J. Arvid Ågren

In biology, individuals can be identified at various levels of organization, ranging from the molecular and cellular—e.g., viruses and protists—to the organismal and societal—e.g., termites and termite colonies. This nested biological hierarchy results from evolutionary transitions in individuality, in which lower-lying entities (particles) come together to form higher-level entities (collectives). These collectives are then taken to be individuals in their own right, capable of being the units on which selection acts and where adaptations may reside. Challenges arise, however, when we recognize the possibility for internal conflict, for example from selfish genetic elements within organisms. Here we develop a quantitative approach to biological individuality, in which collectives qualify as individuals to the extent that their particles possess a unity of purpose—i.e., that they possess aligned fitness interests. We propose a mathematical framework that measures the degree of individuality exhibited by organisms and other biological collectives in the presence of internal conflicts. This framework not only helps pinpoint where in the hierarchy biological individuality sits, but also identifies the level at which adaptation may reside. We develop two metrics: fitness unity, which measures the threat posed by internal conflicts to a collective individual’s status as an optimizing agent; and trait unity, which measures differences in optimal strategies among its particles. Finally, we consider cases where internal conflicts become so disruptive to individualized biological collectives so that it no longer makes sense to think of them as individuals, but instead as compromises of multiple competing particles.

The gene's-eye view of evolution is an influential but contentious perspective on biology. It emerged in the aftermath of the Modern Synthesis and both proponents and detractors have stressed the link between the two. In particular, both the Modern Synthesis and the gene's-eye view have been criticized for overemphasizing the role genes at the expense of organisms in evolutionary explanations. In this chapter, I discuss the connection between the Modern Synthesis and the gene’s-eye view and evaluate the status of genes and organisms in contemporary biology. I show that while the gene’s-eye view traces its origin back to the Modern Synthesis, it can most accurately be said to represent a specific – adaptationist and gene-centric – version of it. To assess the role of genes and organisms, I examine the intimate relationship between the gene’s-eye view and another post-Synthesis development, the concept of inclusive fitness. I argue that the popularity and influence of inclusive fitness theory demonstrate that the individual organism remains safe at the heart of modern evolutionary biology.

Hamilton’s Rule, named after the evolutionary biologist Bill Hamilton, and the related concepts of inclusive fitness and kin selection, have been the bedrock of the study of social evolution for the past half century. In ’The Philosophy of Social Evolution’, Jonathan Birch provides a comprehensive introduction to the conceptual foundations of the Hamiltonian view of social evolution, and a passionate defence of its enduring value in face of the recent high profile criticism. In this review essay, I first outline the version of Hamilton’s Rule defended by Birch, dubbed Hamilton’s Rule General, and its derivation. With this in place, I then navigate through the fierce disagreements that Hamilton’s Rule generates among social evolution researchers and evaluate Birch’s central argument of the book that HRG serves as an organizing framework for social evolution research under which we can compare and interpret more detailed causal models. I then spend the remainder of the review discussing what I take to be three of the most exciting implications of Hamilton’s thinking raised by Birch: the extension of Hamilton’s Rule to mobile genetic elements, maximization of inclusive fitness models and the idea of adaptation as organism design, and the relationship between Hamilton’s approaches to social behaviour and the gene’s-eye view of evolution.