Variation among individual traits is one of the basic requirements for evolution by natural selection. Ecological and evolutionary studies typically focus on the fitness consequences of genetic variation because it is heritable and so it can generate evolutionary change. However, there is accumulating evidence for nongenetic variation across a diverse range of mechanisms and taxa. Because this nongenetic variation is traditionally considered as random “noise” that does not influence a genotype’s fitness, its effects on ecological and evolutionary dynamics remain largely unknown.
Here, we use mathematical theory to ask how nongenetic variation among individual life histories influences a genotype’s fitness. This theory is complimented by simulations of random genotypes that explore how these consequences for a single genotype scale up to determine mean fitness and rates of evolution by natural selection in populations. For direct comparison, both our theory and simulations assume age-structured populations with among-individual variation in survival probabilities and birth rates. To consider the breadth of life histories across real populations, these individual vital rates may be permanent throughout life (fixed condition) or can change at any time (dynamic condition).
Results/Conclusions
Incorporating nongenetic variation in vital rates changes a genotype’s fitness, and this effect scales up to influence mean fitness and evolvability in populations. One important take away is that increasing the strength of nongenetic variation will generally increase fitness for species with early recruitment age, whereas this effect is smaller and can even be reversed for species with later recruitment. The differences between fixed and dynamic condition are amplified for species with later recruitment ages, where in general, mean fitness is greater and the rate of evolution by natural selection is slower with fixed condition compared to dynamic condition.
Our work shows that ignoring the effects of nongenetic variation will generally lead to inaccurate estimates of fitness and evolvability. These results emphasize the importance of not only accounting for nongenetic variation in vital rates, but also for the structure of that variation (fixed or dynamic condition, early or late recruitment), when making predictions about ecological and evolutionary dynamics.
