Its sort of sad when the computer game SimLife provides
more rigorous definitions of scientific terms than a text-
book on the subject. 

(from SimLife, my comments in [])

Evolution: Change in the genetic makeup of a population with
time; a theory that the various types of plants and animals
have their origin in other preexisting types and that the
distinguishable differences are due to modifications in
successive generations.

[Great up to the semicolon, the rest could use a little
help. Bacteria, viruses, fungi and protists also evolve.
Also, the definition hints at, but doesn't explicitly
mention common descent.]

Natural selection: Differential reproduction in nature,
leading to an increase in frequency of some genes or gene
combinations and to a decrease in others.

[_Very_ good. To be slightly nit-picky, they should have
mentioned that it is differential reproduction of
genotypes. And, to be really nit-picky, selection doesn't
necessarily change the frequency of genes (really, really
nitpicky -- try "alleles" instead of "genes"). In balancing
selection and frequency-dependent selection, gene frequencies
stay the same over generations. _Within_ a generation time,
however, they vary. I.e. the gene pool changes from fertilization
to reproductive age each generation, but if you choose one
part of a generation (say repro age), the gene pool is the
same year after year. Consult your local population genetics
text to see how to derive the mathematics of this.]

Genetic drift: Change in the gene pool purely as a result of
chance, and not as a result of selection, mutation or
migration.

[In all, not too shabby for a computer program. The manual also
uses the Vonnegut quote I have been using in my .sig file.]

Chris Colby   ---      email: colby@bu-bio.bu.edu     ---
"'My boy,' he said, 'you are descended from a long line of determined,
resourceful, microscopic tadpoles--champions every one.'"
      --Kurt Vonnegut from "Galapagos"