Fishes are the most diverse vertebrate group, but half of all fishes live in freshwater which accounts for far less than 1% of all habitable water on Earth. My lab combines field and laboratory studies to understand the relative roles of natural and sexual selection in the origin and maintenance of freshwater fish diversity. In particular we address questions about (1) how ecological variation influences evolution across major environmental gradients, (2) how temperature variation influences the maintenance of genetic variation within populations, (3) how genetic variation within populations influences evolutionary pattern and process.