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Transcript
Lecture Outline - Evolution
1. History of Evolutionary thought
2. Principles of Evolution by Natural Selection
3. Modern Synthesis: combining Darwin + Mendelian genetics
4. Sex
5. Evidence for evolution
6. Why the controversy?
Historical Context
Original European belief: Genesis account of creation (6 days)
In 1500’s, explorers started bringing home animals no one
had ever seen before; literal Creationism feel out of favor
Replaced by Special Creationism (held a hint of ecology):
(1) God created every living thing, exactly suited to its
particular environmental niche and location
(2) When one species went extinct, an appropriate successor
was created and installed in its place
- was the belief of most scientists into the 1800’s
Lamarck - “Acquired Characters”
1809 - French biologist proposed that species had changed
over time, and were related by common ancestry
Proposed that individuals changed
to meet the needs of their
environment, and that these
acquired characters were
then passed on to their offspring
Most famous idea - giraffes stretched
their necks to reach high branches

The Birth of Evolutionary Theory
In the early 1800’s, some enterprising young men began
exploring the world’s biodiversity with an eye for pattern
and process in the natural world.
One was Charles Darwin
- educated, from a wealthy family
- destined to become a preacher,
he set off to explore the world after
college (1832-1836)
- collected and observed animal
life from islands on the Beagle
Evolution and Islands
Darwin observed that neighboring islands in the Galapagos
had similar, but different, species of mockingbirds
Evolution and Islands
Darwin observed that neighboring islands in the Galapagos
had similar, but different, species of mockingbirds
Over time, descendents
became different species
on each island
Evolution and Islands
Darwin set off on Beagle with no clear notion
of evolutionary patterns
- returned with basic ideas of his theory and data to support it
However, he knew that proposing how species “came into
existence” would create controversy in Victorian England
- sat on his theory for 20 years, writing Origin of Species
Alfred Russell Wallace
Wallace’s family lost their fortune, so he worked, educated
himself, and took off to explore the world as a young man
While Darwin sat home writing his book, Wallace travelled the
world, financing his epeditions by collecting and selling
animals
Wallace’s collections gave him
insight into:
- variation within a species
(all individuals are not the same)
- geographical distribution of species
(not random)
Alfred Russell Wallace
His observations suggested that geographical boundaries
could explain species distributions
3rd species, as
predicted by
Wallace,
P. pithecia
Different
species
of spider
monkey
P. monachus
one species
of spider
monkey
P. irrorata
- rivers delineated boundaries between related species
Darwin & Wallace
Wallace had the theory to contest Special Creationism
(evolution), but not the mechanism (natural selection)
He and Darwin then began corresponding
In 1858, Darwin received a manuscript from Wallace
outlining the basics of evolution via natural selection
- his friends urged him to quickly write up a summary of
his 20 years’ worth of thoughts + data
- both papers were read aloud together + published
Darwin vs. Wallace
The following year (1859), Darwin published his book
On the Origin of Species by Means of Natural Selection,
or the Preservation of Favoured Races in the Struggle for Life
Major contribution: idea that new species arose by
descent with modification from ancestral species
History forgot about Wallace, but his independent derivation
of natural selection helped convince other scientists of this
revolutionary new theory
2. Evolution by Natural Selection
Darwin’s theory had 4 postulates, or underlying assumptions:
(1) Individuals within a species are variable
(2) Some of this variation is passed to offspring
(in other words, traits are heritable)
(3) In every generation, more offspring are produced than
can survive (due to limited resources)
(4) Survival and reproduction are not random:
- individuals with the most favorable variations survive,
or produce the most young
- this is the principle mechanism of evolution, called
natural selection
Evolution by Natural Selection
Darwin’s 1st postulate:
(1) Individuals within a species are variable
Evolution by Natural Selection
Darwin’s 2nd postulate:
Beak size of offspring (mm)
(2) Some of this variation is passed to offspring
(in other words, traits are heritable)
Beak size of parents (mm)
Evolution by Natural Selection
Darwin’s 3rd postulate:
(3) Every generation, more offspring are produced than
can survive
Organism
aphid
Reproductive potential
524 billion in one year
elephant
19 million in 750 years
a bacterium
cells cover earth 7 feet deep in 2 days
Starfish
1079 offspring in 16 years
Evolution by Natural Selection
Darwin’s 4th postulate:
natural selection
Observed in natural
populations all the time
Beak
depth
Evolution by Natural Selection
Darwin’s 4th postulate:
natural selection
- drought favored deeper
beaks that could crack
the tough seeds that
were available
A change in the genetic
makeup of a population
is…
Beak
depth
Evolution by Natural Selection
Any change in the genetic makeup of a population is termed
evolution
- this can happen by chance, by natural selection, by human
breeding programs, etc.
