Tail of two fish, A: an integrative approach to understand how trade-offs and salinity influence two closely related euryhaline fish

2021 Spring.Includes bibliographical references.It is well understood that adaptive evolution can occur rapidly in nature and that anthropogenic climate change is causing - and will continue to cause - mass extinctions of the planet's biodiversity. These facts represent somewhat of a paradox: rapid adaptation can and does occur in nature, yet many populations are failing to adapt to environmental change. This dissertation lies at the interface of this paradox as it investigates the adaptive process. However, instead of investigating a case of adaptive success, it explores the mechanisms and circumstances underlying a case when evolution appears to be constrained. More specifically, it investigates how a trade-off between salinity tolerance and competitive ability contributes to an evolutionary range limit in Poecilia reticulata. It also investigates how salinity influences genetic variation in a more widespread fish, Poecilia picta.In chapter 1, a conceptual framework of trade-offs as evolutionary constraints that utilizes network/pathway thinking is presented. In chapter 2, it is experimentally shown that P. reticulata experiences a trade-off between salinity tolerance and competition with P. picta, that the trade-off is genetically based, and that it is indeed range limiting. In chapter 3 why this trade-off occurs at the physiological network level is investigated. It is shown that a negative relationship between salinity tolerance and competition arises because salinity exposure in P. reticulata results in the activation of hormonally mediated pathways in the brain associated with ion regulation and a decrease in aggression. Chapter 4 shifts the focus from P. reticulata to P. picta. to investigate how salinity influences the distribution of both neutral and adaptive genetic variation in a species that is found both freshwater and brackish water unlike P. reticulata. It is found that salinity can drive differentiation at putatively adaptive loci despite high levels of population connectivity in populations of P. picta