I am not sure that you understand the actual process here.
An ant, just like a real human, does not ever change his DNA.
Therefore, his system never changes. The HILL improves through the
breeding process.
Understand that the hill is represented by the best ant. The goal of
the hill is to develop a single ant that is the best example of ant-
hood. That is, the best possible ant he can be.
Thus, at the beginning of a new generation we have these 20 ants left
from the previous generation. They were the best 20 ever produced.
Their offspring replace the 80 ants we killed at the end of the
previous generation. Those were the ones that were not good enough to
make it into the top 20.
Through this process some of the children will be better than their
parents and go on to fame and fortune as breeders themselves, passing
their genes onto the future generations. Ultimately, their parents
will move down the ranked list and out of the breeding pool. When they
do so, they will die. Of course, ultimately, the children and their
children will do the same thing.
As for "Why 20%?" Personal experience. The number of breeders versus
the hill population is not an exact science - at least I know of no
studies that discuss it. It would also depend upon the nature of the
data, the complexity of the system one is trying to uncover. In non-
layman's terms that would be called the "search space."
Consider what we are really trying to do... There are 1,000 factors.
Each factor has a potential of 250 values. There are 500 rules. Each
rule has one of 5 "signs," (i.e. >,>=,=,<,<=), 250 different values, 3
branches (take an action, "and" with the next rule, "or" with the next
rule), and 1,000 possible actions.
That would be 1000*250*500*5*250*3*1000 permutations. Which one is
best? That is the space we are searching to find "optimum." Well, not
really optimum... just VERY, VERY good.
How long do you suppose that it would take to try them all?
Consider the chess board... There are 64 squares x 32 pieces - That is
a HUGE problem. This is magnitudes larger.