How about "changes in the genetic composition of populations, including
those associated with splitting of a population into two or more
genetically isolated gene pools". Not much different from Wright's,
except that I prefer "changes" to "fluctuations" as the latter word implies
quantitative increases and decreases of something (eg. allele frequencies)
whereas changes is more general. I specifically include splitting of gene pools
only for clarity as it would in fact be covered by the first part.
I can't claim this is original or anything - but I think it distills what some
others have said. Note: genetic composition includes organelles, rearrangements
duplications etc etc etc.
Hope this helps,
PS Someone talked about correct usage of "necessary and sufficient".
It is true that physicists (and biologists!) talk about necessary and sufficient
causes of phenomena, but the meaning of these two words dont depend on the
physicist's usage of them. Rather the other way around! A definition can in
some sense be sufficient in that it covers the range of phenomena that the
term being defined is intended to cover. Here I think "changes in allele
frequency" is not sufficient to cover the intended meaning of the term
evolution. A definition can be necessary in a utilitarian sense in that it
is needed to facilitate communication or thought about the phenomenon being
defined. I trust the definition I gave above for evolution is both
necessary (maybe not) and sufficient. Cheers. ;-).