Natural selection is dependent upon existing variation. Ultimately,
that variation comes from mutation (broadly defined). Talk of unions
and intersections just is not relevant to the discussion, and probably
will confuse any biologists mightily. Genetic drift could be
considered to be an analogous concept to natural selection with
respect to mutation. Mutation describes events at the individual
level; both NS and genetic drift describe changes at the population
level. Saying that "mutation and NS taken together explain all
evolution" just doesn't make sense. I'd argue that you need the
population-level concept of genetic drift as well as NS to even come
close to explaining a simple majority of evolutionary phenomena.
Gene flow is not the same thing as horizontal transfer. Horizontal
transfer refers to a mutational process that moves genetic material
across taxa. Gene flow is the within-species but inter-populational
exchange of genetic material. Horizontal transfer is, AFAIK, very
rare, being accomplished by such mechanisms as viral transcription.
Gene flow is exceedingly common, and is seen as a nearly completely
effective obstacle to speciational processes.
A clear set of statements of what is known about evolutionary biology
can be found early in Douglas Futuyma's "Evolutionary Biology". I
don't have my copy with me, so I can't repeat his list, but I
recommend it highly.
Wesley
PS: I think that the original "What is evolution?" list gets off
on the wrong foot. Evolution need not reference 6000 year stuff
at all.
Item #1 for evolution in my book would be:
1. Organisms inherit traits from their ancestors.
A couple of additional pieces to get going would be...
2. Traits are inherited via particulate mechanisms.
3. Variation occurs in traits within populations of organisms.