You are anything but stupid. Let's do an analogy. If I release a robot that
is able to move in any direction but I attach a rubber band and attach it
to the east wall, the robot will preferentially move to the east in spite
of it's ability to move in any direction at all. There is a force pulling
it to the east. Selective pressure in a constant direction is like that
rubber band. But if I change the wall that the rubber band is attached to
every second and do it randomly, then the combined motions of my movable
robot plus the pull of the rubber band will make the robot move randomly.
This is what the experimenters have done, changed the experimental
selective pressure with each new experiment. The experiments were not
designed to pull the flies in the same direction.
Consider the situation where you want to find a fly that lives at higher
temperatures. To do that, you must raise each generation of fly in higher
and higher temperature. This is constant selective pressure. If you raise
and lower the temperature randomly, then you probably won't get the fly
genome to get the message that a higher temperature fly is needed.
Selective pressure is different than random mutation. Random mutation will
create a higher or lower temperature fly. The environment and natural
selection will decide what percentage of high or low temp flies reproduce
most effectively
glenn
Adam, Apes and Anthropology
Foundation, Fall and Flood
& lots of creation/evolution information
http://www.isource.net/~grmorton/dmd.htm