A physiological, morphological, or behavioral trait with an underlying genetic basis that enhances the survival and reproduction of its bearers in their environment.
adaptive evolution
A process of evolutionary change in which adaptive traits tend to increase in frequency in a population over time.
adaptive radiation
An event in which a group of organisms gives rise to many new species that expand into new habitats or new ecological roles in a relatively short time.
allele
One of two or more forms of a gene that result in the production of different versions of the protein that the gene encodes.
cline
A pattern of gradual change in a characteristic of an organism over a geographic region.
directional selection
Selection that favors individuals with one extreme of a heritable phenotypic trait. Compare disruptive selection, stabilizing selection.
disruptive selection
Selection that favors individuals with a phenotype at either extreme over those with an intermediate phenotype. Compare directional selection, stabilizing selection.
evolution
(1) Change in allele frequencies in a population over time. (2) Descent with modification; the process by which organisms gradually accumulate differences from their ancestors.
fixation
With respect to the genetic composition of a population, an allele frequency of 100%.
gene flow
The transfer of alleles from one population to another via the movement of individuals or gametes.
genetic drift
A process in which chance events determine which alleles are passed from one generation to the next, thereby causing allele frequencies to fluctuate randomly over time; the effects of genetic drift are most pronounced in small populations.
genotype
The genetic makeup of an individual.
mass extinction
An event in which a large proportion of Earth’s species are driven to extinction worldwide in a relatively short time.
mutation
Change in the DNA of a gene.
natural selection
The process by which individuals with certain heritable characteristics tend to survive and reproduce more successfully than individuals with other heritable characteristics.
phenotype
The observable characteristics of an organism.
speciation
The process by which one species splits into two or more species.
species
A population or group of populations whose members have similar characteristics and can interbreed with one another, but not with members of other such groups.
stabilizing selection
Selection that favors individuals with an intermediate phenotype. Compare directional selection, disruptive selection.