Natural selection causes evolution that results in adaptation,
producing organisms that are better suited to their particular
environment
 adaptation increases..
Problems with Darwin’s Explanation
There were two major problems in Darwin’s theory:
(1) Darwin didn’t have any knowledge of Mendel’s work on
genetics, so he didn’t get how heritability worked
(how were favorable traits passed from parents to offspring?)
(2) mutation had not been discovered, so Darwin couldn’t
explain where variation came from

3. The Modern Synthesis
This was taken care of between 1932-1953 in a series of
books that integrated genetics with the theory of evolution
by natural selection
- this is termed the Modern Synthesis
- explained things in terms of alleles, which are different
versions of a given gene
The Modern Synthesis
Modern synthesis restated Darwin’s 4 postulates:
(1) Individuals in a population are variable for traits, because
in every generation, mutation creates new alleles, and
sexual reproduction creates new allele combinations
(2) Individuals pass some of their alleles to their offspring
(3) More offspring are produced than can survive
(4) Individuals that survive, or reproduce the most, have allele
combinations that best adapt them to their environment
4. Sex and Selection
Nearly every organism has sex at some point in its life - why?
Sex is less efficient than asexual reproduction (cloning)
- sex requires a mate
- wastes half of resources on males, who don’t have babies
Principle advantage of sex: generates variation among
offspring,creating new allele combinations not present in either
parent
Because you don’t know what conditions will be like for the next
generation, you make a bunch of non-identical offspring & hope
at least a few have the right traits to survive
Sexual selection and female choice
male
Peacocks
Fiddler crabs
Males have traits important in mating displays (color, tail, claws)
whereas females lack such ornamentation
- male energy goes into looking sexy, attracting mates
- females invest energy in making large, healthy offspring
Sexual Selection
Darwin’s sequel, The Descent of Man and Selection in
Relation to Sex, was dedicated to this phenomenon
How could traits involved in male display or competition
persist in the face of natural selection?
- shouldn’t natural selection weed out brightly colored
individuals (by predation), or those wasting energy
on big tails?
Sexual Selection
Sexual selection can be:
(a) intersexual - males appeal to female preferences
Females may choose mates based on traits that indicate
his health, or superior genes..
... or possibly just based on arbitrary preferences of the
females of that particular species (what color their eyes
see most clearly, for instance)
Sexual Selection
Sexual selection can be:
(a) intersexual - males appeal to female preferences
(b) intrasexual - males compete with each other for access
to females
Whereas natural selection makes a population more adapted
to its environment, sexual selection does not-- it makes one
sex more successful in mating with the other
- increases fitness by increasing reproductive success
- does not increase adaptation or survival
Darwin’s 3 Forms of Selection
Over his career, Darwin wrote about 3 kinds of selection:
(1) Natural selection
- makes a species better adapted to its environment
(2) Sexual selection
- makes one sex more appealing to the other
- increases mating opportunities
(3) Artificial selection
- we choose desired traits and amplify them through
selective breeding of domestic organisms
In last 150 years, no one else has found another kind
Evidence for Evolution
Evidence for the theory of evolution is found in:
(1) homology among related organisms
(2) the fossil record (intermediate forms in a given lineage)
(3) vestigial structures
(4) molecular evidence
- relatedness of DNA sequences
- conserved action of genes
- conserved genetic code, transcription/translation
Homologous Structures
same set of bones
constructs hands
& forearms in all
mammals reveals
a shared ancestry
- modifications of this shared “scaffold” show how natural
selection altered the ancestral arrangement, increasing
adaptation for different mammal lineages
Evidence for Evolution: the Fossil Record
Evolutionary history of whales is
well recorded in fossils showing
progressive modification over
time, resulting in forms similar to
modern whales
Long-predicted fossils of ancestral
whales with legs were recently
discovered
Evolution is science - we make
testable predictions like, “one day
we will find a fossil whale with legs”
Evidence for Evolution
Evidence for the theory of evolution is found in:
(1) homology among related organisms
(2) the fossil record (intermediate forms in a given lineage)
(3) vestigial structures
- molar teeth in vampire bats
- tailbones in humans
- reduced hind legs in snakes
(4) molecular evidence