 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.
 What does supernatural mean?
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 of species” Darwin argued:
1. life had originated naturally
2. 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
 Early Greek natural philosophers aimed to develop
sets of physical laws to explain the world around
them and how it worked.
 An early example of the Greek approach 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.
Aristotle (384-322 BC)
 Aristotle emphasized 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.
Understanding Nature before Darwin
 During the Age of Enlightenment (begins late 17th
century) there was a great expansion in the
understanding of the natural world.
 E.g. Carolus Linnaeus developed his taxonomy that
grouped all life into a rational hierarchical system.
Understanding Nature before Darwin
Carl Linnaeus (1707-78):
Father of modern taxonomy
Nicolas Steno (1638-86):
Father of geology and
stratigraphy
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, which implies life on earth
has changed over time
An ancient and ever-changing earth
 In Darwin’s era most people believed the earth was
young (only thousands of years old).
 How old is the earth?
 How do we know this?
An ancient and ever-changing earth
 When Darwin was a young man the idea that the
Earth was old was being proposed.
 Great age of the earth based on principle of
Uniformitarianism.
An ancient and ever-changing earth
 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 ready for a theory
of Evolution.
 Multiple people had suggested that evolution had
taken place, but there was no plausible mechanism.
 Enter Jean-Baptiste Lamarck.
Jean-Baptiste Lamarck
(1744-1829)
Jean-Baptiste Lamarck
 In 1809 Lamarck suggested that organisms
descended gradually from older less complex species
and tended to become more complex 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
 But Lamarck’s ideas about mechanism of evolution
contradict current biological knowledge.
 Information flows from DNA to phenotype not in the
reverse direction
 Epigenetics
Charles Darwin (1809-1882)
 Son of a wealthy doctor, but he dropped out of
medical school.
 Studied theology at university, but was more
interested in natural history.
Charles Darwin (1809-1882)
 After graduation Darwin was 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. Lyell’s emphasised 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 slow
deposition of sediment).
The Beagle Voyage (1831-1836)
 The Beagle mapped the coast of South America and
circled the globe.
 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.
What Darwin observed
 Parts of the world with very similar habitats and
climates (e.g. in Australia and South America) were
populated by very different organisms.
 Also the plants and animals found on each continent
are often distinctive.
http://www.focusonnature.com/SouthAm
ericaMammalList.htm
http://upload.wikimedia.org/wikipedia/co
mmons/c/cb/Marsupialia_collage.png
What Darwin observed
 Many species on oceanic islands are found only there
(endemic) and these endemics often closely resemble
species on adjacent mainland.
 Galapagos Marine Iguana (left) Green Iguana (right)
http://www.redorbit.com/media/uploads/ http://iansvivarium.com/viewtopic.php?f=
2004/10/41_8f040a736a1c755a6f0ba08ef 2&t=7193
7a18e44.jpg
What Darwin observed
 These observations taken together don’t make sense
if organisms are specially created.
 They suggested to Darwin that species change over
time i.e., evolution occurs.
Natural Selection
 Convinced 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 rate at which
populations could increase in size was much
greater than the rate at which food resources might
increase.
Natural Selection
 Therefore, most organisms that were born could not
survive to reproduce. There just were not enough
resources.
 Darwin concluded that there must be “a struggle
for existence” in which only some organisms
survived to reproduce and the remainder did not.
Natural Selection
 Darwin also knew that individuals in a population
differ from each other (there is variation).
 If success in surviving and reproducing was related
to variation (i.e., to the traits individuals possessed)
then 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 selectively breeding for particular
traits among Common Rock Doves have produced a
huge variety of different breeds of domestic pigeons.
 These varieties look so different that they would be
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.
 2. That variation is heritable.
Evolution by Natural Selection
 3. In every generation some organisms are more
successful at surviving and reproducing than others.
There is differential reproductive success.
 4. Differences in survival and reproduction are not
random, but are related to variation among
individuals. Those organisms with best
characteristics are ‘naturally selected.’
Evolution by Natural Selection
 Note how postulate 4 “loops back” to postulate 1.
 In each generation the best adapted are chosen so
the organisms become better and better adapted to
their environment.
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 delayed publishing his ideas for fear of the
backlash.
 Only when he received a letter from Alfred Russel
Wallace in 1858 in which Wallace also outlined the
idea of natural selection did Darwin finally 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, but natural
selection not accepted as main mechanism until the
1930’s
 Darwin never understood how heritability worked.
 The “Modern Synthesis” melded population genetics
and natural selection to explain gradual evolution,
speciation, and macroevolution.
The Modern Synthesis
 Gregor Mendel (1822-1884; father of genetics).
 Mendel showed:
1. Genes are passed intact from parent to
offspring.
2. Genes are “particles.” They
do not “blend” with other genes.
The Modern Synthesis
 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.
 How?
The Modern Synthesis
 Many traits result from the interaction of multiple
genes and some genes affect more than one trait.
 Also, minor changes in timing or activity of genes
can lead to large changes in morphology.
 Therefore, complex changes in phenotype can occur
without assuming a large or sudden genotype
change.
The Modern Synthesis
 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