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Stabilizing selection is a type of natural selection that favors the average or intermediate phenotypes in a population, effectively reducing the number of individuals with extreme traits. This kind of selection decreases genetic variance because it weeds out the extremes, leading to a more uniform population.
For example, if you have a population of birds with varying wing lengths, stabilizing selection might favor birds with medium-length wings because they are most efficient for flying. Birds with extremely short or long wings might be less efficient and, thus, less likely to survive and reproduce.
Understand that while stabilizing selection makes a population more adapted to its environment by favoring the 'average' features, it leads to reduced variance. This means less genetic diversity in the population, making it susceptible to changes in the environment.

Directional selection occurs when one extreme phenotype is favored over other phenotypes, causing a shift in the population’s trait distribution towards that extreme. Unlike stabilizing selection, directional selection changes the average value of a trait over time.
Imagine a scenario with a population of rabbits where those with the longest legs can run fastest and escape predators more effectively. If predators are a significant selective pressure, then over generations, the population will shift towards longer-legged rabbits.
While this kind of selection can lead to significant changes in the average trait in a population, it doesn’t necessarily increase overall genetic variance. Instead, it reduces variance at one end of the trait spectrum while potentially increasing it at the other.

Diversifying selection, also known as disruptive selection, favors multiple extreme phenotypes, effectively increasing genetic variance within a population. This means that individuals with traits at both ends of the spectrum are selected over those with average traits.
Take a population of fish, for example, living in an environment with two distinct types of habitats. Small fish might be able to hide in very tight spaces, while large fish might be better at competing for resources in open areas. Medium-sized fish may not fare as well in either habitat type, leading to a selection for both small and large fish.
This type of selection maintains or increases genetic variance because it leads to a greater variety of phenotypes and genotypes within the population.

Positive frequency-dependent selection is a type of natural selection that favors phenotypes that are more common within a population. Unlike the other forms of selection, this one doesn’t directly increase genetic variance but rather maintains it based on common traits.
For instance, in a population of a particular type of butterfly, those with the most common wing pattern might be less likely to be preyed upon by birds who avoid that pattern due to past negative experiences. This reinforces the prevalence of that wing pattern in the population.
While this type of selection can maintain current genetic variance by favoring common traits, it doesn’t contribute to an increase in genetic diversity. Instead, it can lead to a decrease in genetic variance if it strongly favors a particular trait.