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Evolution of resource cycling in ecosystems and individuals.

Crombach A, Hogeweg P - BMC Evol. Biol. (2009)

Bottom Line: If individuals are fixed at lattice sites or direct selection is low, indirect selection result in lineages that structure their local environment, leading to 'smart' individuals and stable patterns of resource dynamics.In a baseline model of ecosystem evolution we demonstrate different eco-evolutionary trajectories of resource cycling.By varying the strength of indirect selection through the spatial setting and direct selection, the integration of information by the evolutionary process leads to qualitatively different results from individual smartness to cooperative community structures.

View Article: PubMed Central - HTML - PubMed

Affiliation: Theoretical Biology and Bioinformatics Group, Utrecht University, Utrecht, The Netherlands. anton.crombach@gmail.com

ABSTRACT

Background: Resource cycling is a defining process in the maintenance of the biosphere. Microbial communities, ranging from simple to highly diverse, play a crucial role in this process. Yet the evolutionary adaptation and speciation of micro-organisms have rarely been studied in the context of resource cycling. In this study, our basic questions are how does a community evolve its resource usage and how are resource cycles partitioned?

Results: We design a computational model in which a population of individuals evolves to take up nutrients and excrete waste. The waste of one individual is another's resource. Given a fixed amount of resources, this leads to resource cycles. We find that the shortest cycle dominates the ecological dynamics, and over evolutionary time its length is minimized. Initially a single lineage processes a long cycle of resources, later crossfeeding lineages arise. The evolutionary dynamics that follow are determined by the strength of indirect selection for resource cycling. We study indirect selection by changing the spatial setting and the strength of direct selection. If individuals are fixed at lattice sites or direct selection is low, indirect selection result in lineages that structure their local environment, leading to 'smart' individuals and stable patterns of resource dynamics. The individuals are good at cycling resources themselves and do this with a short cycle. On the other hand, if individuals randomly change position each time step, or direct selection is high, individuals are more prone to crossfeeding: an ecosystem based solution with turbulent resource dynamics, and individuals that are less capable of cycling resources themselves.

Conclusion: In a baseline model of ecosystem evolution we demonstrate different eco-evolutionary trajectories of resource cycling. By varying the strength of indirect selection through the spatial setting and direct selection, the integration of information by the evolutionary process leads to qualitatively different results from individual smartness to cooperative community structures.

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Distributions of individual 'smartness'. In both series of frequency plots we have taken per 1000 time steps a sample of 100 individuals, over the interval [12.5·104, 25·104]. With 6 × 25 runs this results in 6 data sets of 325000 individuals. A. Frequency plot of smartness defined as the sum of an individual's phenotype. Note the logarithmic y-axis. B. Frequency plot of smartness as an individual's shortest cycle. We calculated the shortest cycle given the presence of the most abundant resources at each sampling point. A minimal number of resources was selected such that the grid was covered by a 0.95 fraction. The three panels show the distribution of shortest cycle lengths of individuals that could actually perform a cycle. For individuals that were incapable of doing so see additional file 4: Figure S3.
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Figure 9: Distributions of individual 'smartness'. In both series of frequency plots we have taken per 1000 time steps a sample of 100 individuals, over the interval [12.5·104, 25·104]. With 6 × 25 runs this results in 6 data sets of 325000 individuals. A. Frequency plot of smartness defined as the sum of an individual's phenotype. Note the logarithmic y-axis. B. Frequency plot of smartness as an individual's shortest cycle. We calculated the shortest cycle given the presence of the most abundant resources at each sampling point. A minimal number of resources was selected such that the grid was covered by a 0.95 fraction. The three panels show the distribution of shortest cycle lengths of individuals that could actually perform a cycle. For individuals that were incapable of doing so see additional file 4: Figure S3.

Mentions: For each setting (3 selection regimes, local and model) we sampled 3.25·104 individuals in the second half of the runs, from 12.5·104 to 25·104. In Figure 9A the resulting distributions of smartness are shown. In all cases there was a peak of 'stupid' individuals close to zero. These were simply mutants that had an extremely deleterious mutation. Secondly, all showed another peak around 300. Thus independent of the selection pressure or spatial setting most individuals were equally smart. Thirdly, it was in the right tail of the distributions that we found distinct behavior for each σ . With increasing σ, individuals became smarter in both models, and even more so in the local model.


