Final answer:
Natural selection occurs when there are heritable variations that affect an organism's survival and reproduction, leading to individuals with advantageous adaptations having a better chance of passing on their traits.
Step-by-step explanation:
Natural selection occurs when variations between individual organisms result in different chances of survival and reproduction. This leads to the event where individuals with advantageous traits, known as adaptations, are more likely to survive and leave more offspring than those without such traits. The key condition that must be met for natural selection to take place is the presence of inheritable genetic variation that affects the organism's ability to survive and reproduce in its environment.