Many students study aspects of natural selection and assume that it is evidence of evolution. This is not the case. There are at least three components to the jigsaw which is evolution.
Natural selection is but one piece of the jigsaw. It can work on the genetic variation present but does not generate it.
The spontaneous generation of life is an insurmountable problem for evolution. Efforts to explain how life can arise do not overcome all the problems. The basic problem is that for a living organism to function it requires too many things to happen simultaneously. Cell membrane, a basic DNA/RNA containing vital metabolic enzymes, replicating capability, ribosomes which translate the DNA/RNA code into polypeptides not to mention the problem of chirality. The list is endless and as each new "condition" for spontaneous generation is added, the odds increase until it becomes not improbable but impossible.
The generation of new genes is a difficult concept to grasp. Without new genes the process of evolution will not work. Mutation is the force which is ascribed the role of generating new genes. It is important at this point to understand the difference between new genes and the modification of existing genes. Perhaps an example from computer software would help. One of the alternative operating systems to windows 98 is Linux. Linux has become very popular because it is free. Not only is it free but it is not copyright, in the sense that anyone is free to alter it. Now nobody can really claim they have created a new operating system, if they simply tinker with an existing one.
Although not an exact analogy it does get over the idea of modifying somebody else's work. In a similar way genes can be altered by mutations to be slightly different. These are usually called alleles and are derived from the gene which was mutated. They are what might be termed derivative alleles. They do not represent an increase in information since they can be re-created by damaging the original gene. A colour television which goes faulty may still give a black and white picture. Is this an advance over the colour TV? The answer is no, because the colour TV represents is more complex. Damage to a colour TV can always produce a black and white picture but a black and white TV cannot be damaged into colour.
What this serves to illustrate is the increase in entropy which mutations represent.
The confident assertion that existing variation is the work of mutation is a circular argument.