Unit 7 Natural Selection: How Evolution Acts on Populations

Population genetics

The study of how the genetic makeup (allele frequencies) of a population changes over time.

Evolution (population genetics definition)

A change in allele frequencies in a population across generations (individuals do not evolve during their lifetimes).

Population

A group of individuals of the same species living in the same area that can interbreed.

Gene pool

All the genetic information (all alleles of all genes) present in a population.

Allele frequency

The fraction (proportion) of all copies of a gene in a population that are a particular allele.

p and q (two-allele system)

Symbols for allele frequencies: p = frequency of allele A, q = frequency of allele a; in a two-allele system, p + q = 1.

Genotype

An individual’s allele combination at a gene locus (e.g., AA, Aa, aa).

Total alleles in a diploid population

For N diploid individuals, there are 2N total alleles at a given gene locus.

Allele frequency from genotype counts

For counts nAA, nAa, naa in a diploid population of size N: p = (2nAA + nAa)/(2N) and q = (2naa + nAa)/(2N).

Microevolution

Evolution within a population; typically measured as changes in allele frequencies over generations.

Natural selection

Nonrandom differential reproductive success due to heritable trait differences, causing alleles linked to advantageous traits to increase in frequency.

Adaptation

A match between organisms and their environment produced by natural selection increasing beneficial traits (selection is the major mechanism that consistently produces adaptation).

Genetic drift

Random change in allele frequencies due to chance events; strongest in small populations and can reduce genetic variation via allele loss.

Bottleneck effect

A form of genetic drift where a population is drastically reduced; survivors may not represent original allele frequencies, reducing diversity.

Founder effect

A form of genetic drift where a small group starts a new population; the new population’s allele frequencies may differ from the source population.

Gene flow

Movement of alleles between populations through migration and interbreeding; can introduce new alleles and tends to homogenize populations.

Mutation (as an evolutionary mechanism)

A change in DNA sequence that creates new alleles; the ultimate source of new genetic variation, usually changing allele frequencies slowly by itself.

Nonrandom mating

Mating that is not random with respect to phenotype (or proximity); often changes genotype frequencies more directly than allele frequencies.

Sexual selection

A type of selection where traits that increase mating success rise in frequency, even if they reduce survival.

Fitness (in population genetics)

Relative reproductive success of a genotype or phenotype in a specific environment (measured by viable offspring produced).

Hardy-Weinberg equilibrium

A null model predicting genotype frequencies when no evolution occurs at a locus; allele frequencies remain constant across generations under the model’s assumptions.

Hardy-Weinberg equation

For two alleles with frequencies p and q: p + q = 1 and expected genotype frequencies are p^2 (AA) + 2pq (Aa) + q^2 (aa) = 1.

Hardy-Weinberg assumptions

No mutation, random mating, no natural selection, extremely large population (no drift), and no gene flow.

Carrier frequency (Hardy-Weinberg)

Under Hardy-Weinberg, the frequency of heterozygotes (carriers) is 2pq; if recessive phenotype frequency is given, it corresponds to q^2.

Directional selection

Selection that favors one extreme phenotype, shifting the population’s average phenotype in one direction.