What are the four reasons that natural selection occurs?

The four reasons are

1) There is an over population.

2) There is a genetic variation in certain offspring that is helpful to survival.

3) The species struggle for survival.

4) The ones that lives will reproduce and pass on their genetic variation.

I hope that's help:0

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.