BIOL 286: Lecture #3 Review (Mechanisms of Evolution & Selection)

fixation

In population genetics

Smaller populations are more at risk of genetic drift

and thus . . .

In the absence of . . . or . . .

any allele must eventually either be lost entirely from the population

Whether a gene will ultimately be lost or fixed is dependent on . . . and . . .

selection coefficients; chance fluctuations in allelic proportions (genetic drift)

The frequency of a beneficial but rare dominant allele may . . . than a beneficial but rare recessive allele during selection (in complete dominance).

increase much more rapidly

Explain how the frequency of a beneficial but rare dominant allele may increase much more rapidly than a beneficial but rare recessive allele during selection in complete dominance.

A rare beneficial dominant allele increases rapidly because its selective advantage is expressed in all genotypes carrying the allele (both heterozygotes and homozygotes). This endows an immediate positive selection coefficient across nearly all its occurrences. By contrast

census population (N)

The number of individuals in a given population

effective population size (Ne)

The subset of a total population that mates randomly

Any characteristic of the population that decreases random mating reduces . . .

the effective population size

How are the effects of genetic drift impacted by decreases in effective population size?

they may become more pronounced

Higher values of Ne indicate that there is . . . the population will lose genetic variation due to genetic drift

a lower chance

Ne can be a long-term predictor of . . .

the health of a population

Explain how effective population size (Ne) can serve as a long-term predictor of population health.

Effective population size (Ne) is a powerful long‑term predictor of population health because it reflects how many individuals are actually contributing genes to the next generation. Populations with small Ne experience stronger genetic drift

Factors that can reduce effective population size (Ne): (3)

(1) unequal sex ratio; (2) some individuals never reproduce; (3) not all individuals reproduce each year; (4) mating is not random (inbreeding

Selection occurs when . . .

some phenotypes are favored over others

artificial selection

A type of selection in which traits are selected for by humans (e.g. crops

natural selection

Organisms between adapted to the environment are more likely to have higher fitness and produce more offspring

For natural selection to take place: (3)

(1) There must be variation in the population; (2) The variation must be heritable; (3) The different phenotypes (and thus genotypes) have differential reproduction

quantitative traits

those traits that demonstrate continuous variation; they do not fit into discrete categories

Quantitative traits are often influenced by . . .

many genes and alleles interacting with the environment

polygenic trait

A characteristic that is influenced by two or more genes. Because multiple genes are involved

selection differential (S)

A measure of the strength of phenotypic selection; describes the difference between the mean of the reproducing members of the population who contribute offspring to the next generation and the mean of all members of a population

Selection differential equation: S = . . .

muS - mu

response to selection (R)

The extent to which a characteristic subjected to selection changes in a single generation (i.e. the change in the population trait value after selection)

Response to selection (R) predicts . . .

how the phenotypic value of the trait will change in a population over time

Response to selection equation: R = . . .

(h^2)*S

heritability

The proportion of variance among individuals in a trait that is attributable to differences in genotype

heritability coefficient (h^2)

A statistic describing the proportion of observed variance in a trait that may be attributed to genetic factors

How does the heritability of a given trait influence its responsiveness to natural selection?

Traits that have higher heritability respond to natural selection faster

Explain why traits with higher heritability respond to natural selection faster.

Traits with high heritability respond more rapidly to natural selection because a larger proportion of their phenotypic variation is attributable to additive genetic variance rather than environmental factors. As per the breeder's equation R = (h^2)*S

phenotypic plasticity

Occurs when one genotype can produce different phenotypes in different environments

genotype by environment interaction (GxE)

Occurs when the effect of a genotype on a phenotype depends on the environment

Distinguish between phenotypic plasticity and GxE interactions.

Phenotypic plasticity refers to the ability of a single genotype to produce different phenotypes across environments. It describes how one genetic background changes its expression when conditions shift. By contrast

Plasticity is . . . and it can be . . .

a trait; heritable

Phenotypic variation equation: Vp = . . .

Vg + Ve + Vgxe

Total phenotypic variation is the sum of the contributions of phenotypic variation due to . . .

. . .

reaction norm

The pattern of phenotypic expression of a single genotype across a range of environments