Quantitative and Population Genetics

Vocabulary practice flashcards covering quantitative genetics, selection modes, Hardy-Weinberg equilibrium, genetic drift, bottlenecks, gene flow, and molecular signatures of adaptation.

Quantitative Trait

A continuously variable phenotypic characteristic that cannot be placed into discrete categories and must be measured on a numerical scale, with phenotypic variation governed by the collective action of many genes and environmental influences.

Broad-Sense Heritability ($H^2$)

The proportion of total phenotypic variation ($V_P$) within a population that is attributable to all genetic variation ($V_G$), calculated using the formula $H^2 = \frac{V_G}{V_P}$.

Narrow-Sense Heritability ($h^2$)

The proportion of total phenotypic variance ($V_P$) that is attributable specifically to additive genetic variance ($V_A$), calculated as $h^2 = \frac{V_A}{V_P}$; it equals the slope of the linear regression comparing mid-parent and offspring trait averages.

Common Garden Experiment

An experimental setup where individuals from distinct geographic populations or latitudes are raised under identical environmental conditions to determine whether observed phenotypic differences are genetically based rather than environmentally induced.

Reciprocal Transplant Experiment

An experimental approach in which organisms from two distinct environments are cross-planted in both home and foreign habitats to test for local adaptation by measuring whether native genotypes outperform foreign genotypes in their respective local environments.

Selection Differential ($S$)

A measure of the strength of selection, defined as the mathematical difference between the average trait value of reproducing breeders ($t^*$) and the average trait value of the entire parental population ($t$): $S = t^* - t$.

Cross-Fostering Experiment

An experimental technique in which newborn or hatched offspring are switched and raised by unrelated foster parents to decouple additive genetic inheritance from confounding shared environmental conditions.

Breeder's Equation

The quantitative genetic model $R = h^2 S$, which predicts the evolutionary response to selection ($R$) as the product of narrow-sense heritability ($h^2$) and the selection differential ($S$).

Selection Gradient

The slope of a regression line relating a quantitative trait directly to relative fitness, providing a continuous metric of selection strength that can analyze multiple traits simultaneously.

Directional Selection

A mode of selection that favors phenotypes at one extreme of a phenotypic distribution, causing a directional shift in the population mean and a reduction in phenotypic variance over time.

Stabilizing Selection

A mode of natural selection that favors intermediate phenotypes over both extremes, maintaining the existing population mean while reducing phenotypic variance.

Disruptive Selection

A mode of natural selection that simultaneously favors individuals at both phenotypic extremes over intermediate forms, increasing phenotypic variance within the population.

Gene Pool

The sum total of all gametes (sperm and eggs) and alleles produced by all reproducing individuals in a population.

Hardy-Weinberg Equilibrium

A null model in population genetics stating that allele and genotype frequencies in a sexually reproducing population remain constant across generations in the absence of evolutionary mechanisms ($p^2 + 2pq + q^2 = 1$ where $p + q = 1$).

Assumptions of Hardy-Weinberg Equilibrium

The five theoretical baseline conditions required for a non-evolving population: no natural selection, no mutation, no gene flow (migration), infinitely large population size (no genetic drift), and random mating.

Genetic Drift

An evolutionary mechanism consisting of random changes in allele frequencies across generations caused entirely by sampling error in finite populations.

Mean Population Fitness ($\bar{W}$)

The average fitness of individuals within a population, calculated as the sum of the relative fitness of each genotype weighted by its frequency: $\bar{W} = p^2(w_{11}) + 2pq(w_{12}) + q^2(w_{22})$.

Recessive Lethal Allele

An allele that results in lethality before birth or reproduction when present in a homozygous state ($w = 0$), but produces a viable phenotype when heterozygous, often generating a characteristic $2:1$ phenotypic ratio among surviving offspring.

Overdominance (Heterozygote Advantage)

A form of balancing selection in which heterozygous individuals have higher relative fitness than either homozygote, maintaining multiple alleles at a stable equilibrium in the population.

Negative Frequency-Dependent Selection

A type of natural selection where the relative fitness of a phenotype or genotype increases as its frequency in the population decreases, maintaining polymorphic behaviors or traits.

Long-Term Evolution Experiment (LTEE)

An ongoing experimental evolution study started in 1988 by Richard Lenski tracking 12 replicate asexual populations of Escherichia coli grown in glucose-limited, citrate-rich media, with daily $1\%$ serial transfers and periodic frozen fossil records every $500$ generations.

Historical Contingency

An evolutionary principle where the realization of a complex novel adaptation (such as aerobic citrate use in E. coli) depends strictly upon the prior accumulation of specific genetic mutations that potentiate the final enabling mutation.

Founder Effect

A reduction in genetic diversity that occurs when a new population is established by a small number of colonizing individuals carrying only a subset of the original population's genetic variation.

Microsatellite

A tract of repetitive non-coding DNA composed of short tandem repeats of nucleotides that mutate rapidly and serve as neutral genetic markers to quantify allelic richness and heterozygosity.

Population Bottleneck

A sharp, temporary crash in the population size of a species that severely depletes genetic variation and heterozygosity due to random genetic drift.

Allele Fixation

The state when an allele reaches a frequency of $1.0$ (100%) in a population, with the probability of neutral fixation by genetic drift alone equal to its current frequency in the population.

Inbreeding Depression

A decline in average fitness, survival, or fecundity caused by mating between closely related individuals, which increases genome-wide homozygosity and exposes rare, deleterious recessive alleles to selection.

Gene Flow (Evolutionary Migration)

The physical movement of alleles between distinct populations through the dispersal of haploid gametes (such as pollen) or diploid individuals, typically acting as a homogenizing force that reduces genetic divergence.

Fixation Index ($F_{ST}$)

A population genetic statistic ranging from $0$ to $1$ that measures the degree of genetic differentiation and allele frequency divergence between subpopulations relative to the total population.

Isolation by Distance

A biogeographical and population genetic pattern in which genetic divergence ($\frac{F_{ST}}{1 - F_{ST}}$) between geographic populations increases as the physical distance between them increases due to restricted dispersal.

$F_{ST}$ Outlier Analysis

A genome-scan method that calculates locus-specific $F_{ST}$ values across the chromosome to identify outlier SNPs exhibiting unusually high differentiation driven by divergent natural selection.

Lactase Persistence

The continued enzymatic production of lactase in adult humans, driven by convergent, independently evolved regulatory mutations located in the introns of the adjacent MCM6 gene in pastoralist populations.

Pairwise Nucleotide Diversity ($\pi$)

A measure of genetic diversity defined as the average proportion of nucleotide differences per site between pairs of homologous DNA sequences sampled from a population.

Selective Sweep

The reduction or complete elimination of linked neutral nucleotide variation (low $\pi$) and extended runs of homozygosity in genomic regions surrounding a beneficial mutation that has rapidly risen to high frequency under positive selection.