The theory is the survival of the fittest, e.g the best suited to an environment will dominate it. Repoduction due to its complex nature has a chance of providing offspring/children/plants which have a differnt property to its adults. This property could be of an advantage or not. But just because it has a better property - stronger, needs less water, better camouflage etc doesn't mean automatically it will survive. The dodo was perfect for its environment till man came along and killed them all.

Yhere ahve been disastourous events where an alien species has been introduced to another country to control pests like toads in sugar cane, and they take over.

This process of change explains why we have the different races. e.g White skin would be of no use for trekking across the kalahari.