How do parental effects introduce variation into individual phenotypes, fitness and population dynamics?

Changes over time in population size arise due to changes in individuals (e.g. via survival or reproduction). Similarly, evolution via natural selection requires differences between individuals, under-pinned by heritable differences. The variation between individuals in their life-history phenotype (the way they grow, mature, reproduce and die) is thus key both to population changes and evolutionary changes. Traditionally, individual variation has been thought to arise because of genetic and environmental differences. Increasingly, we are recognising there is a third cause: past environmental conditions being passed across generations via paternal effects. The most common example of which are maternal influences on offspring condition. If my mother had a lot of food when she was pregnant, I am more likely to have grown in the womb and be born a large and healthy baby. As a result, I am likely to live a long time. Some types of maternal effect are not mediated by nutrition, but by switching on or off genes. Such gene silencing or activation (an 'epigenetic effect') can last several generations. By studying individuals and their life-histories (patterns of growth, maturity, health, survival, longevity) - whether in humans, other mammals, birds, fish, lizards or invertebrates - we are increasingly realising that parental effects are important in determining many aspects of an individual's life. Parental effects can arise through nutrition or epigenetics, and arise through the male or female line. Many studies have been observational (noting patterns and trying to explain them) and so little systematic experimentation has been undertaken. There remain many unanswered questions about the overall importance for parental effects in ecology. How much variation do parental effects create? Over what timescale: can they be outgrown or reversed? How do maternal and paternal effects interact? Do epigenetic effects act differently from nutritional effects? How much do they influence population dynamics by creating variation between individuals? In this grant we first explore the conditions leading to parental effects - by varying male and female age and condition and then looking across the offspring life history from start to finish. Second, we investigate whether the effects arise due to genetics (offspring have different combinations of genes to their parents), nutrition (by looking at the amount of yolk and its chemical composition) and epigenetics (which genes are switched on or off). Third, we build a model to ask the question why did the observed parental effects evolve. Finally, we create experimental populations of hundreds of individuals to see how parental effects created variability between individuals and how much this creates variation in population size and structure. The experiments are conducted using an experimental 'model' animal: a soil mite. This has a fast generation time and a small size, allowing experiments on both individuals and populations.