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Speciation Tempo of Speciation Lack of evolutionary intermediates bothers some people Traditionally interpreted as incom

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

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Speciation Tempo of Speciation Lack of evolutionary intermediates bothers some people Traditionally interpreted as incom

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  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 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

  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

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