The Philosophy of Evolutionary Biology

Philosophy of evolutionary biology is a major subfield of philosophy of biology concerned with the methods, conceptual foundations, and implications of evolutionary biology. It also concerns relationships between evolutionary biology and neighboring fields, such as biochemistry, genetics, cell and molecular biology, developmental biology, and ecology. Initially, many of the questions of central concern to philosophy of biology grew out of general philosophy of science. For instance, one long-standing debate in philosophy of science concerns the matter of what is distinctive of scientific inquiry. Various criteria have been proposed, and much of the early work in philosophy of biology concerned whether evolutionary biology meets these criteria. Another long-standing debate in philosophy of science concerns whether there is any legitimate role for values in science. The study of the evolution of human behavior and cognition has been scrutinized as an instance of both potentially pernicious and positive influence of values in science. More recently, philosophers of biology both collaborate with and draw upon evolutionary biology to either address broader philosophical concerns, such as the nature of consciousness, or engage directly with debates internal to evolutionary biology. For example, philosophers have engaged in conceptual and methodological debates within evolutionary biology over the appropriate conditions for testing hypotheses about adaptation, the units, targets, or levels of selection, mechanisms and measures of inheritance, modes of phylogenetic inference, and classification and systematics. In this category, the line between science and philosophy blurs; participants in many of these debates include both philosophers and biologists. This entry will focus on philosophers’ contributions. To be sure, evolutionary biologists have contributed far more. Please see the Oxford Bibliographies on these topics for scientific contributions to all of these topics. I also urge readers to review the excellent Stanford Encyclopedia of Philosophy entries on topics including but not limited to “Evolution,” “Natural Selection,” “Teleological Notions in Biology,” “Units and Levels of Selection,” “Adaptationism,” “Evolutionary Genetics,” “Evolutionary Psychology,” and “Developmental Biology.”

Adaptation and Natural Selection

Here non-random shifts in allele frequencies over time are introduced, as well as how to incorporate varying levels of selection into a model of a single population through time. This chapter highlights the difference between weak and strong selection, the dynamics of single allele versus genotype-level selection, and how selection strength and population size affect allele frequency distributions over time. Finally the inference of the selection coefficient from allele frequency data is discussed, alongside the concepts of overdominance and underdominance.

Nutrient status shapes selfish mitochondrial genome dynamics across different levels of selection

Cooperation and cheating are widespread evolutionary strategies. While cheating confers an advantage to individual entities within a group, competition between groups favors cooperation. Selfish or cheater mitochondrial DNA (mtDNA) proliferates within hosts while being selected against at the level of host fitness. How does environment shape cheater dynamics across different selection levels? Focusing on food availability, we address this question using heteroplasmic Caenorhabditis elegans. We find that the proliferation of selfish mtDNA within hosts depends on nutrient status stimulating mtDNA biogenesis in the developing germline. Interestingly, mtDNA biogenesis is not sufficient for this proliferation, which also requires the stress-response transcription factor FoxO/DAF-16. At the level of host fitness, FoxO/DAF-16 also prevents food scarcity from accelerating the selection against selfish mtDNA. This suggests that the ability to cope with nutrient stress can promote host tolerance of cheaters. Our study delineates environmental effects on selfish mtDNA dynamics at different levels of selection.

Symbiosis Promotes Fitness Improvements in the Game of Life

We present a computational simulation of evolving entities that includes symbiosis with shifting levels of selection. Evolution by natural selection shifts from the level of the original entities to the level of the new symbiotic entity. In the simulation, the fitness of an entity is measured by a series of one-on-one competitions in the Immigration Game, a two-player variation of Conway's Game of Life. Mutation, reproduction, and symbiosis are implemented as operations that are external to the Immigration Game. Because these operations are external to the game, we can freely manipulate the operations and observe the effects of the manipulations. The simulation is composed of four layers, each layer building on the previous layer. The first layer implements a simple form of asexual reproduction, the second layer introduces a more sophisticated form of asexual reproduction, the third layer adds sexual reproduction, and the fourth layer adds symbiosis. The experiments show that a small amount of symbiosis, added to the other layers, significantly increases the fitness of the population. We suggest that the model may provide new insights into symbiosis in biological and cultural evolution.

Experimental evolution of nascent multicellularity: Recognizing a Darwinian transition in individuality

Major evolutionary transitions in individuality, at any level of the biological hierarchy, occur when groups participate in Darwinian processes as units of selection in their own right. Identifying transitions in individuality can be problematic because apparent selection at one level of the biological hierarchy may be a by-product of selection occurring at another level. Here we discuss approaches to this “levels-of-selection” problem and apply them to a previously published experimental exploration of the evolutionary transition to multicellularity. In these experiments groups of the bacterium Pseudomonas fluorescens were required to reproduce via life cycles involving soma- and germline-like phases. The rate of transition between the two cell types was a focus of selection, and might be regarded as a property of groups, cells, or even genes. By examining the experimental data under several established philosophical frameworks, we argue that in the Pseudomonas experiments, bacterial groups acquired Darwinian properties sufficient to allow the evolution of traits adaptive at the group level.