year vary; (3) as a consequence, the parents of a particular year-class (cohort) do not represent a random sample from the entire population of the previous year; and (4) as a further consequence, allele- frequency differences among the newborns of consecutive years do not necessarily indicate temporal frequency changes for the total population (Jorde and Ryman 1995).
In an age-structured population, the collection of individuals for genetic analysis frequently focuses on a restricted set of age classes. For anadromous Atlantic salmon, commercial catches at sea will not include the youngest age classes, whereas electrofishing on the nursery grounds in streams will primarily sample subadult cohorts. Comparisons of different collections (say commercial catches and nursery-ground collections) are not comparisons of comparable age classes. Allele-frequency dynamics within a generation are such that comparisons among cohorts must be made with care.
To illustrate the different genetic dynamics of populations with discrete versus overlapping generations, Ryman (1997) designed a model of typical Atlantic salmon populations and simulated the effects over 200 years (Figure 5a). The most important observation from Figure 5a is that a population with overlapping generations displays considerably larger allele-frequency shifts from year to year than does a population with discrete generations of the same effective size. Moreover, there is a tendency for temporal correlation in the overlapping generation population; allele frequencies tend to fluctuate in a cyclical fashion that is not expected when generations are discrete.
Clearly, population size is not the only determinant of the amount of temporal allele- frequency change when generations overlap, because the total population does not represent a single genetically homogeneous unit. Rather, it consists of several age classes (six in the Ryman 1997 simulation; Figure 5b) that are produced, partly or completely, from different sets of parents, and, therefore, the different cohorts might exhibit different allele frequencies. The amount of temporal noise in the allele frequencies of the total population thus depends on the age structure (the relative proportions of the different age classes). Likewise, periodicity is introduced, because each cohort of progeny (young fish) is similar not to the cohort of progeny in the previous year but instead to the cohort to which most of the breeding fish of the previous year belonged. Those breeders were young fish several years previously. The magnitude of the variation and the length of the periodic cycle are functions of the number of cohorts present among the breeders (in any given year) and their relative contributions to reproduction. The amount of temporal allele-frequency noise also depends on the age-specific birth and death rates of the particular population (Jorde and Ryman 1995).