Debates concerning the units and levels of selection have persisted for over fifty years. One major question in this literature is whether units and levels of selection are genuine, in the sense that they are objective features of the world, or merely reflect the interests and goals of an observer. Scientists and philosophers have proposed a range of answers to this question. This Element introduces this literature and proposes a novel contribution. It defends a realist stance and offers a way of delineating genuine levels of selection by invoking the notion of a functional unit.

'Unit of selection' is a polysemic expression, meaning interactor, replicator/reproducer or manifestor of adaptation/type-1 agent in today's biological research. This Element presents each of these concepts and demonstrates the necessity of their isolation.

Among the most important innovations in the history of life is the transition from single-celled organisms to more complex, multicellular organisms. Multicellularity has evolved repeatedly across the tree of life, resulting in the evolution of new kinds of organisms that collectively constitute a significant portion of Earth’s biodiversity and have transformed the biosphere. This volume examines the origins and subsequent evolution of multicellularity, reviewing the types of multicellular groups that exist, their evolutionary relationships, the processes that led to their evolution, and the conceptual frameworks in which their evolution is understood. This important volume is intended to serve as a jumping-off point, stimulating further research by summarizing the topics that students and researchers of the evolution of multicellularity should be familiar with, and highlighting future research directions for the field. Chapter 13 of this book is freely available as a downloadable Open Access PDF at http://www.taylorfrancis.com under a Creative Commons Attribution-Non Commercial-No Derivatives (CC-BY-NC-ND) 4.0 license.

The biological world is full of phenomena that seem to run counter to Darwin's insight that natural selection can lead to the appearance of design. For instance, why do organisms in some species divide reproductive labor? The existence of non-reproducing organisms in such 'eusocial' species looks to be at odds with an evolutionary theory which posits traits exist because they help organisms survive and reproduce. What is the evolutionary advantage of an insect being distasteful to its predators? The distastefulness appears designed to deter predators, but can only affect the predator's actions when the insect is eaten; it is hard to see how such a trait could be passed on. This Element will cover the shared foundations of evolutionary explanations for these and other seemingly puzzling phenomena, focusing on the concepts of inclusive fitness and kin selection.

Recent arguments concerning the nature of causation in evolutionary theory, now often known as the debate between the 'causalist' and 'statisticalist' positions, have involved answers to a variety of independent questions – definitions of key evolutionary concepts like natural selection, fitness, and genetic drift; causation in multi-level systems; or the nature of evolutionary explanations, among others. This Element offers a way to disentangle one set of these questions surrounding the causal structure of natural selection. Doing so allows us to clearly reconstruct the approach that some of these major competing interpretations of evolutionary theory have to this causal structure, highlighting particular features of philosophical interest within each. Further, those features concern problems not exclusive to the philosophy of biology. Connections between them and, in two case studies, contemporary metaphysics and philosophy of physics demonstrate the potential value of broader collaboration in the understanding of evolution.

An innovative view of the role of fitness concepts in evolutionary theory. Natural selection is one of the factors responsible for changes in biological populations. Some traits or organisms are fitter than others, and natural selection occurs when there are changes in the distribution of traits in populations because of fitness differences. Many philosophers of biology insist that a trait’s fitness should be defined as an average of the fitnesses of individual members of the population that have the trait. Marshall Abrams argues convincingly against this widespread approach. As he shows, it conflicts with the roles that fitness is supposed to play in evolutionary theory and with the ways that evolutionary biologists use fitness concepts in empirical research. The assumption that a causal kind of fitness is fundamentally a property of actual individuals has resulted in unnecessary philosophical puzzles and years of debate. Abrams came to see that the fitnesses of traits that are the basis of natural selection cannot be defined in terms of the fitnesses of actual members of populations, as philosophers of biology often claim. Rather, it is an overall population-environment system—not actual, particular organisms living in particular environmental conditions—that is the basis of trait fitnesses. Abrams argues that by distinguishing different classes of fitness concepts and the roles they play in the practice of evolutionary biology, we can see that evolutionary biologists’ diverse uses of fitness concepts make sense together and are consistent with the idea that fitness differences cause evolution. Abrams’s insight has broad significance, for it provides a general framework for thinking about the metaphysics of biological evolution and its relations to empirical research. As such, it is a game-changing book for philosophers of biology, biologists who want deeper insight into the nature of evolution, and anyone interested in the applied philosophy of probability.