Natural selection influences the frequencies of particular alleles and phenotypes within populations in several different ways. Primarily, natural selection can be directional, stabilizing, or disruptive. Directional selection favors one extreme trait and shifts the population towards that phenotype while selecting against individuals displaying alternate traits. Stabilizing selection favors an intermediate trait with a narrow range of variation. Deviation from the optimal phenotype towards an extreme one is unfavorable. Finally, disruptive selection favors both extremes of a phenotype, while intermediate phenotypes are selected against.
Directional selection favors one extreme of a phenotype. For example, in sockeye salmon, research has shown that directional selection is favoring seasonally earlier migration. This is thought to be due to predation pressure from fisheries, as fishing increases later in the migration season. Thus, fish arriving and spawning earlier may have a better chance of reaching their destination to reproduce before being caught by fishermen.
When a particular non-extreme phenotype is favored, this is referred to as stabilizing selection. For example, across many species of birds, clutch size (the number of eggs in a single brood) is kept within an optimal window. Lapwings and golden plovers typically lay four eggs. This optimization is a trade-off between keeping the clutch size low enough to ensure enough resources to feed all the chicks and having enough chicks to ensure that at least some survive to adulthood. This is a common theme among bird species.
In some scenarios, two extremes of a trait may be more favorable in the environment than an intermediate trait. The African black-bellied seedcracker (Pyrenestes ostrinus) displays an impressive polymorphism for beak size that is not determined by sex, body size, age or geographic origin. Two major distinct morphs exist, small-billed and large-billed. This trait is controlled by a single autosomal locus, with large bills being dominant. These two distinct bill morphologies allow the seedcrackers to easily eat the seeds of different sedge grasses. The small-billed seedcrackers primarily eat sedge species with softer seeds, whereas the large-billed birds can crack the harder seeds of other species of sedge. However, birds with bills of intermediate sizes cannot easily eat either type and are thus rarely seen.
