Adaptation: Adaptation is the process by which organisms adjust to their environment through genetic or behavioral changes. It allows them to better survive and reproduce in their specific ecological niche.
Variation: Variation refers to differences in traits among individuals within a population. These variations are crucial for natural selection since they provide the raw material upon which selection acts.
Fitness: Fitness, in evolutionary terms, describes an organism's ability to survive and reproduce successfully in its specific environment. The fitter an individual is, the more likely it is to pass on its genes to future generations.