Final answer:
Evolutionary Darwinian fitness is the relative ability of an organism to survive and produce fertile offspring. It is determined by an organism's contributions to the gene pool of the next generation. Fitness is measured by how individuals compare to others in the population.
Step-by-step explanation:
Fitness in the context of evolution by natural selection, is the relative ability of an organism to survive and produce fertile offspring. It is not about being strong or fast, but about how well an organism is adapted to its environment and how successful it is in reproducing. This concept of fitness is measured in terms of an organism's evolutionary (Darwinian) fitness, which is determined by its contributions to the gene pool of the next generation compared to other individuals in the population. Scientists use this concept to study how populations evolve over time.