Population consequences of mutational events: effects of antibiotic resistance on the r/K trade-off

Abstract

What are the effects of a mutational event on population dynamics? This ecoevolutionary question has relevance not only to basic biological theories but also to conservation applications. We evaluated the relationship between maximum population growth rate (r max ) and carrying capacity (K) among strains of the bacterium Pseudomonas fluorescens. Each of 65 strains differed from their common ancestor by one naturally acquired phenotypic change conferring antibiotic resistance, brought about by a single mutational event, and each was grown in isolation in four environments. We found no evidence of a trade-off between r max and K. Rather, strains with rapid growth rates also had high carrying capacity, with little interaction between strain and environment. We conclude that the extensive variation in overall fitness resulting from single mutational events likely masks whatever population trade-offs may exist.

Biophysical mechanisms that maintain biodiversity through trade-offs

“…We determined their r (exponential growth rate) and k (maximal population size) values by fitting the mathematical logistic model d dt x ¼ rxð1 À x=kÞ to growth data, where x(t) denotes bacterial population density at time t (Supplementary Discussion, section C). At low sugar concentrations, ranging from 1 to 8 mg ml À 1 the experiment recapitulates a prior data set 9 , whereby a positive, linear correlation between r and k is observed (Fig. 1a).…”

“…A negative correlation between growth rate and yield has been observed but without the quadratic form 12 . Moreover, positive rate-yield correlations, the antithesis of a rate-yield trade-off, have been observed in other bacterial species 9 and, curiously, this relationship is also predicted to be constrained by a quadratic geometry 10 . We can reconcile these apparent contradictions by following a suggestion 9 that r-k relationships can be manipulated by varying the carbon richness of the environment in which bacteria grow (for clarity, we use k for carrying capacity where others use K).…”

“…Due to its importance to biodiversity theory, a search for trade-off data has become the subject of a burgeoning experimental literature using microbial populations in the laboratory 1,[5][6][7][8][9][10][11][12] . However, despite the significance of trade-off geometry to the theory and the longpostulated hypothesis that trade-offs arise from life-history constraints, we know of no successful derivation of trade-off geometry in living systems from fundamental physical or biochemical principles.…”

Availability of public goods shapes the evolution of competing metabolic strategies

“…Both studies assumed that during the exponential phase of a batch culture, there was excess of substrate and therefore fast growing mutants should become dominant, whereas in either stationary phase or a carbon-limited chemostat, nutrients are limited and mutants that use their substrate efficiently should become dominant. We think that the reason why these and other studies (4,6) failed to identify a yield/rate tradeoff over the course of experimental evolution is that in all cases the experiments were carried out in suspension cultures. In such an unstructured environment resources are a public good and accessible to every individual.…”

“…A lthough the existence of tradeoffs in evolution seems to be undisputable, experimental evidence obtained under controlled conditions is scarce. Several examples failed to show tradeoffs (1)(2)(3)(4), whereas others could find them (5,6) or found general but not universal tradeoffs (7,8). A tradeoff between growth rate and growth yield in microbes (9-11) has direct implications for experiments carried out in liquid cultures.…”