Macroevolutionary Theories
A topic of debate in the field of macroevolution is the way in which evolution
of species takes place over extremely long periods of time. Until recently,
most biologists believed Darwin's theory, called phyletic gradualism, that
evolution was a gradual and continuous process. However, the proposal of a new
theory based on evidence from the fossil record, called punctuated
equilibrium, has raised some doubts in the minds of scientists.
Phyletic Gradualism
Darwin felt that evolution took place gradually, in very small steps. This view
is supported by the change we can observe in species on the scale of human
history. Species seem to be slowly but constantly changing. Unfortunately,
gaps in the fossil record make it impossible to support this view with direct
fossil evidence.
Punctuated Equilibrium
Niles Eldredge and Stephen J. Gould developed the theory of punctuated evolution
within over last thirty years. They propose that the change we can see is
evolutionarily minor and that speciation occurs only in response to major crises
or genetic events. The majority of time, they maintain, species exist in
relative stasis, changing very little, until a crisis or major genetic event
occurs and they undergo rapid change. It is important to understand that the
term "rapid" is meant in the geological sense, meaning over thousands rather
than hundreds of thousands or millions of years. Eldredge and Gould state that
their theory is supported by the fossil record, which does show periods of
sudden change separated by periods of apparent stasis. Unfortunately, the lack
of completeness and detail of the fossil record makes it impossible to prove or
disprove either of these theories.
