Answer:
Reproductive fitness.
Explanation:
The natural selection may be defined as one of the most important factors that plays the main role in evolution. Darwin explains the theory of the natural selection.
The organisms must adapt to their environment and reproduce in the large numbers. The reproductive fitness helps the individual to effectively transfer their genes to the next generation. The successfully adapted and evolved population must show high reproductive fitness. The natural selection increase the chance of individual to survive in harmful environmental conditions and do not remove the conditions.
Thus, the answer is reproductive fitness.
