Abstract
Organisms can affect one another’s phenotypes when they socially interact. Indirect genetic effects occur when an individual’s phenotype is affected by genes expressed in another individual. These heritable effects can enhance or reduce adaptive potential, thereby accelerating or reversing evolutionary change. Quantifying these social effects is therefore crucial for our understanding of evolution, yet estimates of indirect genetic effects in wild animals are limited to dyadic interactions. We estimated indirect phenotypic and genetic effects, and their covariance with direct effects, in North American red squirrels (Tamiasciurus hudsonicus) living in an array of territories of varying spatial proximity. Additionally, we estimated variance parameters and the strength of selection at high and low population densities. Social effects of neighbors on date of spring breeding were weak at low population densities, but stronger at high population densities. Although indirect phenotypic effects were detected, the genetic component to these was not statistically significant. Nevertheless, the estimated effect size was large enough to suggest that indirect genetic effects would alter evolutionary change, giving less change at high densities despite stronger selection. The (uncertain) potential for indirect genetic effects to alter evolution suggests that they have potentially important consequences for any natural systems where organisms interact.