Darwin and Paleontology

Conceptual framework before Darwin regarding species through time

• Lyell
  • Species is a natural unit
  • Species originate in a single place suitable for its survival
  • Species can migrate over time
  • Species go extinct due to environmental change
    • Due to changes in the physical world
  • Overall pattern of gradual turnover
• Bronn: General patterns or laws in nature
  • Species have an objective reality and can only vary within limits
  • Changes in fauna and flora are gradual and piecemeal
  • Gaps in fossil record due to preservation but suffice to show big features
  • Progress to life
  • No evidence of Lamarkian change
  • No evidence of relationships between different classes
• Phenomenological Laws
  • Observational patterns of change, but do not attempt to explain causal relations
  • Typical of the time
  • Apply to all the “laws” of Lyell and Bronn

Darwin’s Theories (after Mayr)

• Species are populations
• Change through time (evolution, strictly speaking)
• Common descent
• Species multiply by splitting of lineages
• Changes are gradual
• Changes are due to natural selection

Natural Selection

• Species are inherently variable
• Some variations will preferentially survive due to environmental factors
  • Habitat, food availability
  • Changing environments may favor some variants
  • Element of chance here: some environmental factors may vary widely
• The variations are inheritable, so survivors pass along morphological traits to offspring
• If a species is split into multiple populations (say by physical barriers), the lineage may split as each population shifts toward most adaptive traits of its environment

Implications

• Meaning of Taxonomic groups change
  • Species within genera: recent common ancestor in the past
  • Genera within family: shared common ancestor, further back in time
  • Taxonomy = outline of ancestory
  • Implications of a past history of branching and adaptation
• Quickly applied to fossil record.
  • Fundamental change in meaning of fossil record
  • Some notable examples:
  • Gundry’s mammal studies (1860s)
    • Connect similar species across Cenozoic
    • Inferred ancestor-descendent relationships
  • Horse evolution(1869-70s)
    • Morphological trends in mainline of horse evolution
    • Interpreted as due to adaptive changes in lineage
  • Archaeopterx (1861)
    • Fossils with features of both reptiles and birds
    • Argued to be an example of evolution between one class and another.

Alternatives to natural selection

• Challenges arose to natural selection in later 19th century
  • Widely adopted by paleontologists
• Two most notable ones:
  • Neo-Larmarkianism
    • Species activities result in changes to morphology due to use
      • Examples: giraffe neck, horse legs
    • These could be inherited
    • Allows lineage/organism to have some control on its fate
  • Orthogenesis
    • Straight-line morphological changes within lineage
      • Examples: horses, Irish Elk
    • Guided by internal forces so that variation was not random but directed
• Attractive because they gave a sense of purpose and reduced element of chance.
• Failed in face of genetics in the early 20th century