Introduction
Macroevolution is the study of long-term evolutionary change. Life has been
present on earth for approximately 3500 million years and in that time has
undergone extensive changes. That the first organisms, simple single-celled
prokaryotes, have given rise
to the current complexity and diversity of over 1 million known species, with an
estimated 10 billion more unknown species, is almost unimaginable. The patterns
by which groups of organisms arise and become extinct are the subject of the
study of Macroevolution.
There are two competing theories as to how species evolve. The older theory is
that of phyletic gradualism. This theory, supported by Darwin, states that
evolution is a gradual process that proceeds slowly but constantly through a
series of small changes. A newer theory that has received a lot of attention is
that of punctuated equilibrium. Proposed by Niles Eldredge and Stephen J. Gould,
this theory states that evolution is not gradual, but rather proceeds by means
of bursts of change separated by long periods of relative stasis. Unfortunately,
the fossil record cannot disprove either of these theories, so the debate over
which is more accurate will likely continue for some time.
Though limited in many ways, the fossil record is able to provide us with a lot
of information. From fossil remains we know that life began on earth about 3500
million years ago. We can also see that a series of mass extinctions has shaped
the organismic landscape, allowing the fall and rise of several different
dominant groups, including dinosaurs and, most recently, mammals.
