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, 3 (3), e1715

Heterogeneous Adaptive Trajectories of Small Populations on Complex Fitness Landscapes

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Heterogeneous Adaptive Trajectories of Small Populations on Complex Fitness Landscapes

Daniel E Rozen et al. PLoS One.

Abstract

Background: Small populations are thought to be adaptively handicapped, not only because they suffer more from deleterious mutations but also because they have limited access to new beneficial mutations, particularly those conferring large benefits.

Methodology/principal findings: Here, we test this widely held conjecture using both simulations and experiments with small and large bacterial populations evolving in either a simple or a complex nutrient environment. Consistent with expectations, we find that small populations are adaptively constrained in the simple environment; however, in the complex environment small populations not only follow more heterogeneous adaptive trajectories, but can also attain higher fitness than the large populations. Large populations are constrained to near deterministic fixation of rare large-benefit mutations. While such determinism speeds adaptation on the smooth adaptive landscape represented by the simple environment, it can limit the ability of large populations from effectively exploring the underlying topography of rugged adaptive landscapes characterized by complex environments.

Conclusions: Our results show that adaptive constraints often faced by small populations can be circumvented during evolution on rugged adaptive landscapes.

Conflict of interest statement

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

Figures

Figure 1
Figure 1. Relative fitness of large and small bacterial populations after evolution on either a simple or complex nutrient environment.
A value of 1 indicates no change.
Figure 2
Figure 2. Fitness trajectories of 12 small (A) and six large (B) populations evolving in the complex environment.
Dotted lines highlight small populations that have attained higher fitness than other small and even the most fit large populations (see text for details).
Figure 3
Figure 3. Simulation results of fitness gain in 50 small (dotted line) and large (unbroken line) populations on either a smooth (a) or complex (b) fitness landscape.
The number of 1-step neighbours, L, is 500 and the mutation rate, μ, is 5e-6. Variation in fitness across treatments and population size is shown in Fig 3c.

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