Understanding the mechanisms and processes that generate biological diversity is a fundamental problem in evolution and ecology. In the past decade, the theory of evolutionary branching and adaptive diversification has provided new perspectives for understanding the evolution of diversity caused by ecological interactions. In models of adaptive diversification, the fitness landscapes change dynamically, so that the likelihood of diversification into different phenotypic clusters increases over time. In contrast, in models with static fitness landscapes, the likelihood of diversification decreases as populations climb fitness peaks, because crossing maladaptive fitness valleys becomes increasingly difficult. We used experimental evolution in bacteria to test how the likelihood of diversification changes over time in a bacterial lineage that has diversified in sympatry from a single ancestral strain. By analyzing the ''fossil'' record of this lineage, and restarting the lineage from different time points in the evolutionary past, we demonstrate that: (i) the lineage has initially undergone a phase of directional adaptation to the competitive environment, and (ii) during this phase, the likelihood of diversification increases significantly over time. These results suggest evolutionary branching caused by frequency-dependent competition as the main mechanism of diversification in our experimental populations.Escherichia coli ͉ evolutionary branching ͉ experimental evolution ͉ frequency dependence D iversification is thought to occur when descendent populations come to occupy different ecological niches. This situation is formalized by adaptive landscapes, in which different peaks correspond to different niches (1-5). When fitness landscapes are static and fixed over time, diversification can occur when an ancestral population is near a trough in such a landscape, and different subpopulations evolve toward different fitness peaks. In this case, it becomes increasingly difficult to cross the fitness valleys if the ancestral population evolves up a fitness peak. As a consequence, diversification becomes less likely with increasing adaptation (1, 6-8).In contrast, if the fitness landscape depends on the current position of the population in phenotype space, i.e., if selection is frequency-dependent, the landscape changes as the population evolves. In this case, new niches (i.e., fitness peaks) may only open up as the population progresses along its evolutionary trajectory, and hence diversification may become more likely over time. An elegant framework for describing diversification when selection is frequency-dependent is provided by the theory of adaptive dynamics (9-11), in which adaptive diversification is described by the mathematical phenomenon of evolutionary branching (10). When a population undergoes evolutionary branching, it first converges under directional selection toward a point in phenotype space at which selection turns disruptive because of frequency-dependent interactions. From this evolutionary branc...

BackgroundThe evolutionary consequences of competition are of great interest to researchers studying sympatric speciation, adaptive radiation, species coexistence and ecological assembly. Competition's role in driving evolutionary change in phenotypic distributions, and thus causing ecological character displacement, has been inferred from biogeographical data and measurements of divergent selection on a focal species in the presence of competitors. However, direct experimental demonstrations of character displacement due to competition are rare.ResultsWe demonstrate a causal role for competition in ecological character displacement. Using populations of the bacterium Escherichia coli that have adaptively diversified into ecotypes exploiting different carbon resources, we show that when interspecific competition is relaxed, phenotypic distributions converge. When we reinstate competition, phenotypic distributions diverge.ConclusionThis accordion-like dynamic provides direct experimental evidence that competition for resources can cause evolutionary shifts in resource-related characters.

Adaptive speciation has gained popularity as a fundamental process underlying the generation of diversity. We tested whether populations respond to similar forms of disruptive selection by diversifying in similar or parallel ways by investigating diversified populations of Escherichia coli B evolved in glucose and glucoseacetate environments. In both environments, the populations have differentiated into two phenotypes, named for their characteristic colony morphologies: large (L) and small (S). Each type is heritable and this polymorphism (or 'diversified pair') appears to be maintained by negative frequency dependence. The L and S phenotypes from different environments are convergent in their colony morphology and growth characteristics. We tested whether diversification was parallel by conducting competition experiments between L and S types from different environments. Our results indicate that replicate diversified pairs from different environments have not diversified in parallel ways and suggest that subtle differences in evolutionary environment can crucially affect the outcome of adaptive diversification.

During a 1000-generation evolution experiment, two types of morphologically and kinetically distinct bacteria repeatedly diverged from a common ancestor in a fully sympatric seasonal environment containing glucose and acetate. To investigate the metabolic modifications associated with this adaptive diversification, we compared transcription profiles of the two derived types and the common ancestor. Both derived types share a suite of common metabolic changes that may represent adaptation to the environment preceding the diversification event. These include improved translation efficiency, glucose uptake capacity via the mal/ lamB genes, upregulation of various transporters during stationary phase, and likely the disruption of the rbs operon. The diversification event is associated with the overexpression of genes involved in the TCA cycle, glyoxylate shunt, acetate consumption, and anaerobic respiration in one type and in acetate excretion in the other. These results reveal that competition for both carbon and oxygen have likely played an important role in the adaptation of Escherichia coli during this adaptive diversification event, where one derived type mainly consumes glucose at a fast rate when oxygen is not limiting, and the other derived type consumes glucose and acetate at a slower rate, even when oxygen is limiting.

BackgroundExplanations for bacterial biofilm persistence during antibiotic treatment typically depend on non-genetic mechanisms, and rarely consider the contribution of evolutionary processes.ResultsUsing Escherichia coli biofilms, we demonstrate that heritable variation for broad-spectrum antibiotic resistance can arise and accumulate rapidly during biofilm development, even in the absence of antibiotic selection.ConclusionsOur results demonstrate the rapid de novo evolution of heritable variation in antibiotic sensitivity and resistance during E. coli biofilm development. We suggest that evolutionary processes, whether genetic drift or natural selection, should be considered as a factor to explain the elevated tolerance to antibiotics typically observed in bacterial biofilms. This could be an under-appreciated mechanism that accounts why biofilm populations are, in general, highly resistant to antibiotic treatment.