In natural selection why do animals die?
Why do some animals die while others thrive?
Why do phenotypes of a population change?
3 answers
it is because each animals have a certain stage that it will die ,e.g old age.
it is because each animals have a certain stage that it will live ,e.g old age can cause the animals death
Availability of food and vulnerability to predators. Natural mutations might make changes in phenotypes that help in dealing with the two factors above. Those would then tend to reproduce.