Answer:
Natural selection
Step-by-step explanation:
Natural selection occurs because less fit individuals are more likely to die before reproducing. Unfit individuals often die before reaching sexual maturity. Fit individuals are more likely to survive long enough to reproduce multiple times, and can win mates when competing with less fit individuals.
Some examples:
A deer with a heart condition is more likely to be caught and eaten than a healthy deer that can outrun it.
A lion born with a weak immune system is more likely to die of disease than a lion that was born with a proper immune system.
A beetle with strong pheromones is more likely to attract a mate than a beetle with weak or no pheromones.