Chapter 12 - Individual Fitness

Abstract

This chapter considers individual fitness, defined by analogy to population fitness, in terms of individual-specific survival and reproductive parameters, determined at birth. In this context, the distinction between realization and capacity is especially important since individuals only live once. Individual fitness is a derived parameter, one that is best evaluated in context of a hierarchical model, and naturally evaluated under the Bayesian māramatanga. A population's fitness is its capacity to persist. It is a summary of potentialities, rather than a summary of outcomes. In discussing fitness a mathematical model of population change is required. Fitness is a latent feature of the population and is defined as a function of model parameters. In genetic terms, fitness is the capacity of a genotype to be propagated into future generations. It is more natural to conceive of genotypic fitness in terms of individuals; genetically identical populations are the exception rather than the rule. It makes sense to think of individual-specific fecundity rates and survival rates, latent features associated entirely with an individual's genetic make-up. Realized fitness is a random variable with distribution determined by latent parameters; latent fitness is a deterministic function of the latent parameters. The problem with realized individual fitness is that it is an estimate based on a sample of size 1, the single realization of an individual's life history. Fortunately, one does not need to operate in this way, but rather may consider individuals in a group context, seeing individual-specific parameters as stochastically related.