 Assigned reading: chapter 2 of Zimmer and Emlen
text
Chapter 2
Biology: from natural
philosophy to Darwin
Natural vs. Supernatural explanations
 All societies have or had creation myths that invoke
the action of supernatural forces to explain the
origins of life and the history of the earth.
 Ideas invoking the role of gods and divine actions are
supernatural (i.e., beyond nature).
Darwin’s book published in 1859
 On the Origin of Species by
Means of Natural Selection, or
the Preservation of Favored
Races in the Struggle for Life
 Descent with modification


All species share common ancestry
Changes occur through natural
selection
Darwin and the origin
 In “The Origin” Darwin presented a detailed
strongly, supported argument, that life had
originated naturally and that over time organisms
had evolved by means of a mechanism he called
natural selection.
 How did this transition from supernatural to a
natural explanation for the diversity of life occur?
Development of Natural Philosophy
 The early Greeks were among the first to develop
natural philosophy to explain the world.
 Natural philosophers aimed to develop sets of
physical laws to explain the world around them and
how it worked.
Methodological Naturalism
 The Greeks’ approach of trying to explain the world
using only natural phenomena is sometimes called
Methodological Naturalism.
 Methodological because this strategy provides a
procedure or method for scientifically explaining the
world and natural because it focused on nature.
Anaximander’s cosmology
 An early example of methodological naturalism is
Anaximander’s cosmology. He proposed the earth is
a disk surrounded by huge wheels on which the
moon and sun rotate around the earth.
 His explanation is mechanistic because it invokes a
natural (although incorrect) mechanism to explain
the presence and motion of celestial objects.
Aristotle (384-322 BC)
 Aristotle was the first Greek philosopher to
emphasize observation and the testing of ideas to
explain those observations (i.e., hypothesis testing).
 Supernatural explanations are inherently untestable.
 The Greeks also emphasized the importance of logic
in moving from observations to general principles.
Understanding Nature before Darwin
 In the two centuries before Darwin’s birth in 1809
there was a great expansion in the understanding of
Nature.
 For example, Carolus Linnaeus developed his
taxonomy that grouped all life into a rational
hierarchical system that is the basis of modern
classification.
Understanding Nature before Darwin
Carl Linnaeus (1707-78):
Father of modern taxonomy
Understanding Nature before Darwin
 Nicolaus Steno in the late 1600’s recognized that
triangular rocks known as “tongue stones” were in
fact shark’s teeth that had been transformed into
stone (fossilized).
Nicolas Steno (1638-86):
Father of geology and
stratigraphy
Understanding Nature before Darwin
 The new science of paleontology led to the discovery
that life on the planet had changed greatly over time.
 George Cuvier, a famous French paleontologist,
documented the fact that many species had become
extinct, which suggested that the fauna and flora of
the earth changed over time.
Paleontology provided evidence that life changed
 Georges Cuvier (1769-1832)
 Fossils resemble but are not exactly the same as modern
species
 Many past species are extinct
An ancient and ever-changing earth
 That the earth is old and undergoes constant change
was not widely accepted until recently in western
thought.
 Most of Darwin’s contemporaries considered the
earth to be young (only thousands of years old rather
than 4.6 billion).
Geologists recognized that change was gradual
 James Hutton
 Observable processes produce small
changes that accumulate over time
 Implies the earth must be old
 William Smith
 Different rock layers contain distinct fossils
William Smith
(1769-1839)
Smith created first geological map
Geologists named rock formations from
different periods based on the fossils they
contained
An ancient and ever-changing earth
 By time Darwin was a young man the idea that the
Earth was young was being challenged.
 Opposition based on principle of
Uniformitarianism.
 Idea that geological processes happening today are
the same as have operated in the past.
An ancient and ever-changing earth
 Uniformitarianism contrasted with Catastrophism
which proposed that current geological formations
had resulted from catastrophic events (such as
biblical flood) which occurred on scale unknown
today.
 Uniformitarianism, first proposed by James Hutton,
was championed by Charles Lyell in his book
Principles of Geology.
