Evolution in the Real World: Stochastic Variation and the Determinants of Fitness in Carlina Vulgaris

Abstract

Empirical studies of life histories often ignore stochastic variation, despite theoretical demonstrations of its potential impact on life-history evolution. Here we use a novel approach to explore the effects of stochastic variation on life-history evolution and estimate the selection pressures operating on the monocarpic perennial Carlina vulgaris, in which flowering may be delayed by up to eight years. The approach is novel in that we use modern theoretical techniques to estimate selection pressures and the fitness landscape from a fully parameterised individual-based model. These approaches take into account temporal variation in demographic rates and density dependence. Analysis of 16 years' data revealed significant temporal variation in growth, mortality, and recruitment in our study population. Flowering was strongly size dependent and, unusually for such a species, also age dependent. Individual-based models of the flowering strategy, parameterized using field data, consistently underestimated the size at flowering, when temporal variation in demographic rates was ignored. In contrast, models that incorporated temporal variation in growth, mortality, and recruitment predicted sizes at flowering not significantly different from those observed in the field. Temporal variation in mortality, which had the largest effect on the flowering strategy, selected for increased size at flowering. An analytical approximation is presented to explain this result, extending the "1-year look-ahead criterion" presented in Rees et al. (2000). A fitness landscape generated by following the fate of rare mutant invaders with a broad range of alternative flowering strategies demonstrated that the observed parameters were adaptive. However, the fitness landscape reveals that approximately equal fitness is achieved by a broad range of strategies, providing a mechanism for the maintenance of genetic variation. To understand how the different parameters that defined our models determine the fitness of rare mutants, we numerically estimated the elasticities and sensitivities of mutant fitness. This demonstrated strong selection on a number of the parameters. Elasticities and sensitivities estimated in constant and random environments were significantly positively correlated, and both were negatively related to the standard error of the parameter. This last result is surprising and, we argue, reflects the genetic and phenotypic responses to selection.

Evolution of flowering strategies inOenothera glazioviana: an integral projection model approach

“…This provides a potential mechanism allowing the maintenance of genetic diversity, and indeed selection experiments have shown that natural populations harbour extensive genetic variation for threshold sizes of owering (Wesselingh & de Jong 1995;Wesselingh & Klinkhamer 1996). Rose et al (2002) draw the same conclusion from the tness landscape of the monocarpic thistle Carlina vulgaris, which is remarkably similar to that calculated here for Oenothera.…”

“…Fitness also increases with increasing variance about the growth curve, as found by earlier studies (Rees et al 1999(Rees et al , 2000Rose et al 2002). This occurs because the variance about the growth curve increases the equilibrium plant size, on an arithmetic scale.…”

“…Second, other factors such as spatial or temporal variation in growth or survival, not included in the model, may allow the maintenance of genetic variation for threshold owering sizes (Sasaki & Ellner 1995). Despite this discrepancy, it is clear that accurate prediction of average life-history phenomena in monocarpic perennials is possible (Rees et al 1999(Rees et al , 2000Rose et al 2002). The elasticity surface for Oenothera demonstrates the importance of growth and reproduction; similar results have been found for monocarpic perennials using matrix models (Silvertown et al 1993), indicating results obtained for Oenothera may be representative of other species.…”

Life‐History Variation in Contrasting Habitats: Flowering Decisions in a Clonal Perennial Herb (Veratrum album)

“…The environment is constant. Several studies have demonstrated that temporal variation in demographic rates can have a substantial impact on the evolution of flowering decisions in monocarpic perennial herbs (Rees et al 1999Rose et al 2002;Childs et al 2004;Ellner and Rees 2007). The ability of a constant environment model to predict the ESS accurately does not imply that stochastic effects are small, because the effects can act in different directions .…”

Integral Projection Models for Species with Complex Demography

“…For example, "the construction of models using both size and age … may be impractical because of the large numbers of categories required" (Caswell 1988, p. 94). Consequently, matrix models with classification have rarely been size # age used, despite numerous studies documenting size-and age-dependent demography (Werner 1975;Gross 1981;Klinkhamer et al 1987;van Groenendael and Slim 1988;McGraw 1989;Lei 1999;Rees et al 1999;Rose et al 2002). In contrast, a size-and age-dependent IPM for Carlina vulgaris required only one extra parameter to describe the effect of age on flowering probability (Childs et al 2003).…”