Final answer:
The biggest impact on natural selection is caused by changes in the environment.
Step-by-step explanation:
In natural selection, the biggest impact is usually caused by changes in the environment.
A sudden change in the environment can lead to different selection pressures, favoring individuals with certain traits that help them survive and reproduce. For example, if the average temperature increases and fruit production declines, individuals with traits that can tolerate higher temperatures or can find alternative food sources will have a higher chance of survival and passing on their genes.