I'll take a stab at defining "founder effect".
New populations are often "founded" by a very small initial
population. Consequently, the genetic variation within the
new population depends strongly on the genetic variation present
in the founding individuals - and this may be substantially
different from the characteristic variation in the source
The upshot is, new populations that are reproductively
isolated from source populations (i.e. no gene flow) can
rapidly diverge from that source population.
Hope this helps (and that it's not blatantly wrong!),
Josh Hayes, Quantitative Sciences HR-20 U of Washington
josh at pogo.cqs.washington.edu 206 543-5004
You'll see my teeth in the stars above