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Separating Spandrels from Phenotypic Targets of Selection in Adaptive Molecular Evolution

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Evolutionary Biology

Abstract

There are many examples of adaptive molecular evolution in natural populations, but there is no existing method to verify which phenotypic changes were directly targeted by selection. The problem is that correlations between traits make it difficult to distinguish between direct and indirect selection. A phenotype is a direct target of selection when that trait in particular was shaped by selection to better perform a function. An indirect target of selection, also known as an evolutionary spandrel, is a phenotype that changes only because it is correlated with another trait under direct selection. Studies that mutate genes and examine the phenotypic consequences are increasingly common, and these experiments could estimate the mutational accessibility of the phenotypic changes that arise during an instance of adaptive molecular evolution. Under indirect selection, we expect phenotypes to evolve toward states that are more accessible by mutation (i.e., states with high mutational entropy). Deviation from this null expectation (evolution toward a phenotypic state rarely produced by mutation) would be compelling evidence of adaptation and could be used to distinguish direct selection from indirect selection on correlated traits. To be practical, this molecular test of adaptation requires phenotypic differences that are caused by changes in a small number of genes. These kinds of genetically-simple traits have been observed in many empirical studies of adaptive evolution. Here, we describe how to use mutational accessibility to separate spandrels from direct targets of selection and thus verify adaptive hypotheses for phenotypes that evolve by molecular changes at one or a few genes.

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Acknowledgements

SAS is grateful to WJ Swanson, J Tyerman, F Breden, and HD Bradshaw for helpful discussions. An NSERC PGSD International Graduate Fellowship to SAS supported this work. MM was supported by NIH grant F32 GM116217. AVM and SAS are grateful to Pierre Pontarotti for his extraordinary hospitality, which enabled the exchange of ideas that ultimately made this work possible.

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Correspondence to Alexandre V. Morozov .

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© 2016 Springer International Publishing Switzerland

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Springer, S.A., Manhart, M., Morozov, A.V. (2016). Separating Spandrels from Phenotypic Targets of Selection in Adaptive Molecular Evolution. In: Pontarotti, P. (eds) Evolutionary Biology. Springer, Cham. https://doi.org/10.1007/978-3-319-41324-2_18

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