This report is part of biology. For students who want to study it.

2. Tempo of Speciation
The tempo of speciation refers to the rate
at which speciation occurs.
There are two models for this.
Phyletic Gradualism
Punctuated equilibrium

3. Phyletic Gradualism
Speciation may occur slowly when many genetic
differences build up between isolated populations.
This is known as gradualism or phyletic gradualism.
In gradualism, species descended from a common
ancestor diverge in morphology more and more as
they acquire unique adaptations. Change is slow,
constant and consistent.
This idea was asserted by Darwin and is
illustrated in Figure on the left.

6. Punctuated equilibrium
First proposed in 1972 by Stephen jay Gould and
Niles Eldredge, another theory states that new
species arise suddenly and rapidly when a few critical
genes change. According to the theory of punctuated
equilibrium, a new species changes the most as it
buds from a parent species and changes little for the
rest of its existence as illustrated in Figure on the
right.

8. The theory of punctuated equilibrium
seemed to account for short,
revolutionary transitions found in fossil
records. Alan Cheetham of the
Smithsonian Institution reported that
in his study of fossilised invertebrates,
long periods of stasis in morphology are
“punctuated” by the relatively rapid
appearance of new species.