The hidden costs of rapid adaptation: experimentally assessing the effects of standing variation on the pace and trajectory of evolution

Abstract

As the planet changes at an alarming rate, there is a great need to understand why some populations are better equipped to rapidly adapt to their new environment than others. Many factors contribute, but populations are ultimately limited in their pace by their genetic makeup—they either have variants that allow them to survive or they do not. But, in the race to adapt, all sources of variation are not equal, and standing genetic variation is theorized to be of the most benefit in contributing to rapid adaptation. Here I explore the role of standing variation, both in a population’s ability to adapt at a rapid pace and in the potential long term evolutionary consequences that occur as a result. My work confirms expectations that increased standing variation in a population allows for a faster rate of adaptation, but although these populations are able to succeed in the short term, but this achievement comes at a significant cost to long term viability. All populations, across all experiments, that utilize standing variation as the genetic basis for rapid adaptation lose the ability to undergo sexual recombination, and therefore lose an important mechanism for maintaining variation in the long term. I begin by determining how the amount of standing variation present in a population correlates to the timing and rate of a successful adaptive response to a stressful environment. I assess how this result is intertwined with loss of sex and explore the mechanism for that loss. Then, I explore how the dynamics of the system change if the environmental shift occurs gradually rather than as a dramatic climactic event. Finally, I compare the variety of evolutionary strategies that develop in populations that began with standing variation versus mutation as their genetic substrate and evaluate their potential for success in the long term. Taken together, these results present a different picture of the role of standing variation than might be assumed. It does indeed allow for rapid adaptation, but the increased degree of genetic variation is not an evolutionary panacea and may send populations down evolutionary trajectories that are short-sighted.