The Evolution of Size in Fish: Evaluation of Natural Selection for Size and Evidence for a Prolonged Cost of Rapid Growth

Abstract

Having a larger body size tends to be positively associated with fitness, as it often leads to increased survival. For my dissertation I evaluated the strength of natural selection and the presence of prolonged trade-offs from an early period of fast growth in order to better understand the evolution of size. I began by reviewing the literature to assess natural selection pressures for size in the early life history of fishes, and calculated standardized selection differentials. I found that the majority, 77%, of standardized selection differentials were positive indicating that larger size was being favored. Because this literature review only focused on the early life history, I then used the Atlantic silverside (Menidia menidia) as my model species to evaluate how selection varies over the life history. Before measuring selection in the field, however, I conducted a lab validation experiment using Atlantic silversides to determine accuracy in otolith back-calculation, a method that can be used to track the traits of survivors. I then collected Atlantic silversides from the field, over their growing season, and found that selection fluctuated considerably between favoring larger size and smaller size. Lastly, I measured a prolonged growth trade-off which has the potential to counteract positive selection for size. Despite evidence for growth costs on some timescales (i. e. immediate, end of life), there has been little evidence of prolonged growth trade-offs which occur in-between these two timescale extremes. I measured 3 traits (swimming ability, muscle morphology, lipid mass) and found that both instantaneous as well as prolonged costs of growth manifested. Interestingly, fish were able to recover their swimming ability after 36-37 days on limited rations. This dissertation has shown that overall selection intensity for size was much weaker when measuring selection over the growing season of Atlantic silversides than when solely looking at the early life history. This result has important implications for better predicting the evolution of size in fish populations due to intense harvesting. Furthermore, I have demonstrated that trade-offs can continue to manifest over a prolonged period and a new life history theory should be developed to take this into account.