Genetic load

Genetic load is the reduction in a population’s average fitness relative to a specified reference genotype or population, caused by deleterious genetic variants.

Mutation–selection balance: The equilibrium between the introduction of deleterious mutations and their removal by natural selection. It explains how recurrent mutation maintains harmful alleles and a persistent load.
Biological fitness: An organism’s contribution of viable, fertile offspring to subsequent generations. Genetic load quantifies a population’s fitness shortfall against a chosen benchmark.
Mutation load: The reduction in population fitness caused by deleterious mutations continually introduced into the population. It isolates the contribution of recurrent mutation to total genetic load.
Minimum viable population: The smallest population size estimated to have a specified probability of persisting for a specified time. Genetic load can reduce survival and reproduction, changing persistence estimates.
Ohta’s nearly neutral theory: A theory that many molecular changes are nearly neutral, with their evolutionary fate strongly influenced by genetic drift. It frames debate about how much mildly deleterious variation small populations accumulate.
Heterozygote advantage: A pattern of selection in which heterozygotes have higher fitness than either homozygote. It can preserve harmful alleles in heterozygotes, producing a segregating load.
Natural selection: Differential survival or reproduction associated with heritable variation among organisms. Selection changes the frequencies of the variants responsible for load.
Segregation load: The fitness loss caused by segregating alleles that produce less-fit genotypes through mating and inheritance. It distinguishes load from polymorphism from losses caused by new mutation.
Genetic rescue: An increase in population fitness after introducing genetic variation from another population. Gene flow may reduce load by restoring variation or masking recessive deleterious alleles.
Hill–Robertson interference: The reduction in selection efficiency caused by linkage among loci under selection in finite populations. It can increase load beyond predictions that treat harmful variants independently.

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