Fossilization

How fossils form. An organism or evidence of it gets buried in sediments or volcanic ash; water and dissolved inorganic compounds infiltrate it; then chemical changes and pressure from accumulating sediments above transform it to stony hardness.

Asteroid

Rocky, metallic body, a few to 1,000 kilometers across, hurtling through space.

Analogous Structures

Body parts that once differed in evolutionarily distant lineages but converged in structure and function as responses to similar environmental pressures.

Stabalizing Selection

Mode of natural selection by which intermediate phenotypes in the range of variation are favored and extremes at both ends are eliminated.

Sexual Selection

A microevolutionary process; a type of selection that favors a trait giving an individual a competitive edge in attracting or keeping a mate (favors reproductive success).

Population

All individuals of the same species that are occupying a specified area.

Polymorphism

The persistence of two or more qualitatively different forms of a trait (morphs) in a population.

Natural Selection

Microevolutionary process; the outcome of differences in survival and reproduction among individuals that differ in details of heritable traits.

Lethal Mutation

Mutation with drastic effects on phenotype; usually causes death.

Inbreeding

Nonrandom mating among close relatives that share many identical alleles.

Genetic Equilibrium

In theory, a state in which a population is not evolving. Compare Hardy-Weinberg rule.

Genetic Drift

Change in allele frequencies over the generations due to chance alone. Its effect is most pronounced in very small populations.

Gene Pool

All genotypes in a population.

Gene Flow

Microevolutionary process; alleles enter and leave a population as an outcome of immigration and emigration, respectively.