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

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

Fig. 24.17

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 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

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