Dissertation: Vertebrates exhibit extensive variation in brain size and architecture, the drivers of which have been contended for over 100 years. This is important because increased brain size is associated with enhanced cognition and survival in novel environments. Due to the high metabolic cost of brain tissue, it follows logically that natural selection plays a role in driving this variation, however, most research has focused on comparisons across species and our understanding of the ecological causes and evolutionary consequences of variation in brain size is lacking. Even less understood is how the brain and its structure lends to cognitive abilities and behaviors of individuals, which are obviously critical aspects to fitness and therefore pertinent to understanding how brains evolve. I aim to discover how the brain, cognition, and behaviors have evolved through natural selection in Trinidad killifish (Rivulus hartii) and experimentally test the impacts on fitness when exposed to novel ecological conditions.