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The evolutionary origin of complex features


A long-standing challenge to evolutionary theory has been whether it can explain the origin of complex organismal features. We examined this issue using digital organisms—computer programs that self-replicate, mutate, compete and evolve. Populations of digital organisms often evolved the ability to perform complex logic functions requiring the coordinated execution of many genomic instructions. Complex functions evolved by building on simpler functions that had evolved earlier, provided that these were also selectively favoured. However, no particular intermediate stage was essential for evolving complex functions. The first genotypes able to perform complex functions differed from their non-performing parents by only one or two mutations, but differed from the ancestor by many mutations that were also crucial to the new functions. In some cases, mutations that were deleterious when they appeared served as stepping-stones in the evolution of complex features. These findings show how complex functions can originate by random mutation and natural selection.

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Figure 1: Digital organism in Avida.
Figure 2: Phylogenetic depth versus time in the case-study population.
Figure 3: Trajectories for two fitness components, showing each genotype in the line of descent for the case-study population.
Figure 4: Functional-genomic array for the first organism to perform EQU in the case-study population.

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We thank A. Bennett, J. Bull, J. Coyne, D. Lenski, M. Lenski and E. Zuckerkandl for comments. The authors' work is supported by the US National Science Foundation Biocomplexity Program and by the MSU Foundation. Part of this work was carried out at the Jet Propulsion Laboratory under contract with the US National Aeronautics and Space Administration.

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Correspondence to Richard E. Lenski.

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Supplementary information


Supplementary Information: Sections I, II and III provide more background information on Avida and the logic functions that digital organisms perform. Section IV provides additional data on phenotypic and genomic evolution along the line of descent in the case-study population, up to the time of origin of the EQU function. (PDF 42 kb)

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Lenski, R., Ofria, C., Pennock, R. et al. The evolutionary origin of complex features. Nature 423, 139–144 (2003).

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