Answer:
natural selection
Step-by-step explanation:
Darwin proposed natural selection as the process that causes evolution.
According to the theory of natural selection, the environment naturally selects for genes that confer fitness to organisms over every other gene. In other words, organisms that are able to adapt to an environment survive and contribute more to the subsequent generation while those that are poorly adapted contribute less and gradually fade away from the population.
For example, when a drug is taken a particular pathogen, the susceptible ones among the pathogen's population will die off while the ones with the resistance gene survive, reproduce, and pass on the resistance gene to the subsequent generation. Consequently, the same drug might not work at all for the new generations of the pathogen because the gene that confers fitness has been selected for.