Darwin’s theory of natural selection is when species over millions of years slowly adapt to their habitat to survive. Some are born with small characteristic changes that make it easier to survive and pass these traits on to their offspring. An example of this is peppered moths. Over a long time their pale colour turned into black so it could camouflage with the black trees around it. This let it have a much higher chance of not being killed by predators and reproduce whereas the pale moths would be spotted instantly and die. That is why we see more darker coloured moths in our civilisation today. This theory is called natural selection not because someone chooses what dies and what does not but because of how the more adapted species live out the others which have not evolved. It is inevitable to happen over time as anything living does whatever it takes to survive.