allele       (uh-LEEL)
An alternative form of a gene.

biotic       (by-OT-ik)
Pertaining to the living organisms in the environment.

Pertaining to nonliving things.

directional selection
Natural selection that favors individuals on one end of the phenotypic range.

diversifying selection
Natural selection that favors extreme over intermediate phenotypes.

All the changes that have transformed life on Earth from its earliest beginnings to the diversity that characterizes it today.

fitness (Darwinian fitness)
A measure of the relative contribution of an individual to the gene pool of the next generation.

One of many discrete units of hereditary information located on the chromosomes and consisting of DNA.

gene flow
The loss or gain of alleles from a population due to the emigration or immigration of fertile individuals, or the transfer of gametes, between populations.

gene pool
The total aggregate of genes in a population at any one time.

genotype       (JEE-noh-type)
The genetic makeup of an organism.

mutation       (myoo-TAY-shun)
A rare change in the DNA of genes that ultimately creates genetic diversity.

natural selection
Differential success in the reproduction of different phenotypes resulting from the interaction of organisms with their environment. Evolution occurs when natural selection causes changes in relative frequencies of alleles in the gene pool.

phenotype       (FEE-nuh-type)
The physical and physiological traits of an organism.

stabilizing selection
Natural selection that favors intermediate variants by acting against extreme phenotypes.