Final answer:
Fitness, in the context of evolution, refers to an organism's relative ability to survive and reproduce. It is not about physical strength, life span, or intelligence, but rather how well an individual can pass on its genetic material. Scientists measure fitness to understand how populations evolve.
Step-by-step explanation:
Fitness, in the context of evolution by natural selection, refers to the relative ability of an organism to survive and produce fertile offspring. It is not necessarily related to physical strength, life span, or intelligence. Rather, it is about how well an individual can pass on its genetic material to the next generation. For example, a small but showy male bird may be selected by female birds to reproduce, while a larger but less colorful one may not. Fitness is often measured by scientists in the field, and it is not an individual's absolute fitness that matters, but rather how it compares to others in the population. This concept, called relative fitness, helps researchers understand which individuals are contributing more offspring to the next generation and how the population might evolve.