http://www.stephenjaygould.org/
Remember that I said that evolution is not
really gradual. Evolution does take a long time
to occur -- many, many lifetimes of the species
in question. However, species can also
reproduce without any significant change in
form or function for many generations also.
How and why does evolution happen? How come
it happened for the finches on the Galapagos,
where many different species of finches evolved
from one common ancestor, but many species
of finches did not evolve on the mainland?
Stephen Jay Gould (1941-2002) proposed
a theory which described when and how
evolution and speciation occurs.
His theory is called
punctuated equilibrium.
http://www.stephenjaygould.org/
Lots of great Gould resources are found (not surprisingly)
at
http://www.stephenjaygould.org/.