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segunda-feira, setembro 11, 2017

Variability in Fitness Effects Can Preclude Selection of the Fittest

Annual Review of Ecology, Evolution, and Systematics

Vol. 48:- (Volume publication date December 2017) 

Review in Advance first posted online on August 28, 2017. (Changes may still occur before final publication.) 

Christopher J. Graves1,2 and Daniel M. Weinreich1,2

1Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island, 02912; email: Christopher_graves@alumni.brown.edu, daniel_weinreich@brown.edu

2Center for Computational and Molecular Biology, Brown University, Providence, Rhode Island, 02912

The fitness of a genetic trait (an allele) may vary over time, rather than remain constant. In this simple model, populations with two different alleles (black or yellow) see-saw between advantage and disadvantage as their relative fitness changes over time (blue line below). Credit: Weinreich et. al. - PhysOrg


Evolutionary biologists often predict the outcome of natural selection on an allele by measuring its effects on lifetime survival and reproduction of individual carriers. However, alleles affecting traits like sex, evolvability, and cooperation can cause fitness effects that depend heavily on differences in the environmental, social, and genetic context of individuals carrying the allele. This variability makes it difficult to summarize the evolutionary fate of an allele solely on the basis of its effects on any one individual. Attempts to average over this variability can sometimes salvage the concept of fitness. In other cases, evolutionary outcomes can be predicted only by considering the entire genealogy of an allele, thus limiting the utility of individual fitness altogether. We describe a number of intriguing new evolutionary phenomena that have emerged in studies that explicitly model long-term lineage dynamics and discuss implications for the evolution of infectious diseases.

Expected final online publication date for the Annual Review of Ecology, Evolution, and Systematics Volume 48 is November 2, 2017. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.





The phrase "survival of the fittest" makes the principle of evolution by natural selection easy to understand—individuals with a trait that adapts them well to their circumstances are more likely to pass that trait along. But as a new study explains, multiple factors make predicting the fate of a trait fiendishly difficult.

Fundamentally, the problem is that a trait conveyed by a gene variant, or allele, may be advantageous for one or a few generations, but provide no advantage or become a liability when circumstances change, said senior author Daniel Weinreich, a professor of ecology and evolutionary biology at Brown University. But most theoretical models of population genetics assume that fitness remains constant.

Perhaps the most obvious way that the fitness of a trait can vary is that the environment can change, not only over time but also over space.

Another dimension that can vary is the "social" life of alleles. Alleles that result in "cheating" are abundant in nature, but they are most effective when they are rare. Once everyone is cheating, it might no longer be an advantage, so the trait over time can become a victim of its own success.

That any of these circumstances can change over time adds yet another layer of complexity, Weinreich said, because the rate at which circumstances change matters.

Indeed, Weinreich said, many models for predicting the fate of alleles have overlooked the possibility that traits can go completely extinct.

Meanwhile, the rate of environmental change is very similar to the rate at which natural selection acts, the math becomes especially tricky.

Weinreich said he plans to delve deeper into the complexities of changes in fitness deriving from varying rates of change in social (e.g. cheaters), genetic (e.g. competing alleles) or environmental (e.g., weather) parameters.

“The overlap between ecological and evolutionary processes—that those two things speak to each other very intimately in a way that’s been overlooked in many models—is the way forward,” Weinreich said. “That’s what’s needed to make critical improvements to models.“