Stable polymorphism

Stable polymorphism describes an evolutionarily stable balance in which different variants persist in a population rather than one simply eliminating the others. In the hawk-and-dove model, this balance can take the form of a stable ratio of hawk and dove genes, showing how the success of a strategy depends on what other members of the population do.

Definition and Population Balance

The term names a genetic state: a stable ratio of competing gene variants in the gene pool. In the model, hawk and dove are alternative behavioral strategies, and the balance between their associated genes persists because neither strategy can be replaced by a deviant alternative once the population has reached its evolutionarily stable state.

The model also allows an equivalent balance without distinct hawk and dove types. If every individual can behave in either way in each contest, all individuals may instead share the same probability of playing hawk. In the particular example, that probability is 7/12, or a 7:5 bias toward hawk. The population-level balance is then expressed as a probability within individuals rather than as a ratio of fixed behavioral types.

How the Model Works

The model’s balance depends on unpredictability as well as on the overall frequency of hawkish behavior. The choice must be random from a rival’s point of view: a predictable sequence, such as playing hawk seven times and then dove five times, would allow opponents to exploit the occasions on which dove was certain. A stable average does not by itself protect a strategy if its pattern can be anticipated.

This treatment belongs to the book’s broader account of behavioral evolution as a process of gene-frequency change. For a behavioral pattern to evolve, a gene influencing the development of nervous systems in ways that make the behavior more likely must persist more successfully in the gene pool than a rival gene or allele. That influence is indirect: genes shape the survival machine and its nervous system, while the nervous system makes moment-to-moment behavioral decisions.

Scope and Significance

Stable polymorphism is a result within a deliberately simplified model, not a claim that animals in nature literally fall into the model’s two behavioral categories. The book presents the hawk-and-dove story as a useful idealization that can be made more complex and brought closer to real situations. Its value is to clarify how population composition can determine which strategies remain viable.

The model’s genetic language also sits alongside an important qualification: genes do not act as independent agents in development. Many genes interact in building a body, and a given gene may influence multiple traits while each trait depends on many genes and environmental conditions. Stable polymorphism therefore describes a population-level selection outcome, not a simple one-gene blueprint for an individual’s conduct.

The concept helps distinguish an evolutionary equilibrium from conscious agreement. Human groups may form pacts that benefit their members over the long term, but such arrangements can be undermined when individuals gain from breaking them. In the book’s account of wild-animal behavior, stable strategies instead arise through selection among alternatives; once an evolutionarily stable balance is reached, selection penalizes deviation from it.