Unit 7: Natural Selection

Evolution (biological)

A change in the genetic makeup of a population over time; specifically, a change in allele frequencies in the population’s gene pool across generations.

Population

A group of interbreeding organisms of the same species living in the same area.

Gene pool

The total collection of alleles present across all individuals in a population.

Allele frequency

The relative proportion of a particular allele among all alleles for that gene in a population.

Acclimation

A short-term physiological adjustment by an individual (e.g., producing more red blood cells at high altitude) that does not usually change inherited DNA.

Genetic variation (genetic variability)

Differences in genes/alleles among individuals in a population; the raw material that selection can act on.

Mutation

A random change in DNA sequence that creates new alleles; most are neutral or harmful in a given environment, and mutations are random with respect to “need.”

Recombination (sexual reproduction)

Processes in sexual reproduction that create new combinations of existing alleles (e.g., crossing over, independent assortment, random fertilization).

Genotype

An organism’s allele combination (its genetic makeup for a trait or gene).

Phenotype

An observable trait; often influenced by both genotype and environment.

Heritability

The degree to which variation in a trait is genetic and can be passed from parents to offspring (required for natural selection to cause evolution).

Natural selection

A mechanism of evolution where individuals with certain heritable traits survive and reproduce more successfully in a particular environment, increasing those alleles over generations.

Four conditions for natural selection

Variation, heritability, overproduction/competition (limited resources), and differential reproductive success.

Evolutionary fitness

Reproductive success—how many viable, fertile offspring an organism contributes to the next generation (relative to others); depends on the environment.

Adaptation

A heritable trait that increases fitness in a specific environment and becomes more common over generations.

Lamarckism (inheritance of acquired traits)

Historical idea that traits acquired during an organism’s lifetime are inherited; not supported as a general mechanism because acquired changes usually don’t alter heritable DNA.

Directional selection

Selection that favors one extreme phenotype, shifting the population mean toward that extreme (e.g., peppered moth color during pollution).

Stabilizing selection

Selection that favors intermediate phenotypes and reduces variation around the mean (e.g., historically, human birth weight).

Disruptive selection

Selection that favors both extremes over intermediates, increasing variation and sometimes contributing to speciation if gene flow becomes reduced.

Sexual selection (intrasexual vs. intersexual)

Selection for traits that increase mating success; intrasexual involves competition within one sex, and intersexual involves mate choice by the other sex.

Artificial selection

Evolution caused by humans intentionally choosing which individuals reproduce to increase desired heritable traits; often reduces genetic diversity.

Human-driven natural selection (e.g., resistance)

Natural selection occurring in environments altered by humans (e.g., antibiotics/pesticides kill susceptible individuals so resistant alleles increase); humans are not directly choosing who reproduces.

Conservation genetics

Applying genetic principles to conservation; emphasizes maintaining genetic diversity to improve adaptive potential and reduce risks from drift and inbreeding in small, isolated populations.

Population genetics

The study of allele and genotype frequencies in populations and how they change over time (linking Mendelian genetics to evolution).

Hardy-Weinberg equilibrium (HWE)

A null model in which allele frequencies and genotype frequencies remain constant across generations; used to test whether evolution is occurring at a gene.

Hardy-Weinberg equations

For two alleles: p + q = 1 and p² + 2pq + q² = 1, where p² (AA), 2pq (Aa), and q² (aa) are expected genotype frequencies under HWE.

Hardy-Weinberg assumptions

Very large population, random mating, no natural selection, no gene flow (migration), and no mutation (or negligible mutation over the timeframe).

Carrier frequency

The proportion of heterozygotes (often 2pq under HWE) who carry a recessive allele without showing the recessive phenotype.

Genetic drift

Random change in allele frequencies due to chance events (strongest in small populations); can fix or lose alleles and reduce genetic variation.

Bottleneck effect

A type of genetic drift where a population size is drastically reduced; survivors may not represent the original gene pool, shifting allele frequencies.

Founder effect

A type of genetic drift where a small group starts a new population; allele frequencies reflect the founders and may differ from the source population.

Gene flow

Transfer of alleles between populations via migration and interbreeding; can increase variation within populations and reduce differences between populations.

Nonrandom mating

Mate choice not based on random pairing (e.g., inbreeding, assortative mating); often increases homozygosity and violates an HWE assumption.

Homologous structures

Anatomical features similar due to shared ancestry, even if they serve different functions (evidence for common ancestry).

Analogous structures

Features with similar function/appearance due to similar selective pressures rather than shared ancestry (a result of convergent evolution).

Convergent evolution

Independent evolution of similar traits in unrelated lineages due to similar environments/selection pressures, producing analogous structures.

Fossil record

Preserved remains or traces showing that species have changed over time; incomplete and biased, but provides evidence for evolution and extinct organisms.

Molecular evidence

DNA/protein sequence similarities that support common ancestry; closely related species tend to have more similar sequences as mutations accumulate over time.

Phylogenetic tree (cladogram)

A branching diagram representing evolutionary relationships and common ancestry, built using fossil and/or molecular data; branching pattern (not tip order) determines relatedness.

Biological species concept

Defines species as groups of populations that can interbreed in nature and produce viable, fertile offspring and are reproductively isolated from other groups.

Reproductive isolation

Barriers that prevent gene flow between populations; central to the formation of new species.

Prezygotic barriers

Reproductive barriers that prevent fertilization (e.g., habitat, temporal, behavioral, mechanical, or gametic isolation).

Postzygotic barriers

Reproductive barriers after fertilization that reduce hybrid survival or reproduction (e.g., reduced hybrid viability, reduced hybrid fertility like mules, hybrid breakdown).

Allopatric speciation

Speciation caused by geographic separation that reduces gene flow; populations diverge via selection, drift, and mutation until reproductive isolation evolves.

Sympatric speciation

Speciation without a geographic barrier; gene flow is reduced by factors such as strong selection, habitat differentiation, sexual selection, or chromosomal changes.

Polyploidy

A chromosomal change (extra sets of chromosomes), especially in plants, that can create near-instant reproductive isolation from the original diploid population.

Reinforcement

When populations in secondary contact produce low-fitness hybrids, selection favors stronger prezygotic barriers, increasing reproductive isolation.

RNA-world hypothesis

Hypothesis that early life used RNA as both genetic material and a catalyst (ribozymes), potentially preceding DNA- and protein-based life.

Oparin-Haldane hypothesis

Proposal that early Earth had a reducing atmosphere (rich in CH4, NH3, H2, and H2O with little free O2) that could facilitate abiotic formation of organic molecules.

Miller-Urey experiment

Experiment that simulated proposed early-Earth atmospheric conditions with electrical sparks (lightning) and produced organic compounds (e.g., amino-acid-like molecules), supporting abiotic synthesis plausibility.