deleterious mutation
Agrawal AF & Whitlock MC 2012 Mutation load: the fitness of individuals in populations where deleterious alleles are abundant. Annu Rev Ecol Evol Syst 43:115-135.
- most populations harbor an abundance of segregating deleterious alleles
- these alleles, most of which are of small effect individually, collectively can reduce substantially the fitness of individuals relative to what it would be otherwise
- the ecological effects of load are not clear-cut
- some conditions (such as selection early in life, sexual selection, reproductive compensation, and intraspecific competition) reduce the effects of load on population size and persistence
- other conditions (such as interspecific competition and load on resource use efficiency) can cause small amounts of load to have strong effects on the population, even extinction
- mutation load is sometimes used loosely to describe a variety of consequences of segregating deleterious alleles including the incidence of genetic disorders and the magnitudes of inbreeding depression and standing genetic variance in fitness resulting from mutational input
- here we use it in its formal sense to refer to the reduction in fitness due to the presence of deleterious mutations segregating at mutation-selection balance
- to some extent, mutation load is not a treatable problem
- it is so diffuse (i.e., many thousands of rare deleterious alleles of small effect affecting fitness in a variety of ways)
- mutation load likely affects equilibrium population densities and could play a role in determining the outcome of interspecific competition
- mutation load can also be a conservation concern
- load (a) lowers initial vital rates and (b) is the source of the segregating deleterious alleles that can drift to fixation (i.e., the conversion of mutation load to drift load), potentially leading to mutational meltdowns
- the low reproductive capacity of humans is for typical (loaded) individuals, not unloaded ones
- it is a mistake to ask whether our current population could withstand a further 90% reduction in fitness
- that would be applying the load twice
- load has no direct relationship to population abundance or persistence
- mutation load refers to the reduction in fitness of individuals, not populations, relative to a mutation-free reference genotype
- it is unlikely that a hominid with fewer than 1,000 deleterious alleles has ever existed
- to sustain a population, each female needs to produce on average only two adult offspring, whereas the world record is 67
- the idea that species cannot persist with high loads, independent of other assumptions, is incorrect
- load can reduce population sizes (even with density-dependent regulation) and possibly cause extinction
- the magnitude of these effects depends heavily on other assumptions