An ancient and ever-changing earth
 Hutton and Lyell inferred the Earth must be very old
based on measurements of rates of ongoing rock
forming processes (e.g., deposition of mud and
sand).
 These developments in geology focused Darwin on
the potential importance of gradual change in
shaping structures.
The intellectual environment
 In the early 1800’s the world was primed for a
comprehensive theory of evolution to be developed.
 Multiple people put forward the idea that evolution
had taken place, what was lacking was a workable
mechanism.
 One influential scientist is this period was JeanBaptiste Lamarck.
Jean-Baptiste Lamarck
(1744-1829)
Jean-Baptiste Lamarck
 In 1809 Lamarck suggested that organisms
descended gradually from older less complex species.
He considered lineages of organisms had an inherent
tendency to become more complex over time and
transformed over time.
 His primary mechanism was called the Inheritance
of Acquired Characteristics (IAC).
Inheritance of Acquired Characteritics
 IAC suggested that traits an organism acquired
during its life could be passed on to its offspring. By
this process of transformation, lineages would
change over time.
 For example, a giraffe stretching its neck during its
life would become slightly longer necked and pass
this slightly longer neck to its offspring.
Inheritance of Acquired Characteristics
 Lamarck’s ideas were widely discussed and
influential in his time as there was no understanding
of how heredity occurs.
 Obviously, Lamarck’s ideas about mechanism of
evolution contradict current biological knowledge.
 Information flows from DNA to phenotype not in the
reverse direction
Charles Darwin (1809-1882)
 Was the son of a wealthy doctor, but he himself
dropped out of medical school.
 Studied theology at university, but was more
interested in natural history.
Charles Darwin (1809-1882)
 After graduation Darwin signed on as the Captain’s
companion for a round the world voyage aboard the
surveying ship HMS Beagle.
The Beagle Voyage
 On the voyage Darwin read Lyell’s Principles of
Geology and was greatly influenced by Lyell’s
emphasis of two major points .
 1. The Earth is very old.
 2. Geological features we see around us can be
explained by the action of the slow, gradual
processes we can observe every day (e.g. the eroding
of rock by waves and wind; the slow deposition of
sediment).
The Beagle Voyage
 Darwin was Captain Fitzroy’s companion during the
voyage (1831-1836).
 The Beagle mapped the coast of South America and
circled the globe.
 Visited South America, Australia, Pacific Islands and
South Africa.
 Famously visited the Galapagos Islands a group of
volcanic (hence quite young islands) off the coast of
Ecuador.
Unique animals on Galapagos include
giant tortoises, marine iguanas, and
Darwin’s finches.
Galapagos Giant Tortoise
Observations convinced Darwin that life evolved
The Beagle Voyage
 On the voyage Darwin noted many things that were
puzzling from the point of view of a creationist
explanation for the diversity of life.
What Darwin observed
 South American fossils resembled living animals.
 Parts of the world with very similar habitats and
climates (e.g. in Australia and South America) were
populated by very different organisms.
 The plants and animals found on each continent are
distinctive.
 Many species on oceanic islands are found only there
(endemic).
 Endemic species on islands often closely resemble
species on adjacent mainland.
What Darwin observed
 These observations taken together don’t make sense
if organisms are specially created. Why should
similar habitats in different parts of the world have
completely different faunas? Why should remote
islands have faunas that differ from but resemble
those on adjacent islands?
 Together these observations suggested to Darwin
that species change over time i.e., evolution occurs.
Natural Selection
 Convinced by the time he returned to England that
evolution occurs, Darwin needed a mechanism to
explain how evolution happens.
 His mechanism is the process of natural selection.
Natural Selection
 An important event in developing natural selection
was when Darwin read Thomas Malthus’ “Essay on
population.”
 Malthus emphasized that the reproductive potential
of animals and the rate at which populations could
increase in size was much greater than the rate at
which food resources might increase.
Natural Selection
 Darwin realized that most organisms that were born
could not survive to reproduce. There just were not
enough resources.
 Thus, there must be “a struggle for existence” in
which only some organisms survived to reproduce
and the remainder did not.
