John Maynard Smith was a pivotal figure in 20th-century evolutionary biology, recognized for fundamentally changing how scientists approach the subject. His primary importance lies in successfully integrating rigorous mathematics into evolutionary theory, providing a quantitative framework for analyzing complex biological phenomena. He moved the field beyond descriptive natural history by developing models that could predict the outcome of natural selection on behaviors, reproductive strategies, and the organization of life itself. His work introduced a new level of analytical precision, formalizing concepts that were previously discussed only qualitatively.
From Engineering to Evolutionary Biology
Maynard Smith’s intellectual path was unusual, beginning with a degree in aeronautical engineering from Cambridge University, a field he practiced during the Second World War. This early training instilled in him a deep appreciation for mathematical modeling and design principles. After the war, he made a sharp career change, deciding that the future of biology was more compelling than engineering. He then pursued a second degree in zoology and genetics under the guidance of the influential biologist J. B. S. Haldane at University College London. This background allowed him to approach evolutionary questions not just as a naturalist, but as an engineer seeking to understand the optimal design and functionality of living systems.
The Foundation of Evolutionary Game Theory
Maynard Smith’s most profound methodological contribution was the application of game theory, originally developed for economics, to the study of evolution. Working with George R. Price in the early 1970s, he introduced the concept of the Evolutionarily Stable Strategy (ESS). An ESS is a behavioral strategy that, once adopted by most members of a population, cannot be invaded by any rare, alternative strategy. This framework models the fitness consequences of an individual’s action based on what other individuals in the population are doing, making fitness frequency-dependent.
The Hawk-Dove game is the classic illustration, modeling a contest over a resource. Individuals adopt one of two strategies: Hawk (fight until injury or victory) or Dove (display but retreat if attacked). The payoff depends entirely on the opponent’s strategy. If the cost of injury is high relative to the resource value, the ESS is often a mixed strategy, where individuals randomly play Hawk a certain percentage of the time. This mixed ESS represents a stable equilibrium, explaining why certain behavioral polymorphisms, such as different mating tactics or foraging strategies, persist within a population.
The Puzzle of Sexual Reproduction
Maynard Smith devoted extensive theoretical work to the persistence of sexual reproduction despite its enormous costs. The most significant of these is the “two-fold cost of sex.” A sexual female passes only half of her genes to each offspring, while an asexual female passes all of her genes to a clone. If all else is equal, an asexual lineage should quickly double the reproductive rate of a sexual lineage, leading to the rapid extinction of sexual species.
The widespread existence of sexual reproduction requires a substantial, compensating advantage to overcome this cost. Maynard Smith proposed that the primary benefit of sex is the generation of genetic novelty and variation. This variation is particularly advantageous in constantly changing environments, such as those where host species are locked in an evolutionary arms race with parasites and pathogens.
This idea, known as the Red Queen hypothesis, suggests that species must continually adapt just to maintain their current relative fitness. The constant mixing of genes allows beneficial mutations to be combined and harmful ones to be purged much faster than in asexual species. His models demonstrated that the advantages of genetic recombination, especially under conditions of fluctuating selection, provide the fitness benefit required to maintain sex in natural populations.
Mapping Major Evolutionary Transitions
In his later career, Maynard Smith collaborated with Eörs Szathmáry to develop a theoretical framework for understanding the history of life through a sequence of major evolutionary transitions. This work focused on defining the events where smaller, independent entities began to cooperate and form larger, more complex entities with a new level of individuality. The transitions are characterized by a shift in how genetic information is stored, transmitted, and replicated, often leading to a new unit of selection.
Key examples include the shift from independent replicating molecules to chromosomes and the move from solitary cells to multicellular organisms. In each case, the formerly self-replicating units lost the ability to reproduce independently and became specialized parts of a larger, integrated whole. The framework explains that these transitions require mechanisms to suppress conflict between the lower-level units, ensuring that cooperation and the fitness of the new, higher-level entity are maintained.