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Darwin not first to suggest evolution
occurs.
Jean Baptiste Lamarck (1744-1829).
Published his theory of evolution in
1809.
Mechanism -- Inheritance of acquired
characteristics.
Inheritance of acquired characteristics:
Characteristics an organism acquires
during its lifetime are passed on to
its offspring.
Example: Giraffes stretch their necks to
reach leaves. As a result their young
born with longer necks.
Lamarck’s mechanism does not work
because DNA is not changed by an
organism’s behavior or experiences.
One-way flow of information from
DNA to body.
Charles Darwin (1809-1882).
Son of a wealthy doctor.
Studied medicine. Dropped out.
Then studied theology.
Keen naturalist and hunter.
Voyage of The Beagle (1831-1836)
Darwin companion for Captain Fitzroy.
(Darwin not the official naturalist.)
Beagle sailed around the World.
Mission to map coast of South America.
Sites visited included Galapagos Islands.
Volcanic islands (hence of recent origin)
off coast of Ecuador.
Unique animals on Galapagos include
giant tortoises, marine iguanas, and
Darwin’s finches.
On voyage Darwin read Lyell’s
Principles of Geology.
Book emphasized:
1. Gradualism: Geological features can
be explained by gradual action of
processes we see around us. E.g. wind,
waves, rivers.
2. Great age of the earth
What Darwin observed
1. South American fossils resembled
living animals.
e.g. Extinct glyptodont (2,000 kg)
resembles modern-day armadillo (2 kg).
What Darwin observed:
2. Parts of the world with similar
climates (e.g. Australia, South America)
populated by very different organisms.
What Darwin observed:
3. Plants and animals on each continent
are distinctive.
E.g. kangaroos in Australia, guinea pigs,
armadillos in South America.
What Darwin observed:
4. Many species on oceanic islands are
found only there (endemic).
Often groups of very similar species
found on these islands.
E.g. Galapagos finches and tortoises.
These are examples of adaptive radiation
Galapagos Giant Tortoise
Large
Cactus
Ground
Finch
Sharpbeaked
Ground
Finch
Warbler Finch
Galapagos Woodpecker Finch
What Darwin observed:
5. Endemic species on islands closely
resemble species on adjacent mainland.
These observations suggested to Darwin
that species are not unchanging.
Instead species change over time i.e.,
Evolution occurs.
But how does evolution take place?
Key breakthrough when Darwin read:
Malthus’s Essay on the Principle of
population.
Malthus: Populations increase
geometrically (multiplicatively), but
resources do not.
Populations will outgrow their food
supply.
Darwin concluded that among organisms
there is a struggle for existence. Only
the best adapted organisms survive to
breed.
Darwin called the mechanism that causes
evolution to occur: natural selection.
Darwin’s logic
5 observations and 3 inferences
(or conclusions).
Observation 1. If all organisms bred
successfully populations would increase
exponentially.
Observation 2. However, populations
generally stay stable over time.
Observation 3. Resources are limited.
Inference 1.
Because resources are limited, more
individuals are born than the
environment can support.
There is a struggle for existence (and
ultimately for reproduction.)
Only some individuals survive to breed.
Observation 4. Individuals differ in
their characteristics. There is variation.
Observation 5. Much of that variation
is heritable.
Inference 2. Survival and reproduction
are not random. The characteristics of
an organism, which it inherits, affect
its chances of survival and reproduction.
Organisms best adapted to the
environment on average leave the most
offspring.
Inference 3. Because variation is
heritable, differences between
individuals in their reproductive success
lead to changes in the characteristics of
the next generation. Evolution occurs.
Darwin developed his ideas in 1842, but
did not publish them immediately.
In 1858 Alfred Russel Wallace wrote
to Darwin. He had independently
developed the theory of evolution by
natural selection.
Joint presentation of their ideas in 1858.
Darwin published On the Origin of
Species in 1859.
Explained in detail his ideas about
evolution and natural selection.
Major difficulty for Darwin was that
he did not understand how inheritance
occurs.
Darwin did not express his ideas in terms
of genes. He had no idea what genes
were or what DNA was.
Basic genetics terminology
DNA is the genetic material. The
instructions for making and “operating”
an organism are written in DNA.
DNA is divided into sections called
genes.
Each gene codes for a protein.
Together the genes determine the
characteristics of an organism.
Alleles are different versions of a
gene.
If a single gene codes for flower
color, white and blue flowers would
be coded for by 2 different alleles.
You possess two copies of each gene in
your body*.
One copy is inherited from each parent.
For a given gene you may have two
different alleles or two copies of the same
allele.
(* excluding genes on sex chromosomes
in males).
A homozygous individual has two
copies of a particular allele. (AA)
A heterozygous individual has two
different alleles. (Aa)
Genotype and phenotype
An organism’s genes (its genotype) play a
large role in determining its physical
appearance (its phenotype).
