Chance and Repeatability
The "big five" mass extinction events decimated a large
fraction of the organisms alive at the time.
Yet many of these extinctions occurred randomly.
Besides being wide-spread, there is no clear attribute
that protected a group of organisms from extinction.
Stephen J. Gould argues that chance has played a
defining role in evolution. He asks us to re-run the tape
of life and imagine possible alternative worlds:
 What if eukaryotes had not evolved?
"Run the tape again, and the first step from
prokaryotic to eukaryotic cell might take twelve
billion instead of two billion years - and
stromatolites, never awarded the time needed to
move on, might be the highest mute witnesses to
Armageddon."
 What if a different collection of metazoans had
survived the Cambrian?
A large number of unusual, high-level taxa evolved
during the Cambrian, & most went extinct. Run the
tape again, and primitive chordates bite the dust
and some other oddball creature sneaks through.
What if an asteroid had not hit the earth at the end
of the Cretaceous? Would dinosaurs have gone
extinct? Would mammals have proliferated?
"Mammals spent their first hundred million years two-thirds of their total history -- as small creatures
living in the nooks and crannies of a dinosaur's
world."
"The divine tape player holds a million scenarios, each
perfectly sensible. Little quirks at the outset, occurring for no
particular reason, unleash cascades of consequences that
make a particular future seem inevitable in retrospect... The
end results are so different, the initial perturbations so
apparently trivial."
Most evolutionary biologists would agree that chance
plays a role in the direction of evolution (drift is all about
the effects that chance events have on evolution!)
The power of the neo-Darwinian view of evolution is in
its ability to understand the responses of living
organisms to the environment & to chance events.
If we played the tape of life again, we might not arrive at
exactly the same place today. But it's likely that some
striking similarities would appear.