First off we need to understand what natural selection is. Natural selection is the mechanism proposed by Charles Darwin in his theory of evolution.

The key points you need to know for this are:

1) that individuals in a species show a wide range of variation - this comes from the differences in their genes

2) Individuals that have characteristics which are more suited to the prevailing (dominant) environment are more likely to survive and reproduce.

3) These successful individuals will pass their genes (and so advantageous characteristics) onto their offspring.

4) Individuals that are more poorly adapted to the environment are less likely to survive and reproduce, so their genes are less likely to be passed on to the next generation.

It is this process that enables species to gradually evolve over time.

We can link this idea to recent human impact on the environment. If the environment changes too rapidly (e.g. severe deforestation, ocean acidification, coral bleaching) species are not able to evolve quickly enough through the process of natural selection to adapt and survive in the changed environment.