Aren't mutating genes the sole mechanism by which organisms are formed?
No. In the early 20th century, geneticists rejected Darwin, in large part because of his mistaken ideas about heredity, and they had discovered mutations. So they pursued the idea that evolution was all through mutation, an idea that has been perpetuated through popular media, such as the X-Men movies.
But then in the 1930's and 1940's, biologists realized that mutations were only one mechanism for increasing the variability of a population's gene pool, whereas it was natural selection that still drove evolution by working on that variability. Thus was the Grand Synthesis and thus was neo-Darwinism.
AndrewPD, we've known for more than 70 years that it's more than just mutations. Isn't it time that you came up to speed on what's what?
One of the products of that Grand Synthesis (or, more properly, part of what had brought about that Synthesis) was
population genetics, the mathematical study of how genes spread through a population. Which is key to answering your other question about new traits getting "diluted" (which harkens back to Darwin's misunderstanding of heredity and which led to his mistaken theory of "pangenes").
May I suggest a book?
Blueprints: Solving the Mystery of Evolution by Maitland A. Eddey and Donald C. Johanson, Penguin, 1989. It goes through the history of the development of the ideas about evolution. It even gets into the reasoning and the experiments behind the different ideas and discoveries. If your Christian college is at all honest, then you should be able to find it in your library. If it's not there, then look for it in a local state-run college or university's library.
Now for a really heretical thought: Truth is truth; facts are facts.
If your theology really embodies Truth, then why does it need to lie about the facts? Does the Truth
ever need to depend on lies?
Now to get really heretical. Instead of using lies and deceptions to attack evolution, why don't you simply learn everything you can about evolution so that you can use evolution's real problems against it? Not the facile lies that creationists teach you, but rather evolution's
real problems. But to learn what those are, you will need to actually learn what evolution actually is. And what the evidence actually is. Are you ready for that challenge?
A word of warning. No creationist has ever been able to do that. So far, every creationist who has actually done all that has, to my knowledge, ended up rejecting creationism for being an outrageous pack of lies and deceptions. And I know of none of them who has not left his religion because of it. Sure, some of them have been able to remain Christian, but not fundamentalist Christians. And many of them have followed the dictates of their fundamentalist training and have become atheists, but that's the fault of their religion, not of evolution.