Macroevolution

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Macroevolution

• Chapter 17

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Lecture Overview

• What is a species? Problems
• Speciation- Modes, Examples, Difficulties
• Barriers Between Species
• The Cambrian Explosion and the Pace of Evolution

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• After describing a set of forms as a distinct
  species tearing up my manuscript, making
  them one species tearing that up making them
  separate, then making them one again (which has
  happened to me), I have gnashed my teeth, cursed
  species, asked what sin I committed to be so
  punished- Charles Darwin

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Inadequacies in Defining a Species

• Appearance alone fails
• Biological Species Concept - Species are groups
  of actually or potentially interbreeding
  populations, which are reproductively isolated
  from other such groups
• Adequacy of Mayers definition?

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Problems with the Biological Species Definition

• Asexual organisms?
• Extinct Organisms?
• Genetics/Geography (Lions and Tigers/ Garter
  Snakes)

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Other Taxonomic Categories

• Races and Subspecies- morphologically distinct
  subpopulations which can interbreed (Song Sparrow
  Example)
• Incipient species?
• Adaptive?
• Biologically Meaningful?

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How do Species Form?

• Reproductive IsolationNo Gene Flow
• Why is this important to generate two species
  from one?

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Barriers Leading to Reproductive Isolation

• Pre-zygotic
• ecological
• temporal
• behavioral
• mechanical
• gametic

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Barriers Leading to Reproductive Isolation

• Post-zygotic- fertilization occurs, but zygote
  dies or fails to reproduce
• hybrid sterility
• hybrid inviability

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Proposed Modes of Speciation

• Allopatric
• Tuft-Eared Squirrel Populations on North and
  South Rim of the Grand Canyon

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Proposed Modes of Speciation

• Sympatric- involves no geographical barrier
• Example of hawthorn fly (Hawthorne fruits to
  apples to cherries 1864 to 1960)
• Is this a good example of speciation?

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Proposed Modes of Speciation

• Parapatric- geographically separated, but still
  has some contact
• Grass subpopulation grows well at mine entrance,
  unlike most other grass in population
• Has gene flow ceased? If so, would it make the
  two populations different species?

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The Cambrian Explosion

• Burst of Diversification Unmatched in History
• Members of all Phyla Appear Very Suddenly, about
  600 million years ago
• No new body plans. Why?
• Evolution Slowed?
• Developmental Constraints?
• Burst of Diversification Brought About by God?

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Mass Extinctions

• Mass extinctions occurred at 5 times in
  geological history
• Are they significant?
• Causes?
• Consequences?

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The Rate of Evolution

• Darwinian- Gradual and Steady. Why?
• Hope to see a fossil representing each small step
  in an evolutionary line
• The Problem with Fossils

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Saltationism

• Evolutionary Change is Abrupt
• Most Species are Static in the Fossil Record
• What is the mechanism?
• Goldschmidts Hopeful Monsters-currently
  rejected by virtually all biologists
• Punctuated Equilibrium
• Species Change Very Little Most of the Time
• Most Evolutionary Change Occurs During a
  Geologically Brief Period of Time