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."
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)
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."
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.