selection and Evolutionary Traits

Transcript for:
Understanding Selection and Evolutionary Traits