Darwin estava errado: a evolução é mais rápida do que ele imaginava

The Properties of Adaptive Walks in Evolving Populations of Fungus

Sijmen E. Schoustra1,2#*, Thomas Bataillon3,4#*, Danna R. Gifford1,2, Rees Kassen1,2

1 Department of Biology, University of Ottawa, Ottawa, Ontario, Canada,

2 Center for Advanced Research in Environmental Genomics, University of Ottawa, Ottawa, Ontario, Canada,

3 Institute of Biology, Bioinformatics Research Centre, Aarhus University, Aarhus, Denmark,

4 INRA, UMR DIAPC, Montpellier, France

Abstract Top

The rarity of beneficial mutations has frustrated efforts to develop a quantitative theory of adaptation. Recent models of adaptive walks, the sequential substitution of beneficial mutations by selection, make two compelling predictions: adaptive walks should be short, and fitness increases should become exponentially smaller as successive mutations fix. We estimated the number and fitness effects of beneficial mutations in each of 118 replicate lineages of Aspergillus nidulans evolving for approximately 800 generations at two population sizes using a novel maximum likelihood framework, the results of which were confirmed experimentally using sexual crosses. We find that adaptive walks do indeed tend to be short, and fitness increases become smaller as successive mutations fix. Moreover, we show that these patterns are associated with a decreasing supply of beneficial mutations as the population adapts. We also provide empirical distributions of fitness effects among mutations fixed at each step. Our results provide a first glimpse into the properties of multiple steps in an adaptive walk in asexual populations and lend empirical support to models of adaptation involving selection towards a single optimum phenotype. In practical terms, our results suggest that the bulk of adaptation is likely to be accomplished within the first few steps.

Author Summary Top

Adaptation is one of the least understood processes in biology because it relies on beneficial mutations, which are often too rare to study. We developed a method to infer the number and size of beneficial mutations substituted during adaptation, a process called an adaptive walk, and used this to test predictions about the properties of adaptive walks in experimental populations of fungus. Our work shows that, in contrast to the gradualist view of adaptation dominant since the 1930s, adaptive walks tend to be fast and short, with beneficial mutations of large effect substituted first, followed by those of smaller effect.

Citation: Schoustra SE, Bataillon T, Gifford DR, Kassen R (2009) The Properties of Adaptive Walks in Evolving Populations of Fungus. PLoS Biol 7(11): e1000250. doi:10.1371/journal.pbio.1000250

Academic Editor: Nick H. Barton, University of Edinburgh, United Kingdom

Received: February 2, 2009; Accepted: October 16, 2009; Published: November 24, 2009

Copyright: © 2009 Schoustra et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This work was supported by an Natural Sciences and Engineering Research Council of Canada Discovery Grant and an Early Researcher Award from the Province of Ontario to RK, a Steno Fellowship from the Danish Council for Natural Sciences (FNU) to TB, and a Fonds Canada-France Award from the French Embassy in Canada to RK and TB. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Abbreviations: EVT, extreme value theory; ML, maximum likelihood

* E-mail: Sijmen.Schoustra@uOttawa.ca (SES); tbata@birc.au.dk (TB)

# These authors contributed equally to this work.

+++++

FREE PDF GRÁTIS [OPEN ACCESS]

+++++

Ênfase em itálico adicionada por este blogger.