But remember an organism’s phenotype is
also affected by the environment.
Today we express evolutionary ideas in
terms of genes because genes are the
only things passed from one generation
to the next.
In the process of natural selection, genes
that help organisms to survive
and reproduce become more common.
Genes that help less or are harmful
gradually are eliminated from the
population.
Only those individuals that are the
most successful, e.g. the fastest, the best
camouflaged, the best foragers, etc.
succeed in breeding.
The genes of other individuals are not
passed on to the next generation.
Natural selection is the term used to
describe the process by which the
best genes are chosen in each
generation.
Evolution occurs when gene frequencies
change from one generation to the next.
As a result of natural selection organisms
become increasingly well adapted to
their environments.
Adaptations are characteristics of
organisms that enable them to survive
in their environments.
Evolution is a population process. An
individual does not evolve.
Natural selection in action.
Peppered Moth and industrial
melanism.
Peppered moth occurs in two forms:
light, speckled form and dark (melanic)
form.
In early 1800’s dark form very rare.
Dark form caused by dominant mutation
that occurs spontaneously.
A mutation is a change in gene’s DNA .
Dominant allele is expressed even if only
one copy is present. (i.e., is expressed
even in heterozygotes).
Peppered moths rest on trees and depend
on camouflage for protection.
In unpolluted areas trees covered
in lichens. Light form of moth hard for
birds to see.
In mid 1800’s air pollution in British
cities covered trees in soot.
In cities dark form of moth
became common and light form rare.
In mid 1950’s pollution controls
reduced air pollution in Britain.
Frequency of dark form has
declined steadily since then.
Darwin’s Finches on Galapagos
Peter and Rosemary Grant of Princeton
have studied Medium Ground Finches
on Daphne Major for almost 30 years.
Banded and measured the island’s
entire population and followed it over
many years.
Rainfall on Galapagos Islands
is unpredictable.
In wet years lots of seeds produced.
In dry years few or no seeds produced.
Finches with larger bills can eat large
seeds and small seeds.
Finches with smaller bills cannot eat
large seeds, but are more efficient at
eating small seeds.
In droughts large-billed finches (and genes
for large bills) survive better than
small-billed finches (and genes for small
bills).
Mean beak depth of the finch population
fluctuates in synchrony with climate.
Sickle Cell Anemia
Sickle cell anemia is a condition common
in West Africans (and African Americans of
West African ancestry).
In sickle cell anemia red blood cells are
sickle shaped.
Usually fatal by about age 10.
About 1% of West Africans have sickle
cell anemia.
A single mutation causes a valine
amino acid to replace a glutamine
in an alpha chain of the hemoglobin
molecule.
Mutation causes molecules to stick
together.
Why isn’t mutant sickle cell gene
eliminated by natural selection?
Only individuals homozygous for
sickle cell gene get sickle cell anemia.
Individuals with one copy of sickle cell gene
(heterozygotes) get sickle cell trait
(mild form of disease).
Individuals with sickle cell trait don’t
get malaria.
Malaria
Malaria is one of the most important diseases in
the world.
About 500 million cases and an estimated 700,000
to 2.7 million deaths occur worldwide each year
(CDC).
Malaria was well known to the Ancient Greeks
and Romans. The Romans thought the disease
was caused by bad air (in Latin mal-aria) from
swamps, which they drained to prevent the
disease.
Malaria is caused by infection with a
single–celled sporozoan parasite called
Plasmodium.
It is transmitted by the bite of an Anopholes
mosquito
Plasmodium has two hosts: mosquitoes and
humans and a complex life cycle.
Sexual reproduction takes place in the mosquito
and the parasite is transmitted to humans when the
mosquito takes a blood meal.
In a human Plasmodium goes reproduces in liver
cells and then red blood cells before being picked
up by a mosquito to continue the cycle again.
The severity of a malaria infection may
range from asymptomatic (no apparent sign
of illness) to the classic symptoms of
malaria (fever, chills, sweating, headaches,
muscle pains), to severe complications
(cerebral malaria, anemia, kidney failure)
that can result in death.
Sickle cell allele protects against
malaria
People with the sickle cell allele are protected
against Plasmodium because their hemoglobin
under low oxygen conditions contracts into
needle-shaped clumps.
This contraction not only causes the sickling of the
cell, but harms the parasite. Parasites are impaled
on the clumps and the cell loses its ability to pump
potassium, which the parasite needs.
Heterozygotes (those with one copy of the sickle cell
Allele have higher survival than
either homozygote. Heterozygote advantage.
Sickle cell homozygotes die of sickle
cell anemia
“Normal” homozygotes more likely
to die of malaria.
Stabilizing selection for sickle cell allele.