In the light of evolution V: cooperation and conflict.

T his paper serves as an introduction to the fifth in a series of Colloquia under the general title “In the Light of Evolution” (ILE; see Box 1). Papers from the first four colloquia in the “In the Light of Evolution” series, titled “Adaptation and Complex Design,” “Biodiversity and Extinction,” “Two Centuries of Darwin,” and “The Human Condition,” are refs. 1–4, respectively. The current volume explores recent developments in the study of cooperation and conflict, ranging from the level of the gene to societies and symbioses. Any student of history knows that humans can be a vicious lot, but paradoxically, we are also among nature’s great cooperators. Which of us, as an individual, can manufacture a cell phone or an airplane? Even our great conflicts—wars— are extremely cooperative endeavors on each side. Some of this cooperation is best understood culturally, but we are also products of evolution, with bodies, brains, and behaviors molded by natural selection. How cooperation evolves has been one of the big questions in evolutionary biology, and how it pays or does not pay is a great intellectual puzzle. If nothing makes sense in biology except in the light of evolution (5), then for the first century after Darwin, cooperation and altruism did not make much sense. We could see that individual organisms sometimes helped others, even at a cost to their own fitness. It was clear that such behavior could benefit the group, population, species, or even other species and whole communities. However, it was not obvious how such effects would be heritable. All our mathematical models—the hard work of the modern synthesis—were about individuals with one allele out-reproducing those with an alternative. This process would favor individuals with higher reproduction but would not be expected to produce self-sacrifice. However, apparent cooperation was routinely attributed to the good of the group, species, or community. This situation changed in the first decade of Darwinism’s second century. Hamilton (6, 7) argued that cooperation was important in nature and that social evolution could be understood in terms of direct gains to the actor’s own fitness or indirect benefits to the fitness of others who share the cooperation allele. There followed an intense period of exploring the indirect effects of cooperation and altruism, reinterpreting sexual selection and many other phenomena in terms of individual advantage, and understanding frequencydependent effects through game theory, efforts that continue to the present. The puzzle of cooperation was the dominant theme of research in the early years, whereas recent work has emphasized its importance and ubiquity. Far from being a rare trait shown by social insects and a few others, cooperation is both widespread taxonomically and essential to life. Major transitions in the hierarchy of life have often involved cooperation among lower-level units to the point where they evolve into higher-level organisms (8, 9). Examples include the assembly of the eukaryotic cell with its symbiotic organelles, the evolution of multicellular organisms, and the organismal colonies of some social insects. Organisms are, at multiple levels, those units that have evolved to have, within their boundaries, extreme cooperation and minimal conflict (10, 11). The depth of research on cooperation and conflict has increased greatly, most notably in the direction of the small organisms. Microbes turn out to have highly developed cooperation (12), and they, along with other model organisms, have proven instrumental in beginning to understand sociality at the genetic and molecular levels, the study of real selfish genes (13). The social evolution approach has given us insights on diseases often caused by microbes (14). At the other end of the spectrum, we are getting a much better understanding of the cooperation and conflict that matters most to our species (15). Cooperation has been central to humanity’s spectacular success and will be central to our shortand long-term fates.