Evolutionary Fitness is how well a species is able to reproduce in its environment. If they're no longer reproducing then they are no longer evolutionarily fit.

Simply put, evolutionary fitness is the ability of an organism to survive and reproduce. It is measured by reproductive success–meaning, how well a genotype or phenotype is passed on to the next generation compared to other genotypes and phenotypes.

The fittest individual is not necessarily the strongest, fastest, or biggest. A genotype's fitness includes its ability to survive, find a mate, produce offspring — and ultimately leave its genes in the next generation.

Most often, measures of fitness involve these key areas: Aerobic fitness, which involves how well the heart uses oxygen. Muscle strength and endurance, which involve how hard and long muscles can work. Flexibility, which is how able joints are to move through their full range of motion.