Adaptive and neutral processes that select on life course diversification

Heterogeneity among individuals in fitness components is what selection acts upon. One of the fundamental questions in evolution is how such heterogeneity arises and is maintained. Fundamental evolutionary theories predict that selection acts against such heterogeneity but contrasting these predictions substantial non-genetic and non-environmental driven variability in phenotypes can be observed; not only under highly controlled lab conditions, but also in less controlled settings. Here we ask, by analysing structured population models across a large range of biological taxa, how selective forces act on the processes that generate variation among life courses. Our findings suggest that non-genetic, non-environmental driven variation is in general neither truly neutral, selected for, or selected against. Much variation exists among species and populations within species, with mean patterns suggesting close to neutral evolution of life course variability. Populations that show greater diversity of life courses do not show, in general, increased or decreased population growth rates, and selective forces acting on the processes of diversification seem not to generally increase or decrease life course variability. Approaches in quantitative genetics that identified similar lack of understanding of maintenance of variability might be extended to include non-genetic and non-environmental driven variability. We believe to be only at the beginning in understanding the evolution and maintenance of non-genetic non environmental variation.