Evolution of resource cycling in ecosystems and individuals.

Crombach A, Hogeweg P - BMC Evol. Biol. (2009)

Distributions of individual 'smartness'. In both series of frequency plots we have taken per 1000 time steps a sample of 100 individuals, over the interval [12.5·104, 25·104]. With 6 × 25 runs this results in 6 data sets of 325000 individuals. A. Frequency plot of smartness defined as the sum of an individual's phenotype. Note the logarithmic y-axis. B. Frequency plot of smartness as an individual's shortest cycle. We calculated the shortest cycle given the presence of the most abundant resources at each sampling point. A minimal number of resources was selected such that the grid was covered by a 0.95 fraction. The three panels show the distribution of shortest cycle lengths of individuals that could actually perform a cycle. For individuals that were incapable of doing so see additional file 4: Figure S3.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2698886&req=5

Figure 9: Distributions of individual 'smartness'. In both series of frequency plots we have taken per 1000 time steps a sample of 100 individuals, over the interval [12.5·104, 25·104]. With 6 × 25 runs this results in 6 data sets of 325000 individuals. A. Frequency plot of smartness defined as the sum of an individual's phenotype. Note the logarithmic y-axis. B. Frequency plot of smartness as an individual's shortest cycle. We calculated the shortest cycle given the presence of the most abundant resources at each sampling point. A minimal number of resources was selected such that the grid was covered by a 0.95 fraction. The three panels show the distribution of shortest cycle lengths of individuals that could actually perform a cycle. For individuals that were incapable of doing so see additional file 4: Figure S3.
Mentions: For each setting (3 selection regimes, local and model) we sampled 3.25·104 individuals in the second half of the runs, from 12.5·104 to 25·104. In Figure 9A the resulting distributions of smartness are shown. In all cases there was a peak of 'stupid' individuals close to zero. These were simply mutants that had an extremely deleterious mutation. Secondly, all showed another peak around 300. Thus independent of the selection pressure or spatial setting most individuals were equally smart. Thirdly, it was in the right tail of the distributions that we found distinct behavior for each σ . With increasing σ, individuals became smarter in both models, and even more so in the local model.

Bottom Line: If individuals are fixed at lattice sites or direct selection is low, indirect selection result in lineages that structure their local environment, leading to 'smart' individuals and stable patterns of resource dynamics.In a baseline model of ecosystem evolution we demonstrate different eco-evolutionary trajectories of resource cycling.By varying the strength of indirect selection through the spatial setting and direct selection, the integration of information by the evolutionary process leads to qualitatively different results from individual smartness to cooperative community structures.

View Article: PubMed Central - HTML - PubMed

Affiliation: Theoretical Biology and Bioinformatics Group, Utrecht University, Utrecht, The Netherlands. anton.crombach@gmail.com

ABSTRACT

Background: Resource cycling is a defining process in the maintenance of the biosphere. Microbial communities, ranging from simple to highly diverse, play a crucial role in this process. Yet the evolutionary adaptation and speciation of micro-organisms have rarely been studied in the context of resource cycling. In this study, our basic questions are how does a community evolve its resource usage and how are resource cycles partitioned?

Results: We design a computational model in which a population of individuals evolves to take up nutrients and excrete waste. The waste of one individual is another's resource. Given a fixed amount of resources, this leads to resource cycles. We find that the shortest cycle dominates the ecological dynamics, and over evolutionary time its length is minimized. Initially a single lineage processes a long cycle of resources, later crossfeeding lineages arise. The evolutionary dynamics that follow are determined by the strength of indirect selection for resource cycling. We study indirect selection by changing the spatial setting and the strength of direct selection. If individuals are fixed at lattice sites or direct selection is low, indirect selection result in lineages that structure their local environment, leading to 'smart' individuals and stable patterns of resource dynamics. The individuals are good at cycling resources themselves and do this with a short cycle. On the other hand, if individuals randomly change position each time step, or direct selection is high, individuals are more prone to crossfeeding: an ecosystem based solution with turbulent resource dynamics, and individuals that are less capable of cycling resources themselves.

Conclusion: In a baseline model of ecosystem evolution we demonstrate different eco-evolutionary trajectories of resource cycling. By varying the strength of indirect selection through the spatial setting and direct selection, the integration of information by the evolutionary process leads to qualitatively different results from individual smartness to cooperative community structures.

Show MeSH