Speciation

1

• Speciation
• Tempo of Speciation
• Lack of evolutionary intermediates bothers some
  people
• 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

2

• Speciation
• Tempo of Speciation
• 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

3
Fig. 24.17
4

• Speciation
• Tempo of Speciation
• Proponents of punctuated equilibrium
• Speciation events observed over years, not
  centuries
• Proponents of gradualism
• Periods of stasis result from accumulation of
  mutations without altering phenotype (preserved
  by fossil record)
• Speciation may be reflected in physiology or
  behavior, which arent 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

5

• 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

6
Limpet
Slit-shell
Nautilus
Squid
Murex
Fig. 25.26