Evolution of maternal age effects

Maternal age effects, in which offspring survival or fertility decreases with increasing maternal age, has been demonstrated in many species, including humans. Despite decades of phenotypic studies, we still do not understand how maternal age effects influence population dynamics or evolutionary fitness. After all, if maternal age effects are so negative, shouldn’t they be selected against? How do they still exist? 

Event history diagram, showing lifespan daily reproduction, for 272 individual rotifers. Note the large variability in life history between individuals. Some, but not all, of this difference is caused by maternal age effects. From van Daalen et al., 2022, The American Naturalist. 

To understand the influence of maternal effects senescence on population dynamics, fitness, and selection, we are working with mathematical ecologists Chrissy Hernandez, Silke van Daalen, Chrissy Hernandez, and Hal Caswell (each from one or more of from Woods Hole Oceanographic Institution, the University of Amsterdam, and Cornell University). We developed matrix population models and fit them to our laboratory data on maternal effects in Brachionus manjavacas. Our work shows that because the strength of selection decreases with both age and maternal age, negative maternal age effects are not selected against, and thus will continue to exist and evolve. 

Selection gradients on survival and fertility in a high-growth environment, with and without the presence of maternal age effects. The top row shows the selection gradients on fertility, and the bottom row shows the selection gradients on survival. Left column shows the selection gradients with maternal effects, and Right column shows the selection gradients without maternal effects. Bars are colored by the magnitude of the selection gradient. Note that the z axis is log-scaled. These results show that selection decreases with increasing age and with increasing maternal age. From Hernandéz et al, 2020, PNAS. 

In a separate modeling study investigating the source of variability between individuals in lifetime reproductive output (LRO), and whether this is due to heterogeneity explained by maternal age effects, or due to stochasticity (“luck”), we found that in high-growth rate environments, maternal age effects could account for around 25% of variability in LRO. Interestingly, our simulations show that for both the strength of selection and for variance in LRO, the relative importance of maternal age effects depends strongly on environmental conditions.  

Contribution (%) of maternal age heterogeneity to the variance in lifetime reproductive output for a range of virtual environments. The values on the color bar are on a log scale, with a minimum of 0.0006% and a maximum of 41.4%. The thick black line represents combinations of reduced survival and fertility where the population is stationary (i.e., lambda = 1). The maximum contribution of maternal age heterogeneity to variance in lifetime reproductive output is around 26% is highest when both fertility and survival are high. From van Daalen et al., 2022, The American Naturalist.