Stabilizing selection removes extreme variants, as they do not provide a survival advantage in a given environment, and increases the frequency of the intermediate phenotype. For example, the bone density of a species of bird is likely to experience stabilizing selection. Bones that are too dense will inhibit the bird's ability to fly, and bones that are too light will be brittle and prone to injury. Stabilizing selection moderates the influence of these two factors and selects for the intermediate phenotype that is neither too heavy, nor too weak.

Disruptive selection shows an increase the in the frequency of extreme traits and a decline in the intermediate trait. For example, if two extremes are white and black coloration, disruptive selection will act against grey coloration and favor both white and back. Directional selection favors only one extreme, for example favoring black over grey and grey over white. Artificial selection occurs when humans interfere with breeding habits to promote the inheritance of a specific trait.

In this example the extreme beak sizes are favored over the mean; however the mean beak size does not change. There is selection against the mean, without affecting the value of the mean. This type of selection is known as disruptive selection.

This relationship displays a frequency-dependent selection. Frequency-dependent selection occurs when the fitness of a phenotype depends on how common it is in the population. When blue-bellied lizards are preyed on the most because they are most abundant, the yellow-bellied lizard population grows quickly. However, as the blue-bellied lizard population declines, predators will begin to prey on the yellow-bellied lizards. As the yellow-bellied lizard population declines, the blue-bellied lizard population will increase, and so on. The number of blue or yellow-bellied lizards depends on the relative amount of lizards of a different phenotype, demonstrating frequency-dependent selection.  

Mutations are changes to a cell’s genome and include inversions, duplications, translocations, and deletions, among other unplanned changes to the DNA. Mutations can be harmful to the organism, but can also have beneficial or neutral impacts. Mutation is an important means of evolution because it introduces new genetic combinations into a genome, allowing for the potential of new functions. 

Mutations are changes in a cell’s genomic DNA sequence. Types of mutation include insertion, deletion, translocation, inversion, and duplication. An insertion is the addition of a nucleotide somewhere in the DNA; a deletion is the opposite. A translocation mutation involves the physical movement of DNA segments, and can occur within a chromosome or between different chromosomes. Genetic drift, on the other hand, is a type of sampling error that leads to the perceived allele frequency changing among generations.

Horizontal gene transfer is a method of transferring genetic material from one organism to another and is common among bacteria. Horizontal gene transfer is an example of gene flow, a mechanism of evolution that involves the flow of genes among populations, species, and organisms. Natural selection involves uneven survival and evolutionary fitness due to the advantages granted to certain individuals through genetic variation. A mutation is an unplanned change in the DNA sequence of a cell or organism. Genetic drift is the change in an allele frequency in a population due to random sampling.

Stabilizing selection is a type of selection that decreases genetic diversity and stabilizes an optimum trait. In this situation, there is selection against extreme phenotypes. It is the opposite of disruptive selection and similar to purifying selection.

Directional selection is a type of natural selection in which one extreme phenotype is selectively advantageous. In other words, individuals with this trait are more likely to be reproductively successful. In this type of selection, one advantageous allele increases in frequency over time.

Disruptive selection is a type of natural selection in which the extremes of a phenotype are favored. This means that there is active selection against intermediates. It increases genetic diversity and is the opposite of stabilizing selection.

Positive selection is a type of natural selection in which the frequency of an allele increases.