Darwin & Natural Selection

Charles Darwin & the Origins of Natural Selection
    Who is Darwin & what did he do?
    What is Natural Selection & how does it work?
    How does Natural Selection explain 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

     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?

Change has occurred, how do we explain it?
The Enlightenment favors rational explanation.

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

The Darwinian Revolution

     Charles Darwin (1809 -1882)
            BSc (Cambridge): pre-med
            Naturalist on board HMS "Beagle" (1831-36) over father's objections
                Capt. Robert FitzRoy, RN
                South America: extinction is real
                Galapagos Islands: variation is real
           "The Voyage of the Beagle" (1839): a best-seller

             Examined collections closely:
                    transmutations in time & space are real (March 1837)
             Read Robert Malthus "On Population" (Sept - Oct 1838):
                    population increases exponentially, resources increase arithmetically
             Extensive study of Artificial Selection by plant & animal breeders

             Married Emma Wedgwood (Jan 1839)
                   Materialism: "I deserve to be called a theist" ;
                        [Death of Annie Darwin ,1851]

Sketches of 1842 & 1844: "Natural means of selection" / "Picking"
"It is like confessing a murder."

Letter from Alfred Wallace (1823-1913) in June 1858
"On the Origin of Species" (1859) [online text]

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 heritable: offspring tend to resemble their parents.
["Hard inheritance" is sufficient: Mendel and genetics were unknown in 1859).

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

Putting it another way....

      "Natural Selection" describes an evolutionary process in which
            "adaptation" occurs in such a way that "fitness" increases.
            Under certain conditions, this results in descent with modification.

      If:     variation exists for some trait, and
                a fitness difference is correlated with that trait, and
                the trait is to some degree heritable (determined by genetics),
      Then: the trait distribution will change
                over the life history of organisms in a single generation,
                    and between generations.

The process of change is called "adaptation"

That's all.

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 (1871) "Descent of Man")
               "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."
       Thomas Huxley (1825-1895) "Man's Place in Nature" (1863)
                 established similarity and relationship to Great Apes

"Nothing in Biology makes sense, except in the light of Evolution." (Th. Dobzhansky, 1975)

For further reading:

   Janet Browne (1995). Charles Darwin: Voyaging. Knopf
                            (2002). Charles Darwin: The Power of Place. Knopf.
    Daniel C. Dennet (1995). Darwin's Dangerous Idea. Beacon
    *Loren Eisley (1959). Darwin's Century. Doubleday.
    Stephen Jay Gould (2002). The Structure of Evolutionary Theory. Harvard.
    Richard Hofstader (1955). Social Darwinism in American Thought, 1860 -1915 (rev. ed.). Beacon.
    William Irvine (1955). Apes, Angels, and Victorians: Darwin, Huxley, & Evolution. McGraw-Hill.
    Ernst Mayr (1994). One Long Argument [see especially Chapter 4: Darwin's Path]. Harvard University Press.

Web resources:
    John van Wyhe (ed.), The writings of Charles Darwin on the web (http://pages.britishlibrary.net/charles.darwin/)
    Brief biography of Darwin by John van Wyhe [http://pages.britishlibrary.net/charles.darwin3/darwin_bio.htm]
    Course notes for Bio2900 - Principles of Evolution & Systematics
    Extract from PBS special "Darwin's Dangerous Idea": Natural Selection in 10 minutes