Chapter 21

Bio 101

Sexual dimorphism

Differences between the secondary sex characteristics of males and females of the same species.

Adaptive evolution

A process in which traits that enhance survival or reproduction tend to increase in frequency in a population over time.

Neutral variation

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

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.

Frequency-dependent selection

Selection in which the fitness of a phenotype depends on how common the phenotype is in a population.

Disruptive selection

Natural selection in which individuals on both extremes of a phenotypic range survive or reproduce more successfully than do individuals with intermediate phenotypes.

Sexual selection

A process in which individuals with certain inherited characteristics are more likely than other individuals of the same sex to obtain mates.

Gene pool

The aggregate of all copies of every type of allele at all loci in every individual in a population.

Bottleneck effect

Genetic drift that occurs when the size of a population is reduced, as by a natural disaster or human actions. Typically, the surviving population is no longer genetically representative of the original population.

Genetic variation

Differences among individuals in the composition of their genes or other DNA sequences.

Heterozygote advantage

Greater reproductive success of heterozygous individuals compared with homozygotes; tends to preserve variation in a gene pool.

Gene flow

The transfer of alleles from one population to another, resulting from the movement of fertile individuals or their gametes.

Microevolution

Evolutionary change below the species level; change in the allele frequencies in a population over generations.

Balancing selection

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

Genetic drift

A process in which chance events cause unpredictable fluctuations in allele frequencies from one generation to the next. Effects of genetic drift are most pronounced in small populations.

Population

A group of individuals of the same species that live in the same area and interbreed, producing fertile offspring.

Directional selection

Natural selection in which individuals at one end of the phenotypic range survive or reproduce more successfully than do other individuals.

Hardy-Weinberg equilibrium

The state of a population in which frequencies of alleles and genotypes remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.

Founder effect

Genetic drift that occurs when a few individuals become isolated from a larger population and form a new population whose gene pool composition is not reflective of that of the original population.

Stabilizing selection

Natural selection in which intermediate phenotypes survive or reproduce more successfully than do extreme phenotypes.