Equivalence, Interactors, and Lloyd’s Challenge to Genic Pluralism

  • Ryan Ketcham


On a train ride in 1986, Elisabeth Lloyd convinced Stephen Jay Gould he had been wrong about species selection. The mistake had to do with differentiating what Lloyd came to call the “interactor question” from three other distinct questions implicit in the units of selection controversy. Lloyd’s efforts to answer this question led her to propose “emergent fitness” with Gould, develop the additivity criterion, and allowed the dissection of the units debates that was endorsed by George Williams and John Maynard Smith. It also led her into conflict with contemporary genic pluralists who argue that higher and lower level models are equivalent. Lloyd’s approach to the interactor question led her to show why this is a mistake and is key to understanding the controversy.


Interactors Pluralism Additivity criterion Group selection Contextual analysis Multilevel selection 



I would like to thank Elisabeth Lloyd for her generosity and candid insight over the course of multiple extensive discussions regarding the development of her ideas and how they were received by a host of distinguished figures she engaged with over the course of her career so far. I would also like to thank Michael Wade for discussion of some of the technical details, Ciprian Jeler for helpful editorial insight, and especially Lisa Ferrier for all of her crucial support.


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© The Author(s) 2018

Authors and Affiliations

  • Ryan Ketcham
    • 1
  1. 1.Indiana UniversityBloomingtonUSA

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