Final answer:
Selection in biology, determined by fitness, leads to increased frequency of advantageous genotypes and decreased frequency of disadvantageous ones, potentially leading to allele fixation.
Step-by-step explanation:
All forms of selection involve individuals with inherited genetic differences competing within the same population. Success (selection) is determined by fitness, which includes at a minimum, the individual's ability to survive and reproduce, thereby passing on alleles to the next generation. When a particular genotype is positively selected, this genotype will increase in frequency over time. When a genotype is disadvantageous to fitness, this is called negative selection because the genotype will be selected against and thus decrease in frequency over time. Another term used in these scenarios is "fixation," which means that a particular set of alleles that give rise to a particular phenotype are the only ones present in the population (all others have been selected out of the population).
Selection can affect the population in different ways, such as stabilizing selection, which favors average phenotypes, directional selection, which favors phenotypes at one end of the spectrum, and diversifying selection, which favors extreme phenotypes at both ends of the spectrum. Frequency-dependent selection includes both positive frequency-dependent, favoring common phenotypes, and negative frequency-dependent selection, favoring rare phenotypes. All these mechanisms can influence how allele frequencies, and thus genetic variance, change in a population, reflecting the population's genetic structure.