Final answer:
Differences in traits influencing reproductive success, variation among individuals, and the ability for traits to change during an individual's lifetime are all critical for Darwin's idea of natural selection.
Step-by-step explanation:
Darwin's idea of natural selection is based on several critical factors. One of these is that differences in traits among individuals can influence their reproductive success. This means that some individuals with certain traits may be more likely to survive and reproduce compared to others. Another critical factor is that individuals exhibit variation, meaning that no two individuals are exactly the same. Additionally, Darwin believed that the traits individuals exhibit can change during their lifetime, which is also important for natural selection. Overall, all of these factors are critical for Darwin's idea of natural selection.