Tuesday, April 03, 2007

A fitness floor?   posted by Razib @ 4/03/2007 12:29:00 AM

PLOS Biology has an interesting new paper out, Understanding the Evolutionary Fate of Finite Populations: The Dynamics of Mutational Effects:
In any population, two factors determine whether the average fitness of individuals will increase...or decrease: the size of the population and the distribution of mutational effects...Although it is relatively simple to get quantitative information on population size, it is much harder to gain insight into the distribution of mutational effects...Here, we use laboratory evolution of a bacterial virus to quantify the distribution of mutational effects. Our results reveal that the average impact of a mutation is approximately constant with respect to fitness, that most mutations have small effects, and that the rate of beneficial mutation depends on the fitness of the organism. Our study demonstrates the simple, but perhaps underappreciated fact that mutational effects are dynamic. It also proposes and tests an explicit model of adaptation in which organismal fitness specifies both the rate and distribution of deleterious and beneficial mutations, and it presents specific and testable predictions of the circumstances under which populations will adapt.

From the abstract: "The most consistent result in more than two decades of experimental evolution is that the fitness of populations adapting to a constant environment does not increase indefinitely, but reaches a plateau. Using experimental evolution with bacteriophage, we show here that the converse is also true. In populations small enough such that drift overwhelms selection and causes fitness to decrease, fitness declines down to a plateau." In regards to fitness reaching a plateau, this is intelligible via R.A. Fisher's argument that mutations of large effect will tend to overshoot the fitness optimum. The "plateau" occurs simply because a population is presumably converging upon the local fitness optimum, and as it approaches that optimum mutations which increment it toward that state must by the nature of the landscape be of small enough effect so as not to overshoot the peak. Conversely, the authors of this paper argue that the fitness floor which occurs when a population size is small and random genetic drift overwhelms the purifying power of selection (so that deleterious mutations may fix in the genetic background) may be generated by the emergence of compensatory epistasis which acts as a break upon further accumulation of "deleterious" alleles. By this, they mean that alleles at loci which in the "normal" genetic background (i.e., closer to the fitness optimum) would be detrimental toward fitness may now actually favorable because of interactions with other normally "deleterious" alleles at other loci.

Consider a set of loci, numbered from 1 to 10. Assume that they are fixed for a range of alleles in the normal genetic background. If loci 1 mutated from a to b in the normal genetic background it might be deleterious. But, if loci 3 were mutated from a to b then one might imagine that the interaction between two b alleles across the loci might result in a less deleterious outcome than either one in the conventional genetic background. Any thoughts on the relevance of this to the mutational meltdown hypothesis?

Related: Rugged Roads of Gene Land.