Evolution - Exam 3 - Lecture 9

directional selection

A mode of selection in which one extreme phenotype has the highest fitness

stabilizing (or normalizing) selection

A mode of selection in which the intermediate phenotype is most fit

disruptive (or diversifying) selection

A mode of selection in which two or more phenotypes are more fit that the intermediates

absolute fitness

The per capita growth rate of each phenotype

relative fitness

The fitness of a genotype relative to that of a reference genotype (i.e., to the genotype with the highest relative fitness)

mean fitness

The average fitness of individuals in a population relative to the fittest genotype

coefficient of selection

The amount by which the fitness of a genotype is different from the reference genotype; abbreviated as (s)

purifying selection

Selection that lowers the frequency of (or eliminates entirely) a selectively disadvantageous allele

balancing selection

Maintenance of polymorphism by natural selection

heterozygote disadvantage ( or overdominance)

When a heterozygote has higher fitness than either homozygote

heterozygote disadvantage ( or underdominance)

Occurs when the heterozygote has lower fitness than either homozygote

antagonistic selection

Opposing selective forces; may or may not maintain polymorphism in a population; may result from a temporally or spatially fluctuating environment

extended phenotype

A phenotype that is not limited to the individual body in which a gene is housed (e.g., a termite mound that houses a termite colony, a beaver dam, protective cases made
by caddisflies)

frequency-dependent selection

A type of balancing selection in which the fitness of a phenotype depends on its frequency relative to other phenotypes in a given population

inverse (or negative) frequency-dependent selection

The rarer a phenotype, the greater its fitness

positive frequency-dependent selection

The fitness of a genotype is greater the more frequent it is in a population

adaptive landscape

A metaphor for the relationship between mean fitness of a population and the allele frequencies at one or more loci that affect fitness; possible populations with allele
frequencies that maximize mean fitness are represented as peaks on the metaphorical landscape

adaptive peak

The allele frequency, or combination of allele frequencies at two more loci, at which the mean fitness of a population has a local maximum; the mean phenotype for one or more characters that maximizes mean fitness

adaptive valley

A set of allele frequencies at which mean fitness is at a minimum

shifting balance theory

Adaptive evolution may proceed quickly when a population divides into subpopulations with limited gene flow; explains how populations can move through an adaptive valley from one peak to another