title: There is no fitness but fitness, and the lineage is its bearer.
abstract: Inclusive fitness has been the cornerstone of social evolution theory for more than a half-century and has matured as a mathematical theory in the past 20 years. Yet surprisingly for a theory so central to an entire field, some of its connections to evolutionary theory more broadly remain contentious or underappreciated. In this paper, we aim to emphasize the connection between inclusive fitness and modern evolutionary theory through the following fact: inclusive fitness is simply classical Darwinian fitness, averaged over social, environmental and demographic states that members of a gene lineage experience. Therefore, inclusive fitness is neither a generalization of classical fitness, nor does it belong exclusively to the individual. Rather, the lineage perspective emphasizes that evolutionary success is determined by the effect of selection on all biological and environmental contexts that a lineage may experience. We argue that this understanding of inclusive fitness based on gene lineages provides the most illuminating and accurate picture and avoids pitfalls in interpretation and empirical applications of inclusive fitness theory.
authors.affiliation: Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA eakcay@sas.upenn.edu.
authors.forename: Erol
authors.identifier: http://orcid.org/0000-0001-8149-7124
authors.initials: E
authors.lastname: Akçay
authors.affiliation: Department of Biology, University of Kentucky, Lexington, KY 40506-0225, USA.
authors.forename: Jeremy
authors.identifier: http://orcid.org/0000-0003-3656-4257
authors.initials: J
authors.lastname: Van Cleve
mesh_terms: D000818:Animals; D005075:Biological Evolution; D003299:Cooperative Behavior; D004777:Environment; D005260:Female; D056084:Genetic Fitness; D006801:Humans; D008297:Male; D008954:Models, Biological; D008957:Models, Genetic; D009154:Mutation; D012098:Reproduction; D012641:Selection, Genetic; D012919:Social Behavior; D014422:Turkeys
keywords: Hamilton's rule; class structure; inclusive fitness; invasion fitness; lineage; non-additive interactions