American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Drivers of diversification in individual life courses”

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Raisa Hernández-Pacheco and Ulrich K. Steiner

Variability in life courses does not depend on ecological mechanisms underlying population structural composition

A male rhesus macaque (<i>Macaca mulatta</i>) grooms a female during mating season in Cayo Santiago, Puerto Rico.<br />(Credit: Raisa Hernández-Pacheco)
A male rhesus macaque (Macaca mulatta) grooms a female during mating season in Cayo Santiago, Puerto Rico.
(Credit: Raisa Hernández-Pacheco)

The evolution of populations is shaped by demographic differences among individuals in their life courses. Yet thorough examination of the mechanisms behind individual differences remains undone because current population level analyses center on mean factors that average over such mechanisms.

In this study, the authors use a novel approach to investigate principal drivers of individual differences in Cayo Santiago rhesus macaques. Using 40 years of demographic data on over 3,000 rhesus females, researchers study the relationship between population density and the rate at which the diversity of female life courses diversify with age. The authors show that density regulates the annual distribution of reproductive success in the population, but the rate of diversification is independent of density. For example, they find that as population density increases, the proportion of successful breeders decreases. However, such change in density does not affect the life course of these females. This suggests that an increase in density neither drives individuals to a certain optimal life course, nor forces individuals to explore new niches by diversifying life courses.

This study may be the first to demonstrate that year-to-year variation in the rate of diversification of life courses is independent of the well-known ecological mechanisms underlying structural composition in a population, such as density-dependence. The study also illustrates how the measure of the rate of diversification provides a more integrated estimate of variability compared to the population structural composition, giving us new insights about the underlying drivers of individual differences within populations and potential evolutionary mechanisms.

The authors, Raisa Hernández-Pacheco and Ulrich K. Steiner, are at the University of Puerto Rico and the University of Southern Denmark, respectively. Read the Article