  Fitnesses, which should be understood simply as the aggregation of probable-growth factors that drive the dynamics of large populations, may depend on a variety of factors. Fitness components differ from variation components in that they affect population frequencies proportionally to those frequencies, that is to say, multiplicatively. Fitness components in biological evolution include mortality and reproductive rate. In cultural evolution, they include transmission probability and rejection probability. Within either sort of model, what matters is how fitnesses change as a result of other changing factors within the model. In the simplest cases, fitnesses are fixed and the type with the highest fitness inevitably dominates the population. In more complex cases, fitnesses may depend on variable factors like who one plays against, or the state of a variable environment. Most commonly, variable fitnesses are calculated using a payoff matrix like the one above. In general, to calculate the expected fitness of a type, one multiplies the fitness a type would have in each situation times the likelihood that individuals in the population will confront that situation and adds the resulting products. 