Michigan Medicine - University of Michigan Apr 4 2025 In evolutionary terms, fitness is defined as an organism's ability to survive and reproduce its genes into the next generation. Genes influence fitness, sometimes competing against each other within an organism.  This competition, or arms race, is typically hard to observe–except when the genes in question live on the X and Y chromosomes, which determine the sex of offspring in mammals. In mice, this arms race can result in broods that have