In evolutionary theory, _______________ refers to the reproductive success of an individual organism relative to the average reproductive success in the population. A. natural selection B. gene flow C. adaptation D. fitness

The definition given refers to the reproductive success of an individual organism relative to the average reproductive success in the population.

Go over each of the available options. A. Natural selection: This is the process by which organisms with traits that improve their chances of survival and reproduction are more likely to pass these traits on to the next generation. Though important in evolutionary theory, this definition isn't referring to natural selection. B. Gene flow: This is the transfer of genetic material from one population to another. Again, while important, this term doesn't match the given definition. C. Adaptation: This is the process by which organisms develop adaptive traits to better survive and reproduce in their environment. The given definition doesn't quite match the definition of adaptation either. D. Fitness: This term looks at the reproductive success of an individual within a population based on traits that are inherited. It's a measure of how well an organism can survive and reproduce in its environment, relative to other individuals in the population, which matches the given definition.

Based on the review of each term, the correct answer is D. Fitness, as it directly refers to the reproductive success of an individual organism relative to the average reproductive success in the population.