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Darwin gets a fresh boost

Press release: Ottawa November 19, 2009&ndashAs the Origin of Species celebrates its 150th anniversary, a team of evolutionary biologists from the University of Ottawa put adaptation under the microscope, and once again Darwin comes off looking very good. In a study to be published in the journal PLoS Biology, the team of researchers that included postdoctoral fellow Dr Sijmen Schoustra, grad student Danna Gifford, and led by Dr Rees Kassen, Associate Professor at the Department of Biology, tracked adaptation in a rapidly reproducing fungus. The goal of this work was to understand how adaptations that allow an organism to out-compete and out-reproduce its kin are built up from occasional mutations in DNA sequences.

"Darwin didn't know about DNA, and so he couldn't have understood what raw materials were used by natural selection to craft an adaptation. In other words, he didn't know how to build an adaptation" said Dr Rees Kassen. To see what was happening -- step by step -- the researchers examined adaptations over the course of 800 generations in two different sized fungal populations made up of more than 100 fungi lineages. The experimental results revealed that almost all the evolving lines adapted using just a few mutations.

"Darwin was right about natural selection, but it doesn't need to be as slow as he thought. Adaptation can happen quickly because just a few mutations are involved and of the largest benefits tend to happen early on." he added. "These results have important implications and provide the first experimental confirmation of predictions about adaptation from modeling," said Dr Kassen.

The research conclusions demonstrate that it should not be a surprise to see rapid adaptation in nature, whether it be to novel antibiotics in our hospitals or to a rapidly changing climate. The key is for natural populations to adapt quickly enough to avoid being on the wrong end of natural selection.

Research in our lab concerns the most fundamental and persistent puzzle of nature: the origin and maintenance of biodiversity. Or, put another way, we are trying to answer the age-old question, why are there so many species in the world? To do this requires that we study the evolutionary process as it unfolds, keeping track of the variety of genotypes and phenotypes in a population or community through time, a task that would be very challenging with any large, long-lived organism. For this reason, we use microbial populations of bacteria and protists to study the evolutionary process in the laboratory.

Author summary of paper in PLoS Biology

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.

Abstract

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.

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