Drivers of Diversification in Individual Life Courses

Abstract

Heterogeneity in life courses among individuals of a population influences the speed of adaptive evolutionary processes, but it is less clear how biotic and abiotic environmental fluctuations influence such heterogeneity. We investigate principal drivers of variability in sequence of stages during an individual’s life in a stage-structured population. We quantify heterogeneity by measuring population entropy of a Markov chain, which computes the rate of diversification of individual life courses. Using individual data of a primate population, we show that density regulates the stage composition of the population but that its entropy and the generating moments of heterogeneity are independent of density. This lack of influence of density on heterogeneity is due to neither low year-to-year variation in entropy nor differences in survival among stages but is rather due to differences in stage transitions. Our analysis thus shows that well-known classical ecological selective forces, such as density regulation, are not linked to potential selective forces governing heterogeneity through underlying stage dynamics. Despite evolution acting heavily on individual variability in fitness components, our understanding is poor whether observed heterogeneity is adaptive and how it evolves and is maintained. Our analysis illustrates how entropy represents a more integrated measure of diversity compared to the population structural composition, giving us new insights about the underlying drivers of individual heterogeneity within populations and potential evolutionary mechanisms.

Hurricane-induced demographic changes in a non-human primate population

“…The stable stage distribution (SSD; w ) computed from Q t correlated with the observed stage-specific distribution of females as reported previously ([ 33 ], electronic supplementary material), indicating that our annual models based on stable stage theories accurately represent the observed changes in stage distributions. The mean stable stage distribution w for non-hurricane years ( G ) was biased towards NB females with 14.8% I, 13.1% Y, 11.1% J, 42.3% NB, 2.19% FB and 16.6% B ( figure 5 a ).…”

Rhesus macaques compensate for reproductive delay following ecological adversity early in life

“…Although food provisioning, and thus census taking, resumed after 1 (Hugo) and 2 (Georges) days (Morcillo et al, 2020 ), major hurricanes are significant events that also cause changes in behavior involving an increase in the proximity of social networks (Testard et al, 2021 ). Given negative density dependence in reproduction driven by the annual number of adult females alive in the population (Hernández‐Pacheco et al, 2013 ; Hernández‐Pacheco & Steiner, 2017 ), we also considered the experienced female adult density at birth as a second source of early‐life ecological adversity. For this, we defined population density as the total number of adult females (≥3 years of age) alive at the onset of each birth season.…”

Natural disasters generate heterogeneity in individual life histories

“…Following Hernández-Pacheco and Steiner (2017), we defined the annual structure in our analysis from 1 June at time t to 31 May at time t + 1 to avoid significant overlap of birth seasons. Models were based on annual transition probabilities among developmental and reproductive stages (Hernández-Pacheco & Steiner, 2017;Morcillo et al, 2020). In a given year, we classified sexually immature females in one of three age-specific developmental stages: infant (I; < 1 year of age), yearling (Y; 1-2 years of age) and juvenile (J; 2-3 years of age).…”

“…This sequence defines an individual's reproductive trajectory (ω) and population entropy, H, describes the rate at which these trajectories diversify with age (Tuljapurkar et al, 2009; Table 1). We quantified this variation using matrix Re, a 3 x 3 submatrix of Pe including adult stages only, weighted by its corresponding quasi-stationary stage distribution, ω' (Hernández-Pacheco & Steiner, 2017;Steiner et al, 2010). Here, we employed H as a proxy of the expected annual rate of diversification in life courses of mature individuals (Hernández-Pacheco & Steiner, 2017).…”

“…Population entropy measures the rate at which these stage trajectories diversify with age (Hernández-Pacheco & Steiner, 2017;Tuljapurkar, 1982;Tuljapurkar et al, 2009). Persistence in any life stage instead measures the correlation time between an individual's current life stage and their stage at a later age.…”