Life History And Fitness

Life history is the progress of an individual throughout his or her life. An individual is first born, then grows into an adult or fails to survive to adulthood. If the individual survives to adulthood, then the individual perhaps mates and reproduces at specific ages and finally dies at some age. In Chapter 11, we defined the fitness components of viability, mating success, and fertility/fecundity. To examine life history and its evolutionary implications, we must first make each of these fitness components an explicit function of age. We start with the fitness component of viability. Viability can be measured in an age-specific fashion by the age-specific survivorship, lx, the probability of an individual surviving to age x. Ideally, age should be measured from fertilization in order to cover the diploid individual's entire life history, but in practice a time point well after conception is used in many species. For mammals, and humans in particular, the initial time point is usually birth. This obviously misses any deaths that occur between conception and birth.

Table 15.1 illustrates the concept of age-specific survivorship through the example of females from the United States as determined by the 2000 U.S. census data. As shown in this table, age is often treated not as a continuous variable but rather as a series of consecutive categories or ranges of ages. This represents the practical constraint of how such data are gathered, but in theory one could treat age as a continuous variable. However, in this chapter we bow to the reality of actual data and will treat age as an ordered categorical variable.

Table 15.1. Life History Table for U.S. Females Based on 2000 Census Data

Age Range (years)

Assigned Age, x




x l x ^M xbx


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