Driving Forces of Evolution

What is the driving force of evolution??
What makes one species evolve into another??
Why does evolution occur??
Is it natural selection? or is it perhaps mutation?

The points below are what I understood after a bit of reading and thinking.

Natural Selection drives evolution. Since irrespective of the repertoire of different genes present in the gene pool, if one gene is not chosen above another gene (based on some criteria) then the relative frequencies of genes in gene pool will remain the same and consequently there wouldn't be any evolution.
So clearly some gene has to get selected over another, this selection criteria is called Natural Selection.

However for Natural Selection to work there must be sufficient diversity in the gene pool itself so that some or the other gene is good enough to pass the test proposed by selection. Thus rate of mutation also is a factor at driving evolution. Its observed that viruses mutate at a higher rate and (thus?) evolve at a higher rate too.

However, mutations can either make a gene better at passing the present selection test, or make it worse at it. If they make the gene worse then it will fail the test and disappear into oblivion. But generally mutations don't cause any significant change in the capability of a gene to pass a selection test. These 'neutral' mutations can accumulate to a large number over the span of time. Random drifts in such neutral mutations can result in some gene being replaced by another as result of random event. Might seem a little far fetched, but in the timescales that evolution works under its definitely plausible. And so random drifts in neutral mutations may drive evolution too.

Is there any bias for or against some particular mutation? Is it more likely for an 'A' to convert into 'G' or 'C' rather than a 'T'. Or for that matter any other combination of 'ATGC' in the above sentence. I don't know if such bias is proven, but this bias has been implicated as the reason for the observed GC content variations among closely related prokaryotic species.

Of course all these mechanisms are not mutually exclusive w.r.t gene or time. For different genes different factors would be responsible for evolution. But these forces probably are the reason why one species evolves into another with passage of time.