Balancing selection is when no one allele is favored over the other, and this can sometimes be due to what's called the heterozygous advantage, where heterozygous individuals actually have an advantage over homozygous ones. Now a great example of this is with a condition called sickle cell anemia. Sickle cell anemia is caused by having 2 copies of a recessive allele for a particular gene. Now, you would think that because this disease is very harmful to a person's health that these alleles would be selected against. That this disease would become very rare in the human population. And that's actually the case in many parts of the world, but notably, in parts of the world where malaria, the mosquito-borne disease, is prevalent, the presence of this allele is much more common, and that's because of the heterozygous advantage. It turns out that people who are homozygous dominant have a disadvantage to those who are heterozygous. Heterozygotes have both copies of the allele, and their blood cells look like this. They have a mix of sickle cells and normal cells. So it turns out that these individuals fare better with malaria because the sickle cells give them a sort of natural defense to the disease, whereas homozygous dominant individuals do not have that same advantage and they are far more susceptible to the disease malaria. So that is the heterozygous advantage illustrated through sickle cell anemia.
Now balancing selection can also occur due to frequency dependent selection, which is when certain alleles are favored only when they are rare, meaning that certain alleles are favored when they are uncommon in the population. An example of this is the alleles for coloration in guppies. Yes, I know, strange example, but particular alleles that give guppies a rare coloration, a color that's not common in the population, gives them an advantage when it comes to predation. You see, predators tend to learn to recognize the familiar shapes and colors of their prey. So, guppies that have the common coloration, the common allele in the population are not favored. But the alleles that give rare colorations, those are favored. However, if those alleles for rare coloration become more common in the population, then they become less favored. So frequency dependent selection is when certain alleles are favored when they are rare and only when they are rare.
Ecological selection is natural selection without any sexual selection. So essentially, it's when only ecological influences affect selection because sexual selection is out of the equation, meaning that we have relatively random mating. That means that only ecological factors are going to affect the natural selection process. So gene flow is something we talked about in the Hardy-Weinberg model, and this is the transfer of alleles from one population to another, potentially altering gene frequencies. And you have a nice example of that here. Two populations of the same species of birds that are geographically separated by this mountain, but you have a little gene flow, you have one of these red birds mate with someone from the blue bird population and sneak its little h allele into that population. You have one of these blue birds mate with a red bird and sneak its capital H allele into the red bird population, and that is gene flow, the transfer of alleles from one population to another. Now, let's turn the page and talk about some other influences on genetic variation.