near neutrality - senecio7の日記

Tachida H 1991 A study on a nearly neutral mutation model in finite populations. Genetics 128:183-192.

as a nearly neutral mutation model, the house-of-cards model is studied in finite populations
if 4Nσ is large compared to one, a few advantageous mutants are quickly fixed in early generations
then most mutation becomes deleterious
very slow increase of the average selection coefficient follows
both advantageous and neutral (including slightly deleterious) mutations are fixed

Ohta and Tachida (1990) proposed a model of protein evolution in which effects of random genetic drift and very weak selection are incorporated
in this nearly neutral mutation model, the distribution of the effect of mutant allele on selection coefficient is fixed
a motivation for the model of Ohta and Tachida (1990) is our biological intuition that there must be a limit in the improvement of a protein and that after major improvements there would be some fine tuning of the function
in this model, the proportion of advantageous mutations decreases as the population accumulates advantageous mutations and in consequence has higher average fitness

the fixed mutation model is the same as the "house-of-cards" model of Kingman (1978)
the model is also adopted in the studies of evolution of quantitative characters and selection limits (Cockerham and Tachida 1987; Zeng, Tachida and Cockerham 1989)

in the house-of-cards model, only several advantageous fixations bring the population fitness to a high value for larger 4Nσ
in this state most mutations become deleterious
in the shift model continuous deterioration of the population fitness results if there is no advantageous mutation
in the house-of-cards model, there is a stochastic equilibrium to which the population approaches
the population fitness goes up and down through time according to this distribution in the equilibrium