Population Genetics and Natural Selection

Flashcards summarizing vocabulary, key concepts, and case studies on population genetics, natural selection, and mechanisms of evolution.

Evolution

The change in a population over generations, with natural selection acting as its primary mechanism through descent with modification.

Fitness

An organism's evolutionary ability to survive and reproduce more offspring, measured quantitatively by counting metrics such as eggs or seeds.

Adaptation

A physical trait or characteristic resulting from natural selection, such as a longer beak or a color change.

Conditions for Natural Selection

The three necessary conditions for natural selection: $1$) pre-existing heritable genetic variation in a population, $2$) competition for limited environmental resources, and $3$) differential survival and reproduction of organisms possessing advantageous variations.

Phenotypic Plasticity

Non-heritable variations in an organism's physical appearance or phenotype caused by environmental effects, such as changes in body size due to parasites or nutrition, and tanning from sun exposure.

Milkweed Adaptation

An evolutionary adaptation where caterpillars mutated to tolerate poisonous milkweed by altering their $\text{Na}^+/\text{K}^+$ pump composition, allowing them to ingest cardiac glycosides and store toxins to deter predators.

Clausen, Keck, and Hiesey Null Hypothesis

The assumption that there are no genetic differences among populations, predicting that all plants would grow equally well across all experimental gardens regardless of origin.

Cinquefoil Ecotype Study (1940)

An experiment by Clausen, Keck, and Hiesey transplanting P. glandulosa across Lowland ($30\,m$ at Stanford), Mid-elevation ($1{,}400\,m$ at Mather), and Alpine ($3{,}050\,m$ at Timberline) gardens; its results rejected the null hypothesis and showed that growth depended on both genetic local adaptation and environmental factors.

Stabilizing Selection

A type of natural selection that culls extreme variations and narrows the width of distribution by selecting for the most common intermediate phenotype.

Directional Selection

A type of natural selection that eliminates one extreme phenotype while selecting for another extreme, shifting the overall population distribution left or right.

Disruptive Selection

A type of natural selection that eliminates the common intermediate phenotype and favors both extreme phenotypes, resulting in a bimodal distribution.

Three Types of Selection

The three modes of natural selection on phenotypic distribution: stabilizing selection (narrows width), directional selection (shifts distribution left or right), and disruptive selection (creates a bimodal distribution).

Genetic Drift

A numbers-game phenomenon involving evolutionary change in allele frequencies due to random chance, which narrows genetic diversity in small populations.

Population Bottleneck

A form of genetic drift occurring when a random event drastically reduces population size by chance, killing individuals disproportionately regardless of genotype and altering genotype frequencies.

Founder Effect

A form of genetic drift occurring when a small subset of a population establishes a new colony, causing the loss of original genotypic diversity.

Forms of Genetic Drift

The two primary mechanisms of genetic drift: Founder Effect (subset founding a new colony) and Population Bottleneck (random event drastically reducing population size).

Hardy-Weinberg Equilibrium

The principle stating that no evolutionary change occurs in a population unless disrupted by selection, non-random mating, mutations, immigration, or small population size.

Pesticide Treadmill

A cycle where repeated pesticide or herbicide applications select for resistant organisms, leaving individuals in the population that are no longer affected by the chemical.