The primary determinant of evolutionary fitness is an organism's ability to survive and reproduce in its environment. This encompasses various factors such as access to resources, avoidance of predators, and adaptation to changing conditions. Organisms that possess advantageous traits are more likely to pass them on to subsequent generations, enhancing their overall contribution to the gene pool. This concept is central to the theory of natural selection, as it drives the evolution of species over time. In essence, those best suited to their environments thrive, influencing the dynamics of evolutionary processes.