| The coexistence of diverse life histories within natural populations may arise from a combination of ecological and evolutionary processes. As yet, a complete theory of life history evolution that integrates multivariate selection, genetics, and age-structured populations does not exist and few empirical studies of natural populations have been conducted. Therefore, I use a demographic matrix model and an analysis of phenotypic selection on life history traits to address the maintenance of winter annuals and biennials within populations of the monocarpic plant, Campanula americana. The frequency of the types within two Michigan populations, survivorship and fecundity over four generations, and the presence of a seed bank are documented.; I describe a general and systematic procedure for deriving demographic loops for the life history types from the structure of the life cycle graph. Biennial loop elasticities are over 90% in 6 of the 8 data sets examined, indicating higher relative fitness of biennials and consistent with the greater frequency of biennials in these two populations. However, winter annuals had higher survivorship in at least one year in one of the populations, suggesting temporal variation in the relative performance of the life history types.; Experimental seed cages indicated that newly dispersed seed delayed germination, suggesting that the fall emerging winter annuals may be older than one year. Simulations of the seed bank dynamics demonstrate that the fraction of spring vs. fall emergence plays an important role in determining the relative contributions of winter annual and biennial life history pathways to the population growth rate.; Selection on size and time of emergence varied in magnitude and direction among populations, episodes in the life cycle, and within populations. In one population, selection favored earlier emergence in the experimental cages but later emerging seedlings in the area surrounding the cages. Selection acting early in the life time favored larger rosettes, generally biennials, but later this was offset by stabilizing selection on plant height.; I conclude that temporal and spatial variation in selection acting in concert with the age structure produced by the seed bank maintains the two life history types within populations. |
