ALTERNATIVE INTERPRETATIONS OF EVOLUTIONARY
PATTERNS BASED ON THE FOSSIL RECORD
See King p. 113 on
Gradual Evolution. Phyletic gradualism. This is the theory of Darwin and Wallace. They
believed all evolution took place gradually, with one species turning into another by natural
selection.
However, some species appeared suddenly in the fossil record without any obvious
ancestors. How might the origin of such creatures be explained?
Alternative theory. Punctuated Equilibrium developed in the past thirty years by
Stephen J. Gould and others. This suggests long periods of stability (stasis) interrupted by
sudden change.
Punctuated equilibrium theory suggests that a group of interbreeding animals stays the
same for a length of time (stasis). Suggestions are that stasis is more likely to occur in
situations where environmental conditions are unstable. Elsewhere, e.g. for trilobite, in
deep sea environments, there is more opportunity for experiment, and new species evolve
i.e. evolutionary change occurs when environmental conditions are constant. If the
new species is more successful than the old one, eventually it will take over – e.g. grey
squirrels have almost ousted red squirrels from Britain,
If this theory is valid, what would geologists expect to observe?
More recent studies have suggested that both these theories are two simplistic (King p.
114). Studies of Micraster suggest gradual changes in shape as Micraster burrowed
deeper (for different food; to cope with different sediments, or to escape predators?). This
graduated change was punctuated by more abrupt changes.