Natural selection, in the view of most evolutionists, is the primary mechanism controlling the direction of evolution. It was first proposed in the middle of the 19th century by Charles Darwin and Alfred Russell Wallace, though Darwin is, justifiably, given most of the credit for proposing natural selection.
An important point to make is that Darwin (and Wallace) really contributed the idea of natural selection more than they did the idea of evolution. Many people had been convinced that all life forms had evolved before the idea of natural selection came along. However, no one had developed a reasonable mechanism for that evolution before natural selection.
Natural selection is really a very simple idea, once it is explained to people. It works very much like the “artificial selection” mechanism used by dog breeders, except that the selection is controlled by nature rather than by an intelligent individual. That’s why it is called “natural”.
In summary, organisms within a population inherit different characteristics due to random variations (called mutations). We surely see this in humans. Some people are taller than others; some can run faster than others; some understand mathematics better or more easily than others, etc. Some of those genetic characteristics increase the organism’s ability to survive and/or reproduce in comparison to those that don’t have that particular characteristic. Over time - over many generations - these beneficial genetic characteristics become part of the entire population’s genetic characteristics. In the same way, genetic characteristics that decrease the organism’s ability to survive and/or reproduce will be eliminated.
Here’s how Charles Darwin explained it:
As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.
Selection is really a very simple concept. Anyone can figure it out — and many people have. Farmers have known about selection for millennia. They do so when they set aside particularly fruitful seed stock or especially robust farm animals for breeding and eat the rest.
Darwin’s genius was in understanding that nature would perform the same sort of selection that farmers do, but nature would do so naturally.
When natural selection was explained to Thomas H. Huxley, a friend and associate of Darwin’s, Huxley exclaimed, “Why didn’t I think of that?” Such is the reaction of many people.
Some people – generally creationists only – call natural selection “Darwinism”. That would seem to be a reasonable name to apply to the idea, and that word is found in many dictionaries. But the motives of the creationists are a bit malicious. By putting an “ism” at the end, they imply that it is nothing but a sort of creed or dogma accepted on faith alone without any real supporting evidence. The use of Darwin’s name even implies a sort of cult implying people who accept evolution are followers of the man rather than believers in his ideas.
Note that no one calls the Law of Universal Gravitation “Newtonism”, though it would seem to apply just as well as “Darwinism” applies to evolution through natural selection.
Because of that, most scientists reject the word “Darwinism” as it applies to natural selection. For those same reasons, I reject the use of that word.
Some creationists really believe that “Darwinism” really is a sort of cult. Darwin was wrong about some things. For example, and as we have already discussed, he really knew nothing about heredity and thought that some acquired characteristics could be inherited. (A blacksmith who has built himself up to have strong arms could pass strong arms to his children.) He was wrong about that.
Some creationists attack those places where Darwin was wrong and imply that ALL of evolution is wrong as a result. Since creationists themselves believe in an inerrant book and many say that if any part of the book is wrong, then none of it can be trusted, this sort of argument makes sense to them.
Creationists are also prone to attack Charles Darwin’s character. For example, though he was very liberal for his time and was a strong opponent of slavery even before the Civil War in the United States, his letters show that he felt that people with white skin, particularly upper-class Englishman (he was himself an upper-class Englishman), were superior to people with dark skin. He was wrong about that as well. In those views, he was simply a product of the times in which he lived.
Creationists are often surprised when scientists shrug their shoulders at Darwin’s infrequent mistakes and his politically incorrect views on some things, at least when heard by 21st century ears. But, in fact, those things have no influence on the scientific validity of evolution. Nearly every scientist who comes up with a new idea is wrong in some detail. Newton’s ideas had to be “corrected” by Albert Einstein. It’s no surprise that Darwin’s ideas have been modified over time. The current synthesis is called “neo-Darwinism”. The number of modifications to Darwin’s ideas that have been made over the last century and a half are surprisingly small and are due, not unexpectedly, almost completely to a better understanding of genetics.
Darwin was indeed a genius, but he was also an imperfect human being. His personal flaws, perceived or real, have no influence on the validity of evolution through natural selection.
 “The Origin of Species”, Charles Darwin, p. 459. Also available on-line at http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=477 and elsewhere.