A chapter from the Ph.D. thesis of Pengyao Jiang was recently published in the Journal of Evolutionary Biology. Dr. Jiang, a newly minted Asst. Professor of Evolutionary Biology at Arizona State University, earned her Ph.D. in the Department of Ecology and Evolution at U. Chicago. Dr. Jiang's paper addresses a long-standing question in the field: whether canalization is required for, or tends to prevent, adaptation. The former idea was first set forth by C. H. Waddington in his 1942 article, while the latter completely contrasting idea is the consensus of most modern evolutionary biologists. Dr. Jiang's paper, entitled "The effect of mutational robustness on the evolvability of multicellular organisms and eukaryotic cells,” uses a Boolean population model to analyze the relationship between robustness and evolvability. It "established several new insights into the relationship between canalization and evolution in multicellular and other eukaryotic organisms," and, as the authors noted, resolves the "tension" between the two different views on how canalization affects adaptation. The paper was co-written by Jiang and her thesis advisor Professor John Reinitz, with additional contributions from Professor Emeritus Martin Kreitman.
Paper Abstract
Canalization involves mutational robustness, the lack of phenotypic change as a result of genetic mutations. Given the large divergence in phenotype across species, understanding the relationship between high robustness and evolvability has been of interest to both theorists and experimentalists. Although canalization was originally proposed in the context of multicellular organisms, the effect of multicellularity and other classes of hierarchical organization on evolvability has not been considered by theoreticians. We address this issue using a Boolean population model with explicit representation of an environment in which individuals with explicit genotype and a hierarchical phenotype representing multicellularity evolve. Robustness is described by a single real number between zero and one which emerges from the genotype-phenotype map. We find that high robustness is favoured in constant environments, and lower robustness is favoured after environmental change. Multicellularity and hierarchical organization severely constrain robustness: peak evolvability occurs at an absolute level of robustness of about 0.99 compared with values of about 0.5 in a classical neutral network model. These constraints result in a sharp peak of evolvability in which the maximum is set by the fact that the fixation of adaptive mutations becomes more improbable as robustness decreases. When robustness is put under genetic control, robustness levels leading to maximum evolvability are selected for, but maximal relative fitness appears to require recombination.