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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

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Darwin not first to suggest evolution


Jean Baptiste Lamarck (1744-1829).

Published his theory of evolution in


Mechanism -- Inheritance of acquired



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.












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


Malthus: Populations increase

geometrically (multiplicatively), but

resources do not.

Populations will outgrow their food



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



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


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



Each gene codes for a protein.

Together the genes determine the

characteristics of an organism.


Alleles are different versions of a


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


(* 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
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



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


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.

peppered moth and industrial melanism
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
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


Mean beak depth of the finch population

fluctuates in synchrony with climate.

sickle cell anemia
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


Mutation causes molecules to stick



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 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
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.