Ivory from elephant tusks is a valuable commodity on the world market. As a result, male African elephants with large tusks have been heavily hunted for the past few centuries. Today, male elephants have significantly shorter tusks at full adulthood than male elephants in the early 1900s. This is an example of                  . a. diversifying selection; b. stabilizing selection; c. directional selection; d. chance; e. more than one of the above is correct

Video transcript

Hi everyone. Let's look at our next problem. The environment created selection pressure that favored giraffes with longer necks that could reach more food in the trees over those with shorter necks. This is a typical example of which of the following. And we have four types of selection stabilizing directional disruptive and balancing. So let's think about what kind of selection pressure this is and what it's an example of. So I'm just going to put up a brief little quick little graph here imagining our scenario. And we've got a situation. Let's imagine we had some sort of, you know, beginning population of giraffes and we graft a number of giraffes versus their neck length. And let's say we started out with some sort of bell curve um of neck lengths where you had the two extremes. Very short necks, very long necks. Well in this case our selection pressure, which is that giraffes with longer necks can reach more food and therefore more likely to survive and have more offspring. So this favors the extreme phenotype of long necks over anything shorter. So over time we see our curve will shift where we'll have more individuals with the long necks and fewer individuals with average or short necks. So our selection pressure was in just one direction towards long necks favoring that extreme phenotype on that side. So with that in mind, let's look at our four answer choices and see which one describes our giraffe situation choice a says stabilizing selection. One stabilizing selection. Extreme phenotype sis are selected against and therefore their numbers go down average phenotype sis selections in favor of them. And so we see their numbers go up. So that's when you see um a disadvantage to phenotype at either end of the distribution. And there's a loss of genetic diversity because the majority of the population is showing sort of similar average phenotype with the extreme ones at either end being their numbers going down. So that's not what we have here. Because we're selecting in favor of the one extreme phenotype of long neck. So choice A. Is not our correct answer. Choice B. Says directional selection. And in directional selection as you might guess by the word direction we see china types of the population shift in one direction usually favoring an extreme phenotype. And that's exactly what we have here. The phenotype shifting in the direction of longer necks. So that's our correct answer here. Choice. Bi directional selection is what's happening with the draft neck link. But to be thorough, let's look at our remaining two choices we have. See disruptive selection disruptive is the opposite of stabilizing. That's when you have extreme phenotype sis are being selected for and therefore their numbers go up. The numbers of individual. Extreme phenotype sis and there's a disadvantage to the average phenotype. So the numbers of individuals, the average phenotype decrease. So let's imagine a population of birds of different sizes and under disruptive selection, the very small birds and the very large birds have an advantage. But the birds with sizes somewhere in the middle are at a disadvantage. So we see the growth of the population of very small and very large. With a decrease in the numbers of those birds in the middle. That's obviously not what's happening here. Because we're only favoring one extreme, not both. So Choice C. Not a correct answer. And finally, choice d. balancing selection is when no one a little is favored over another. Well, that's obviously not the case here because we are favoring the longer neck. So balancing selection judy is not our answer. So again, when we say what is it? This is an example of when we have a selection pressure favoring drafts with longer necks that would be bi directional selection. See you in the next