Chapter 6: Genetic Drift Evolution at Random

random genetic drift

Random changes in the frequencies of two or more alleles or genotypes within a population.

genetic drift

Random changes in the frequencies of two or more alleles or genotypes within a population.

effective population size

The effective size of a real population is equal to the number of individuals in an ideal population (i.e., a population in which all individuals reproduce equally and population size is constant in time) that produces the rate of genetic drift seen in the real population.

population bottleneck

A severe, temporary reduction in population size.

founder event

A population bottleneck that results when a new population is founded by a small number of individuals

gene tree

A diagram representing the history by which gene copies have been derived from ancestral gene copies in previous generations. The copies may or may not differ in their sequences.

coalesce

Looking backward in time on a gene tree, the merging of two lineages in their most recent common ancestor.

purifying selection

Elimination of deleterious alleles from a population.

selective constraint

bounded variation in one or more characteristics, due to natural selection against more pronounced variants.

background selection

Elimination of deleterious mutations in a region of the genome; may explain low levels of neutral sequence variation.

codon bias

The more frequent usage of certain synonymous codons to encode a given amino acid.

positive selection

Selection for an allele that increases fitness. See purifying selection.

inbreeding load

The decline in a population’s mean fitness that results from the fixation of deleterious mutations by drift.

mutational meltdown

Increasing inbreeding load caused by the fixation of deleterious mutations by genetic drift, leading to the accelerated decline of population size as drift becomes increasingly strong.

molecular clocks

The relatively constant accumulation of changes over time that occurs in some DNA and protein sequences. Molecular clocks provide the basis for dating the time of divergence of lineages based on their molecular differences.

neutral theory of molecular evolution

The hypothesis that most alleles that are polymorphic within populations and that differ between species do not significantly alter fitness and evolve by genetic drift.

pseudogene

A nonfunctional duplicate of a functional gene.

d_N/d_S ratio

The ratio of the number of nonsynonymous substitutions per nonsynonymous site (d_N) and the number of synonymous substitutions per synonymous site (dS). Values of this ratio smaller than one are consistent with purifying selection, while values greater than one suggest the action of positive selection.

MK test

statistical test for evidence of positive selection in the coding region of a gene, named after its inventors John McDonald and Martin Kreitman.

genome scan

A search in the genome for features of interest, for example as performed during a genome-wide association study.

local adaptation

Of an allele, trait, or population, the state of being differentially adapted to conditions that prevail in a spatially restricted area.