There are four types of natural selection: stabilizing, directional, disruptive, and frequency-dependent. Each type of selection can act on different levels within a population, from the gene to the entire species.
Stabilizing selection is a type of natural selection that results in the stabilization of a particular non-extreme trait value in a population. This is thought to be the most common mechanism of action for natural selection, as most traits do not appear to change drastically over time. Stabilizing selection typically uses negative selection to select against extreme values of the character.
Stabilizing selection can act on traits like plant height. If a plant is too short, it may not be able to compete for sunlight. However, if a plant is too tall, it may be more susceptible to wind damage. Stabilizing selection leads to an increase in the number of plants of medium height, and a decrease in the number of short and tall plants.
Directional selection is a type of natural selection where an extreme phenotype is favored over other phenotypes. This causes the allele frequency (gene frequency) to shift over time in the direction of that phenotype. The advantageous allele increases as a result of differences in survival and reproduction among different phenotypes.
The beak size in a population of finches is an example of how the environment can affect future generations. The depth of the birds’ beaks ranges from large and tough to small and smooth, depending on what they eat.
During wet years, when small seeds are more common, the finches usually eat more large seeds. However, during dry years, when none of the seeds are in great abundance, the birds usually eat more large seeds. This change in diet affects the depth of the birds’ beaks in future generations.
Disruptive selection occurs when extreme values for a trait are favored over intermediate values. This results in an increased variance of the trait and the population being divided into two distinct groups. In this scenario, more individuals acquire peripheral character values at both ends of the distribution curve.
Frequency-dependent selection is a type of natural selection where the fitness of a phenotype depends on how common it is in the population. This can lead to stable polymorphism, where multiple phenotypes are maintained in a population at equilibrium because no one phenotype has an advantage over the others.
One example of frequency-dependent selection is the “snowball effect” seen in some predator-prey relationships. If all the individuals in a population of prey are the same color, then predators can easily find and eat them. However, if there is genetic variation for coloration in the prey population, then those individuals who are less common colors (and therefore more camouflaged) will be less likely to be eaten by predators. This gives them a fitness advantage, which leads to an increase in their frequency over time. As they become more common, they become easier for predators to find and their advantage decreases until they reach equilibrium again with the other phenotypes in the population.