Abstract
Individuals differ substantially in fitness components, but such realized variation in lifespan and reproductive success need not be visible to natural selection, because fitness is not a property of individuals but of groups or genotypes. I will present formulas—rooted in Markov Chain theories—to compute exactly the variance in lifespan and in lifetime reproductive success among individuals with identical (genotypic) vital rates. This expected neutral variance is solely driven by individuals transitioning stochastically among stages. I illustrate how the neutral expected variation in fitness components matches those observed in real populations, how ignoring this stochastic variability and labeling it undesired noise can lead to false demographic predictions, and present data from isogenic bacteria that illustrates that the overwhelming amount of variation in fitness components among individuals is neutral and that the amount of variance detected in the lab does not differ much from natural populations.