Natural Selection
 Darwin knew that individuals in populations are
different from each other (there is variation).
 His insight was in realizing that if success in
surviving and reproducing was related to variation
(i.e., to the traits individuals possessed) that
beneficial traits would become more common and
populations would change over time.
Darwinian Natural Selection: Artificial Selection
 Artificial Selection. Humans have selectively bred
for desirable traits in domestic animals and plants
for millenia.
 Process has produced our crop plants, garden plants,
pets, and domestic animals.
 Darwin closely studied pigeon breeding.
Pigeon breeding
 Pigeon fanciers by selecting for particular traits
among Common Rock Doves by breeding from only
certain individuals had been able to produce a huge
variety of different breeds of domestic pigeons.
 These varieties were so different in appearance that
they would have been considered to be different
species if encountered in the wild.
Common Rock Dove
http://ibc.lynxeds.com/photo/rock-dovecolumba-livia/perched-my-balcony
Hungarian Buga
www.PigeonBreed.com
Holle Cropper
www.PigeonBreed.com
Pomeranian Pouter
www.PigeonBreed.com
Frillback Crested
www.PigeonBreed.com
Evolution by Natural Selection
 Darwin envisaged a process similar to artificial
selection that had produced organisms we see today.
He called it Natural Selection.
 Instead of humans deciding who would breed, nature
would.
Evolution by Natural Selection
 Darwin proposed evolution was the inevitable
outcome of 4 postulates:

 1. There is variation in populations. Individuals
within populations differ in their traits (color, size,
hunting ability, etc).
 2. Variation is heritable. Parents pass on their traits
to their offspring.
Evolution by Natural Selection
 3. There is differential reproductive success. In
every generation only some organisms survive to
reproduce and they produce different numbers of
offspring.
Evolution by Natural Selection
 4. Survival and reproduction are not random. It is
not luck that determines who survives to reproduce.
Those organisms whose traits fit the environment
best are ‘naturally selected’ and they pass their genes
on to the next generation.
 Because only some individuals reproduce it is only
their genes that make it into the next generation.
Thus the gene pool changes and the traits of the
population change in the next generation.
Evolution by Natural Selection
 If these 4 postulates are true then the population will
change from one generation to the next.
 Evolution will occur.
Darwin and Wallace
 Darwin realized his ideas would be controversial and
delayed publishing them for fear of the backlash that
would result. Instead he continued to gather
evidence to support his ideas.
 It was not until he received a letter from Alfred
Russel Wallace in 1858 in which Wallace outlined
the idea of natural selection was Darwin finally
prompted to publish his work.
Darwin and Wallace
 1858 Charles Darwin and Alfred Russel Wallace
jointly proposed natural selection as the mechanism
of evolution in a paper presented to the Royal
Society.
 1859 Origin of Species published
The Modern Synthesis
 Idea of evolution accepted rapidly
 However, a lack of understanding of heredity and
population genetics prevented natural selection
being accepted as the primary mechanism until the
1930’s.
 The “Modern Synthesis” of genetics and Darwin’s
ideas melded an understanding of population
genetics and natural selection to explain gradual
evolution, speciation, and macroevolution.
The Modern Synthesis
 Genes are passed intact from parent to offspring.
Genes are “particulate.” They do not “blend” with
other genes.
 Some mutations are beneficial and these increase in
frequency as a result of selection.
 Mutation is a major source of genetic variation, but
sexual reproduction also produces massive amounts
of genetic variation.
The Modern Synthesis
 Many traits result from the interaction of multiple
genes and some genes affect more than one trait.
Thus, complex changes in phenotype can occur
without assuming a large or sudden genotype
change.
 Many genotype changes affect the phenotype
(physical appearance of an organism). This variation
in phenotype is the raw material for natural
selection.
The Modern Synthesis
 The synthesis showed there was no conflict between
modern genetics and Darwin’s idea that evolution
was largely a slow process driven by natural
selection.
Darwin’s theory has been expanded
 Sexual selection
 Selection for traits that provide a
mating advantage
 Genetic drift
 Change in frequency of traits due to
chance events