Abstract
In order to characterize the dynamics of adaptation, it is important to be able to quantify how a population’s mean fitness changes over time. Such measurements are especially important in experimental studies of evolution using microbes. The Long-Term Evolution Experiment (LTEE) with Escherichia coli provides one such system in which mean fitness has been measured by competing derived and ancestral populations. The traditional method used to measure fitness in the LTEE and many similar experiments, though, is subject to a potential limitation. As the relative fitness of the two competitors diverges, the measurement error increases because the less-fit population becomes increasingly small and cannot be enumerated as precisely. Here, we present and employ two alternatives to the traditional method. One is based on reducing the fitness differential between the competitors by using a common reference competitor from an intermediate generation that has intermediate fitness; the other alternative increases the initial population size of the less-fit, ancestral competitor. We performed a total of 480 competitions to compare the statistical properties of estimates obtained using these alternative methods with those obtained using the traditional method for samples taken over 50,000 generations from one of the LTEE populations. On balance, neither alternative method yielded measurements that were more precise than the traditional method.
Highlights
The concept of fitness is central to evolutionary biology
The experiment uses a Davis Minimal salts medium with 25 μg/mL glucose (DM25), which supports densities of ~3–5 x 107 bacteria per mL
The only suggestion of a meaningful difference was that the Altered Starting Ratio (ASR) method appeared worse than the other two methods in the early generations, when the fitness gains of the evolved bacteria were still fairly small
Summary
The concept of fitness is central to evolutionary biology. Genotypes with higher fitness will tend to produce more offspring and thereby increase in frequency over time compared to their less-fit competitors. Fitness is often difficult to measure, especially for long-lived organisms. Unlike traits such as color, fitness cannot be observed at a single point in time, but instead it must be measured and integrated across the lifespan of the individuals. Researchers typically measure fitness components—such as the number of seeds produced or offspring fledged—and use them as proxies for fitness.
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