Genetic Variation II: Models & Measurement

Vocabulary flashcards covering models and measurements of genetic variation, evolutionary forces, the Classical vs. Balance schools, allozyme electrophoresis, the Neutral Theory, and molecular tests of selection.

Heterozygosity ($H$)

The fraction of individuals in a population that are heterozygous, averaged across gene loci.

Polymorphism ($P$)

The proportion of gene loci in a population that have two or more alleles.

Mutation

The ultimate source of genetic variation, arising from non-directed replication errors and increasing genetic diversity within populations.

Recombination

The genetic process that generates new combinations of mutations, forming novel haplotypes and increasing genetic variation in populations.

Haplotype

A set of alleles located on a single chromosome that is inherited together from one parent.

Genetic Drift

The change in the frequency of an existing gene variant (allele) due to random chance, which decreases genetic variation and is more pronounced in small populations.

Negative (Purifying) Selection

A form of natural selection that removes mutations that reduce fitness, thereby decreasing genetic variation in populations.

Positive (Directional) Selection

A form of natural selection that favors mutations that increase fitness until they reach fixation, decreasing genetic variation in populations.

Fixation

The loss of all but one allele at a locus, causing a polymorphic locus to become monomorphic due to natural selection or genetic drift.

Balancing Selection

Natural selection favoring the long-term maintenance of multiple alleles at a locus (e.g., through heterozygote advantage or frequency-dependent selection), which increases or retains genetic variation.

Gene Flow (Migration)

The movement of genetic material from one population to another, which decreases differences between populations and increases genetic diversity within populations.

Classical School (Mutation-Selection Balance)

A population genetics model championed by T. H. Morgan and H. J. Muller predicting low heterozygosity and polymorphism, where the wild type is the normal genotype and most variation consists of deleterious, transient mutations purged by purifying selection.

Balance School

A population genetics model championed by E.B. Ford and T. Dobzhansky predicting high heterozygosity and polymorphism maintained actively by balancing selection, such as heterozygote advantage.

Allozymes

Different allelic forms of the same protein or enzyme that can be separated and visualized through gel electrophoresis.

Neutral Theory of Molecular Evolution

A theory developed by Motoo Kimura proposing that the vast majority of molecular genetic variation is selectively neutral, because deleterious mutations are rapidly purged and advantageous mutations rapidly fixed.

Silent Mutation

A nucleotide change in a protein-coding sequence that does not alter the encoded amino acid.

Missense Mutation

A nucleotide change in a protein-coding sequence that alters a codon to specify a different amino acid.

Nonsense Mutation

A nucleotide change in a protein-coding sequence that converts an amino acid codon into a premature stop codon.

McDonald-Kreitman Test

An analytical method that compares the ratio of non-synonymous to synonymous fixed differences between species with polymorphisms within species to test for adaptive protein evolution.

Neo-Darwinism (The Modern Synthesis)

The 1920s–1950s mathematical evolutionary framework initiated by R.A. Fisher, J.B.S. Haldane, and S. Wright demonstrating that continuous variation and Darwinian natural selection are fully consistent with Mendel's Laws.