I.
Speciation
D.
Tempo of Speciation
•
Lack of evolutionary intermediates bothers
some people
•
•
1.
Traditionally interpreted as incompleteness of fossil
record
Two models to explain evolution
Gradualism
•
•
•
Evolution proceeds at constant rate
New species evolve gradually, with distinct
intermediate forms
Lack of intermediates in fossil record due to gaps
I.
Speciation
D.
Tempo of Speciation
2.
Punctuated Equilibrium
•
•
•
•
•
First proposed in 1972 by Gould & Eldredge
Fossil record accurately reflects evolutionary history
Species evolve relatively rapidly
Long periods of stasis between evolutionary events
Lack of intermediates due to short time over which
intermediates persist
Fig. 24.17
I.
Speciation
D.
Tempo of Speciation
•
Proponents of punctuated equilibrium
•
•
Proponents of gradualism
•
•
•
•
•
Speciation events observed over years, not
centuries
Periods of stasis result from accumulation of
mutations without altering phenotype (preserved by
fossil record)
Speciation may be reflected in physiology or
behavior, which aren’t preserved
Pace of evolution may not be uniform in all
instances
Most mutations may accumulate at constant
rate without causing speciation
Major mutations may cause rapid speciation
II.
Macroevolution
•
•
•
•
•
Major changes over long periods
Often includes development of novel
features
Ex: Increased brain size and upright gait
in humans
Ex: Development of feathered, winged
birds and mammals from reptiles (no
feathers, no mammary glands)
Ex: Complex camera-type eye
Limpet
Slit-shell
Nautilus
Squid
Murex
Fig. 25.26