Abstract
Colonization of new habitats exposes populations to novel selective pressures, and the evolutionary dynamics of invasive species are determined by how the founding environment interacts with the phenotypes of invaders. Measuring these processes depends on measuring natural selection immediately upon colonization, which has been a logistical hurdle for understanding how selection operates in newly established populations. We established six brown anole (Anolis sagrei) populations on spoil islands in Florida and manipulated initial sex ratios to understand how natural selection differs based on the composition of founding populations. We found unpredictable spatial and temporal variance in natural selection, and selection increased in strength over time and with increasing adult density. The relationship between phenotypes and fitness in newly established populations may not be consistent over time. As a result, each colonization event by an invasive population provides the opportunity for a novel evolutionary trajectory towards either establishment or extirpation.