Climate variability increases northward along the latitudinal gradient from the equator, and organisms tend to have larger range distributions at higher latitudes. Genetic diversity also tends to decrease with increasing latitude. The Climate Variability Hypothesis (CVH) offers a potential mechanism to explain these macroecological patterns. The CVH posits that increased climate variability at higher latitudes selects for organisms with larger thermal breadth, the total range of temperatures an organism can tolerate. The ability to withstand a wider range of temperatures is hypothesized to allow organisms to move between environments with different climates more easily. Lower barriers to dispersal for thermal generalists at higher latitudes is predicted by the CVH to decrease the levels of genetic differentiation between populations at higher latitudes. The CVH has been used to explain why the tropics are more diverse than temperate climates and why tropical organisms are more vulnerable to climate change. However, there is a substantial latitudinal gradient in climate variability within North America alone. I seek to determine whether the CVH is supported on a finer scale within the temperate latitudes of North America. Ectotherms are especially sensitive to abiotic conditions; therefore, I first assess the genetic predictions of the CVH in the literature across vertebrate ectotherms using a meta-analysis. I find that increased temperature seasonality, a measure of climate variability, is positively correlated with increased population connectivity in reptiles and amphibians. I then narrow my focus to spotted salamanders (Ambystoma maculatum), using double digest restriction-site associated DNA sequencing (ddRAD) and find that increased temperature seasonality is also positively associated with increased population connectivity in northern salamander populations. Next, I test the thermal breadth assumption of the CVH by quantifying maximum and minimum thermal tolerance limits of salamanders from New York and Georgia, reared at common temperatures. I find that salamanders from a more seasonal climate have larger thermal breadth, and continued selection for cold tolerance is the driving force contributing to latitudinal differences in thermal breadth. I also find that salamanders from higher latitudes develop faster and grow larger, and I focus on the potential fitness consequences and physiological mechanisms underlying cold tolerance and rapid growth. I use slow motion videography and find predation avoidance is improved for larger salamanders. Last, I use waterborne hormone assays and determine that adaptation to colder climate and exposure to lower temperatures during development facilitate more robust corticosterone activation in response to 24-hour cold stress, which is beneficial for tolerating short-term stressors. These studies collectively suggest that the Climate Variability Hypothesis can be used as a framework for understanding thermal evolution in ectotherms with populations that are widely distributed along latitudinal gradients. Furthermore, southern populations of salamanders and other North American amphibians may be at increased risk in the face of climate change, relative to northern conspecifics.