Why is selection on allele frequency often more complex than simple “favored allele increases”?
Because alleles rarely affect a single trait independently
(pleiotropy, epistasis, codominance, polygenetic)
Pleiotropy
one gene influences multiple phenotypic traits. Selection on one trait can indirectly alter others.
Antagonistic pleiotropy
when a mutation with beneficial effects for one trait also causes detrimental effects on other traits
(mosquito example and drought resistant plant example)
Epistasis
when an unrelated gene modifies the phenotypic expression of another SINGLE DINGLE gene
Define additive effects (additive traits).
the cumulative, independent contributions of multiple genes or alleles to a trait’s phenotype. like height
Heterozygotes are intermediate.
frequency-dependent selection
Fitness depends on how common or rare a phenotype is.
(purple and yellow flowers)
Negative frequency-dependent selection
Rare phenotypes have higher fitness → maintains variation
Positive frequency-dependent selection
Common phenotypes favored → reduces variation, drives fixation.
How do genetic drift and selection interact in diff population sizes?
In large populations, selection dominates; allele frequency change reflects fitness differences.
In small populations, genetic drift (random sampling error) can overpower selection, even fixing deleterious alleles.
Why can selection fix recessive and additive alleles but not dominant alleles easily?
Recessive beneficial alleles, hidden in heterozygotes, must be homozygous to be expressed
Additive alleles, additive alleles are always exposed to selection bc theyre expressed in heterozygotes and homozygotes
Dominant beneficial alleles hide recessive alleles in heterozygotes
how do recessive beneficial alleles become fixed
Slow initial increase (hidden in heterozygotes), but once common, selection efficiently drives them to fixation.
how do additive alleles become fixed
Always exposed to selection bc theyre expressed in both heterozygotes and homozygotes; increase steadily
do dominant beneficial alleles become fixed?
no
Increase quickly when rare, but recessive deleterious alleles persist in heterozygotes, preventing complete fixation.
heterozygous advantage
heterozygotes have higher fitness than either homozygote
over time, both alleles are maintained at a stable equilibrium frequency
average excess of fitness (a)
The difference between the average fitness of individuals carrying a particular allele and the mean fitness of the population.
It predicts Δp, the change in allele frequency.
quantitative trait
measurable phenotypes influenced by multiple genes (polygenic) and environmental factors, showing continuous variation within a population rather than distinct, “either-or” categories
Difference between categorical and continuous variation
Categorical variation: Discrete phenotypic classes (e.g., blood type).
Continuous variation: Gradual distribution (e.g., height), typically normally distributed.
How does continuous variation arise?
Polygenic inheritance
Additive genetic effects
Environmental variation (VE)
how does polygenic inheritance lead to continuous variation
polygenic traits are influenced by alleles at many loci
more loci=more phenotypes
Variance
average squared deviation from the mean
On a graph, greater variance = wider distribution curve.
VG
genetic variance
VE
environmental variance
How is phenotypic variance calculated?
VP=VG+VE
VA
variance from additive effects of genes
