Fundamentals exam3 Pt 2

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 advantage (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

BIO 265: Fundamentals of Evolution
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