Death or survival are the two outcomes of an infection. Pathogen infections perturb the resource and energy balance of hosts and in response hosts adjust metabolism and resource assimilation. Host resistance against infection relies on the allocation of resources, in particular to mount sufficient immune responses. We hypothesize that interactions between host resources, host immune responses and pathogens determine host homeostasis and result in contrasting infection outcomes. In this study, we mathematically formulate these interactions and find that the temporal dynamics lead to two distinct trajectories of disease progression. The pathogen load responds in a bistable manner by either proliferating or persisting, if not being eliminated, which corresponds to the death or survival of the host. We identified two key negative feedbacks causing bistability: 1) host resources are either directly consumed or indirectly inhibited by pathogens; 2) pathogen elimination by the host immune response. The dynamics is also sensitive to “initial conditions”, such as resource availability, immunocompetence, and pathogen load at the onset of infection. Our result provides a mechanistic intuitive explanation for the bistable outcome of pathogen infection based on resource reallocation. It also provides a quantitative framework to monitor disease trajectories and forecast the physiological tipping point of death and survival.

Extreme climatic events may influence individual-level variability in phenotypes, survival, and reproduction, and thereby drive the pace of evolution. Here, we quantify how experiencing major hurricanes influences individual life courses in the Cayo Santiago rhesus macaques. Our results show that major hurricanes increase heterogeneity in reproductive life courses despite an average reduction in mean fertility and survival, i.e. shortened life courses. In agreement with this, the population is expected to achieve stable population dynamics faster after a hurricane. Our work suggests that natural disasters force individuals into new niches to potentially reduce strong competition during poor environments where mean reproduction and survival are compromised. We also demonstrate that variance in lifetime reproductive success and longevity are differently affected by hurricanes, and such variability is mostly driven by survival.

Heterogeneity among individuals in fitness components is what selection acts upon. Evolutionary theories predict that selection in constant environments acts against such heterogeneity. But observations reveal substantial non-genetic and also non-environmental variability in phenotypes. Here we examine whether there is a relationship between selection pressure and phenotypic variability by analysing structured population models based on data from a large and diverse set of species. Our findings suggest that non-genetic, non-environmental variation is in general neither truly neutral, selected for, or selected against. We find much variation among species and populations within species, with mean patterns suggesting nearly 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. Our analysis suggests we are only at the beginning in understanding the evolution and maintenance of non-genetic non environmental variation.