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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 generally 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.
The relationship between genes and
evolution
We express evolutionary ideas in terms of
genes because genes are the only thing that
are passed from one generation to the next.
Process of Natural Selection
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.
Process of Natural Selection
Individuals that are the best adapted to their
environments (the best camouflaged, best at
finding food, etc.) will generally be more
successful at breeding than less well adapted
individuals.
As a result, their genes (which make them well
adapted) will be commoner in the next generation
than the gens of less well adapted individuals.
Natural Selection leads to Evolution
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.
Definitions of evolution
Microevolution: Change from generation to
generation in the genetic structure of a
population.
Macroevolution: Cumulative change in the
characteristics of organisms that occurs over the
course of successive generations and that can lead
to the origin of new species over time.
Natural Selection leads to adaptation
As a result of the ongoing process fo natural
selection organisms become increasingly
well adapted to their environments.
An adaptation is any characteristic of an
organism that enables it to survive and
reproduce in its environemnt.
Evolution is a population process
It’s important to remember that an
individual can be selected for or against, but
only populations evolve.
Evolution involves change in the
characteristics of a population over the
course of generations.