Natural selection is any consistent difference in fitness among different phenotypes or genotypes. Evolution caused by natural selection has been observed directly many times.
Fitness is the number of offspring that an individual leaves to the next generation, or the average number that an allele, genotype, or phenotype leaves. Selection causes evolution when there is a correlation between a phenotype and fitness, and a correlation for that phenotype between parents and offspring.
The rate at which a beneficial allele spreads through a population is determined by how strongly it is favored, measured by its selection coefficient, and by the amount of genetic variation at that locus in the population. Even if it is favored by selection, a new beneficial mutation can be lost by chance when it is still rare.
An allele that has no effect on fitness can spread if it is associated (in linkage disequilibrium) with an allele at another locus that is favored by selection. One consequence is that a selective sweep reduces genetic variation in the region of chromosome near the selected locus.
Several kinds of selection can act to maintain genetic variation. One is overdominance, the situation in which heterozygotes have highest fitness. Other kinds are negative frequency dependence, multiple niche polymorphism, and spatial variation in selection.
Positive frequency-dependent selection occurs when fitness increases with the frequency of genotypes or phenotypes in a population. Unlike balancing selection, this situation eliminates variation. Which allele becomes fixed depends on the initial allele frequency.
With underdominance, heterozygotes have lower fitness than both homozygotes. Underdominance does not preserve genetic variation, and one of the alleles will either be lost or spread to fixation, depending on its initial frequency. An allele that is underdominant can spread when rare only if some evolutionary factor other than selection is at work.
The mean fitness of a population evolves as allele frequencies change. Fisher’s fundamental theorem of natural selection states that selection causes the mean fitness to increase. In Wright’s adaptive landscape, allele frequencies change in the direction that increases mean fitness. These conclusions hold only under certain conditions. Evolution can cause a population’s mean fitness to decrease when fitnesses are frequency-dependent.
Deleterious mutations occur frequently. They are maintained in populations by mutation even though selection acts to remove them. Their combined effects across the entire genome contribute to senescence.