Hi. In this video, we're going to talk about genetic variation in populations and different types of natural selection. Now, even though the goal is usually to increase genetic variation because that tends to help populations, sometimes, there are circumstances that actually decrease the genetic variation in a population, and inbreeding is a great example of this. Inbreeding is mating between relatives, and it tends to increase homozygosity in the population. This can also lead to something called inbreeding depression, which is a decline in the average fitness as homozygosity increases and heterozygosity decreases. Now, homozygosity increasing would mean that there is less genetic variation, right? Because that's going to mean that one allele, in particular, is going to be pretty dominant in the population. Dominant, not necessarily in terms of whether it's dominant or recessive, but dominant in the sense of it's going to be prevalent, it's going to be widespread in our population. So here, we have a nice model of a pedigree for Shetland Ponies. And you will see that there is the capital A dominant allele, this lowercase a a recessive allele, and over generations when you have inbreeding, that tends to result in more homozygous individuals whereas outbreeding tends to result in more heterozygous individuals. Now, it is advantageous, generally, to have more heterozygous individuals and to increase the genetic variation in a population. Homozygosity tends to decrease fitness in the long run. Now there are exceptions to this, I'm just talking about general trends here. And inbreeding is why it's so difficult to bring back species that are on the brink of extinction. Because their populations are so small it's really difficult to avoid inbreeding. So generally, when trying to revive those species, those endangered species, inbreeding happens, and the overall fitness of those to face to face.
Now, sexual selection is a type of natural selection. That's where some individuals outcompete others because they're better at securing mates. And often, this can pertain to sexual dimorphism, which is a phenotypic difference between the males and females of a species. And you're probably very familiar with this in terms of bird mating, which is a great example of intersexual selection, which is selection between the sexes, usually when a female chooses a male. And many birds display a great degree of sexual dimorphism or a great degree of variation between the appearance of the males and females. And usually, this comes in the form of males having very decorative, perhaps a kind of lavish or grandiose appearance, that they use to attract mates. So here, you see an example of the peacock with its very large impressive tail feathers and the, no offense, rather unimpressive peahen, the female of the species. Peacocks and peahens have a, use a form of intersexual selection. The males display their impressive feathers, and the females decide whether or not they're impressed.
Now, you can also have intrasexual selection. An intrasexual selection is selection within a sex, and this is usually in the form of males competing with one another for mating rights. Like you see these two elephant seals right here that are squaring off for a fight, they're competing for mating rights. Now, the interesting thing about intersexual selection, not to backtrack too much, is that intersexual selection can actually result in some kind of funny appearances for organisms. So you might think peacock's feathers are kind of lavish and grandiose, and guess what? They are. They're a little ridiculous. Right? Why does a bird need such crazy tail feathers? Well, the reason is that the females tend to pick males who have the most impressive display of plumage. Meaning, that over generations, the males of the species that are going to be producing the most offspring because they're going to be securing the most mates, are those with the most impressive tail feathers. Meaning, that over generations, the males of the species are going to, because of this intersexual selection, they're going to, the alleles in the population are going to, the alleles for more impressive tail feathers are going to increase. So over generations, you're going to have a larger and larger percentage of males who have crazier and crazier tail feathers, and this can actually go awry. This can actually go awry for organisms. There's an example of a particular type of extinct deer who, scientists think, mating was a type of intersexual selection for this organism and these deer, their antlers grew so big. They grew bigger and bigger and bigger because the females, they picked the males with big antlers. So over generations, the males evolved to have bigger and bigger antlers to try to compete for those female mates, and it got to the point where their antlers got so ridiculously big that it was actually very hard for these organisms to even lift their necks. It basically, the intersexual selection pressures made, drove this species to extinction because the males of the species eventually ended up with antlers that were so big they couldn't even function properly, they couldn't live normal lives. So, it just goes to show that sexual selection isn't always driving organisms to be more fit. Sometimes it actually hurts the fitness of those organisms. Whereas intrasexual selection tends to favor males, you know, again, we're just speaking generally now, it tends to favor males who are more fit, who are bigger, stronger, more bad, scary. Right? The guys who are going to win the fight. Now, let's flip the page and talk about some other types of natural selection.