What is "Evolution"?

Fact: current life forms differ from those of previous times,
          and are 'descended' from them.
    Evolution is descent with modification.
                  ["Descent" in the genealogical sense]

 Fact: current life forms are extremely variable,
          both within and among species.
    Evolution accounts for variation among living organisms.
                  [Variation occurs in time and space]

What is the "Theory of Evolution"?
      The explanation of observed patterns of temporal and spatial variation
            in terms of biological and physical processes.
     Theory: an organized set of facts, principles, and hypotheses;
            not a "guess", not "just a theory"

Course themes:

   Quantification and interpretation of organismal variation
            Taxonomic patterns have been described in the second-year core.
       Laboratory exercises introduce quantitative & statistical methods.

     Genetic & ecological processes underlying variation within species:
            Short-term, small-scale (microevolution)
       Populations genetics models evolution in terms of allele frequencies.

     Patterns in the fossil record describe origin of variation among species:
            Long-term, large-scale (macroevolution).
       Systematics & the Comparative Method as a means of analyzing evolution.

     Th. Dobzhansky (1900-1975):
    "Nothing in biology makes sense except in the light of evolution."


Now I am ready to tell how bodies are changed
Into different bodies.

I summon the supernatural beings
Who first contrived
The transmorgifications
In the stuff of life.
You did it for your own amusement.
Descend again, be pleased to reanimante
This revival of those marvels.
Reveal, now, exactly
How they were performed
From the beginning
Up to this moment.
                                        Ovid: Metamorphoses (trans. Ted Hughes)


History of evolutionary thought: Darwin's Century

Biology in the 18th century
     The Classical Tradition: Plato & Aristotle (4th cent. BCE)
       Theory of Forms (essences, eidos)
                  'real' objects are manifestations of 'ideal' forms
                  variation is illusory  [see Plato "The Republic"]
       Dichotomy: the world is composed of paired opposites
                  "A" versus "not A" classes
                  good / bad, right / wrong, up / down, light / dark, male / female, etc.
           e.g., vertebrates vs. invertebrates
       Aristotle - "Father of Biology"
                Five books on zoology ("Generation of Animals")
                Biological structures have purpose: Efficient versus Final Causes

     Natural Theology: "The Wisdom of God, Manifested in His Creation"
            'Ideal' forms exist in the Mind of God:
                  'real' world created by God (Genesis 1:1)
       Scala Naturae: the "Great Chain of Being"
                  Creation is an infinitely graduated progressive series
                  Time scale is short (ca. 6,000 years)
                  Species are static: no new forms, no change, no extinction
            The study of nature is a pious activity

     Linnean Taxonomy (Carl von Linne [Carolus Linneaus] 1707-1778)
            "Systema Naturae" (1735; 10th ed. 1758)
                  4,162 animals described
           binomial nomenclature: genus + species names
                  "ad majorem Dei gloriam": for the greater glory of God

     Exploration creates a Scientific Crisis
       New forms are discovered that don't fit the Scala
       Extinctions have evidently occurred
       Variation is real in space: what about over time?

Biology in the early 19th century:
      Change has occurred, how do we explain it?
     The Enlightenment favors rational explanation.

     Jean Baptiste de Lamarck (1744-1829): "Zoological Philosophy" (1809)
            New features arise due to persistent "besoin" (need / want)
                  (teleological: a goal-directed explanation)
            Use and disuse alter morphology:
            Altered morphology is passed on to offspring
                  (Lamarckism: inheritance of acquired characteristics)
             Ex.: Giraffes stretch their necks to feed on leaves.
                           Successive generations gradually acquire longer necks.
                            [or, trees become taller to escape giraffes]
            Therefore, organisms change (evolve) over time

     Uniformitarianism replaces Catastrophism in geology
       Charles Lyell (1797-1875): "Principles of Geology" (1830)
       Observable, gradual processes + enormous time = world geology

The Darwinian Revolution [extended lecture on Darwin]

     Charles Darwin (1809-1882)
            B.Sc. (Cambridge): pre-med
            Naturalist on board HMS "Beagle" (1831-36)
                "The Voyage of the Beagle" (1839) a best-seller
            Read Robert Malthus "On Population" (1838):
                population increases exponentially, resources increase arithmetically
            Letter from Alfred Wallace (1823-1913) in June 1858
            "On the Origin of Species" (1859)


The theory of evolution by natural selection
        (after pp. 80-81 of "Origin")

Observation: In any species, more young are born than can possibly survive.

Observation: Yet a species' numbers do not increase without limit.

CONCLUSION: There is a Struggle for Survival,
        and differential survival and reproduction occur within species.
        [Darwin: "I use 'struggle' in a large and metaphorical sense..."].

Observation: Individuals within species show variation
          that affects the probability that they will survive this struggle and leave offspring.

CONCLUSION: Those individuals that survive and reproduce do so in consequence
      of their "adaptively superior" variation (they are "more fit")
      This process of differential survival and reproduction is called Natural Selection.

Observation: Variation is heritabile: offspring tend to resemble their parents.
            [Remember that Mendelian genetics was unknown in 1859).

CONCLUSION:
       Adaptively superior variation will be inherited by the offspring generation.
      That is, evolution occurs as descent with modification.


Implications of Darwin's Theory

     Natural Selection provides a mechanism for Evolution:
            Modern evolutionary theory seeks to clarify this mechanism.

      The observable order in Nature is due to common descent from an ancestor:
            Organisms resemble each other because they are related.

      The degree of relationship provides a basis for "natural classification":
            Taxonomy should reflect the phylogeny of organisms.

     All living things are related (the basic fact of biology):
       Humans have evolved from other animals (Darwin's "Descent of Man" 1871).
               "The main conclusion arrived at in this work,
                  namely that man is descended from some lowly organised form,
                  will, I regret to think, be highly distasteful to many."


For further reading:

    Loren Eisley (1959). "Darwin's Century." Doubleday.
    William Irvine (1955). "Apes, Angels, and Victorians: Darwin, Huxley, & Evolution." McGraw-Hill.
    Ernst Mayr (1994). "One Long Argument". Harvard University Press.
    Gordon Ratray Taylor (1963). "The Science of Life." McGraw-Hill.


All text material © 2006 by Steven M. Carr