Charles Darwin wrote about natural selection in On The Origin of Species. Natural selection, in fact, involves more than one concept. Darwin was ambiguous about what natural selection actually meant.
A well-known explanation for natural selection is that it involves the survival of the fittest. The weak die while the strong reproduce and leave more offspring. In nature many more offspring are produced than survive, and even a small advantage can give the edge to the lucky individuals.
If selection pressures are strong, a change can be observed in a short space of time, may be even as little as 30 years. For example, Darwin’s finches show different shapes and lengths of beaks depending on which seeds they eat. If there are droughts year after year birds with short beaks die out while birds with stronger beaks live because the food source has changed. Environmental change drives species change.
Another meaning of natural selection is the selection of modified traits such that plants and animals become adapted to new conditions and thus avoid extinction.
I fully endorse both of these meanings.
The Principles of Creation 