Chap 23.3 & 23.4

Frequency-Dependent Selection

A decline in the reproductive success of a morph resulting from the morph's phenotype becoming too common in a population; a cause of balanced polymorphism in populations.

Intrasexual Selection

A direct competition among individuals of one sex (usually the males in vertebrates) for mates of the opposite sex.

Sexual Selection

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

Sexual Dimorphism

A special case of polymorphism based on the distinction between the secondary sex characteristics not directly associated with reproduction or survival.

Natural Selection

Alleles passed to next generation in proportions different than from present generation, process by which individuals that are better suited to their environment survive and reproduce most. successfully; also called survival of the fittest

Bottleneck Effect

Changes in the gene pool caused by a rapid reduction in population size. Reduces genetic variability.

Stabilizing Selection

Form of natural selection by which the center of the curve remains in its current position; occurs when individuals near the center of a distribution curve have higher fitness than individuals at either end.

Disruptive Selection

Form of natural selection in which a single curve splits into two; occurs when individuals at the upper and lower ends of a distribution curve have higher fitness than individuals near the middle.

Directional Selection

Form of natural selection in which the entire curve moves; occurs when individuals at one end of a distribution curve have higher fitness than individuals in the middle or at the other end of the curve.

Neutral Variation

Genetic variation that does not appear to provide a selective advantage or disadvantage.

Heterozygote Advantage

If individuals who are heterozygous at a particular gene locus have greater fitness than the homozygous, natural selection will tend to maintain two or more alleles at that locus, sickle-cell anemia is an example: survival of heterozygotes out-weighs death in homozygotes.

Gene Flow

Movement of alleles into or out of a population due to the migration of individuals to or from the population. Reduces differences between populations over time.

Balancing Selection

Natural selection that maintains stable frequencies of two or more phenotypic forms in a population.

Diploidy

One of the factors that contribute to the preservation of genetic variation in population. 2 sets of chromosomes; eukaryotes can hide recessive alleles.

Intersexual Selection

Selection whereby individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex; also called mate choice.

Polymorphism

The coexistence of two or more distinct forms in the same population.

Relative Fitness

The contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals in the population.

Genetic Drift

Unpredictable allele change due to chance (founder effect and bottleneck effect)

Founder Effect

When a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool isn't reflective of the source population.