31.2
Types of Selection
Natural selection changes the frequencies of particular alleles and phenotypes within a population through directional, stabilizing, and disruptive selection.
In a typical population, most individuals have intermediate traits, while a few individuals have extreme traits. In directional selection, environmental or sexual pressures favor one extreme phenotype, causing the population mean to shift toward that trait.
Imagine a population of lizards living on an island with no competitors or predators.
In this population, females prefer bright, colorful, and fit males. This preference is called sexual selection.
As females mate more often with bright, colorful males, the frequency of this phenotype increases. Over time, the population shifts toward the bright phenotype. This is directional selection.
Now, consider a different situation. A predator arrives on the island, and bright, colorful males become easy prey.
At the same time, dull males survive but attract few females.
Because both extremes face disadvantages, males with intermediate color survive and reproduce more successfully. This is called stabilizing selection.
The last type of selection is disruptive selection, in which both extreme phenotypes are favored, and the intermediate phenotype is selected against.
In this lizard population, females prefer bright males, so these males mate more often.
At the same time, dull males that look similar to females avoid aggression from dominant males and gain mating opportunities without direct competition. Intermediate males do not have either advantage.
Because both bright and dull males reproduce successfully, their frequencies increase, while the frequency of intermediate males decrease.
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
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.
Stabilizing Selection
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.
Disruptive Selection
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.
Natural selection changes the frequencies of particular alleles and phenotypes within a population through directional, stabilizing, and disruptive selection.
In a typical population, most individuals have intermediate traits, while a few individuals have extreme traits. In directional selection, environmental or sexual pressures favor one extreme phenotype, causing the population mean to shift toward that trait.
Imagine a population of lizards living on an island with no competitors or predators.
In this population, females prefer bright, colorful, and fit males. This preference is called sexual selection.
As females mate more often with bright, colorful males, the frequency of this phenotype increases. Over time, the population shifts toward the bright phenotype. This is directional selection.
Now, consider a different situation. A predator arrives on the island, and bright, colorful males become easy prey.
At the same time, dull males survive but attract few females.
Because both extremes face disadvantages, males with intermediate color survive and reproduce more successfully. This is called stabilizing selection.
The last type of selection is disruptive selection, in which both extreme phenotypes are favored, and the intermediate phenotype is selected against.
In this lizard population, females prefer bright males, so these males mate more often.
At the same time, dull males that look similar to females avoid aggression from dominant males and gain mating opportunities without direct competition. Intermediate males do not have either advantage.
Because both bright and dull males reproduce successfully, their frequencies increase, while the frequency of intermediate males decrease.
From Chapter 31:
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31.2 : Types of Selection
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