Natural selection in populations
- Natural selection can cause microevolution (change in allele frequencies), with fitness-increasing alleles becoming more common in the population.
- Fitness is a measure of reproductive success (how many offspring an organism leaves in the next generation, relative to others in the group).
- Natural selection can act on traits determined by alternative alleles of a single gene, or on polygenic traits (traits determined by many genes).
- Natural selection on traits determined by multiple genes may take the form of stabilizing selection, directional selection, or disruptive selection.
Quick review of natural selection
- Organisms with heritable (genetically determined) features that help them survive and reproduce in a particular environment tend to leave more offspring than their peers.
- If this continues over generations, the heritable features that aid survival and reproduction will become more and more common in the population.
- The population will not only evolve (change in its genetic makeup and inherited traits), but will evolve in such a way that it becomes adapted, or better-suited, to its environment.
Natural selection can cause microevolution
Example: Rabbit coat color
Fitness = reproductive success
Fitness depends on the environment
Natural selection can act on traits controlled by many genes
How natural selection can shift phenotype distributions
- Stabilizing selection. In stabilizing selection, intermediate phenotypes are more fit than extreme ones. For example, medium-green beetles might be the best camouflaged, and thus survive best, on a forest floor covered by medium-green plants. Stabilizing selection tends to narrow the curve.
- Directional selection. One extreme phenotype is more fit than all the other phenotypes. For example, if the beetle population moves into a new environment with dark soil and vegetation, the dark green beetles might be better hidden and survive better than medium or light beetles. Directional selection shifts the curve towards the favorable phenotype.
- Disruptive selection. Both extreme phenotypes are more fit than those in the middle. For example, if the beetles move into a new environment with patches of light-green moss and dark-green shrubs, both light and dark beetles might be better hidden (and survive better) than medium-green beetles. Diversifying selection makes multiple peaks in the curve.
- Stabilizing selection: Intermediate phenotypes have the highest fitness, and the bell curve tends to narrow.
- Directional selection: One of the extreme phenotypes has the highest fitness. The bell curve shifts towards the more fit phenotype.
- Disruptive selection: Both extreme phenotypes have a higher fitness than intermediate phenotypes. The bell curve develops two peaks.