Natural Selection

natural selection

A process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates than other individuals because of those traits.

disruptive selection

favors individuals at both extremes of the phenotypic range

stabilizing selection

Natural selection that favors intermediate variants by acting against extreme phenotypes

directional selection

occurs when natural selection favors one of the extreme variations of a trait

gene flow

Movement of alleles into or out of a population due to the migration of individuals to or from the population

genetic drift

A change in the allele frequency of a population as a result of chance events rather than natural selection.

founder effect

genetic drift that occurs after a small number of individuals colonize a new area

bottleneck effect

Genetic drift resulting from the reduction of a population, typically by a natural disaster, such that the surviving population is no longer genetically representative of the original population.

Biochemical evidence of evolution

DNA or protein comparisons

Morphological evidence

evidence of evolution from the study of external features

analogous structures

structures that do not have a common evolutionary origin but are similar in function

prezygotic isolation

prevents individuals of different species from mating

behavioral isolation

isolation between populations due to differences in courtship or mating behavior

temporal isolation

When two species breed at different times of day, season, or years.

geographic isolation

isolation between populations due to physical barriers

habitat isolation

populations live in different habitats and do not meet

Mechanical isolation

Morphological differences prevent fertilization.

gamete isolation

Sperm of one species may not be able to fertilize eggs of another species

postzygotic isolation

reproductive isolation that occurs after members of two different species have mated and produced a hybrid offspring. Such hybrids are usually unable to reproduce

Hybrid viability

hybrid offspring do not develop normally and die as embryos

reduced hybrid fertility

Sterile hybrids due to uneven chromosome number.

hybrid breakdown

Hybrid is fertile, but when they breed the next generation is sterile.

sympatic speciation

occurs when a species evolves into a new species in an area without a geographic barrier

sexual selection

A form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates.

allopatric speciation

The formation of new species in populations that are geographically isolated from one another.