The Theory of Evolution
The theory of evolution

Is evolution JUST a theory?

What is a theory?

A scientific theory is defined as “a scientifically
accepted general principle supported by a
substantial body of evidence offered to provide
an explanation of observed facts and as a
basis for future discussion or investigation.”
The theory of evolution

Example – the theory of
heredity

Mendel and his peas –
Laws of particulate
inheritance, dominance,
and recessiveness
The theory of evolution

Other scientific
“theories”
• The Heliocentric
•
•
•
hypothesis – Copernicus
Atomic theory
Quantum theory
Theory of plate tectonics
The theory of evolution


Thus evolution can be considered a fact that
is explained by the theory of evolution.
Two major hypotheses with respect to the
theory of evolution.
1.
Descent, with modification, from common ancestor
2.
The cause of descent with modification is that
natural selection sorts among variations.
The evolution of evolutionary thought

Ancient Greeks – various
ideas about the origins and
diversity of life including
•
Empedocles of Acragas (in
Sicily), proposed that the
universe was composed of four
basic elements -- earth, air, fire,
and water.
Jean-Baptiste Lamarck (1744-1829)
•In 1809 Lamarck published
the Philosophie Zoologique,
where he proposed his
hypothesis for evolutionary
change.
God
Angels
Kings/Queens
Archbishops
Dukes/Duchesses
Bishops
Marquises/Marchionesses
Earls/Countesses
Viscounts/Viscountesses
Barons/Baronesses
Abbots/Deacons
Knights/Local Officials
Ladies-in-Waiting
Priests/Monks
Squires
Pages
Messengers
Merchants/Shopkeepers
Tradesmen
Yeomen Farmers
Soldiers/Town Watch
Household Servants
Tennant Farmers
Shephards/Herders
Beggars
Actors
Thieves/Pirates
Gypsies
Animals
Birds
Worms
Plants
Rocks
Lamarck

Lamarck believed that
every species had
originated individually
by spontaneous
generation at the
bottom of the chain.

Because species are
continuously arising,
the patterns we see
now are the result of
variation in the age of
species.
Lamarck

Lamarck recognized that not all species fit into
the chain of being, and he believed that
adaptation caused some species to deviate
from the chain.

Species in different environments had different
needs, and those organs and appendages that
were used more often attracted “nervous fluid”
which enlarged them.
Lamarckism

These changes acquired over a lifetime are
inherited

Laid foundation for Lamarck’s
Theory of Inheritance of Acquired
Characteristics
Key Features

Desire to change

Use/Disuse

Passing on of Acquired Characteristics

•
Ex. Long neck to giraffe offspring
Lamarck was on the right track, but had
evolution happening much too fast / within short
time frames
Artificial Selection

Humans select traits of organisms
• breeds and characteristics of dogs
Artificial Selection

Agriculture
• corn kernel characteristics
Thomas Malthus (1766-1834)

An Essay on the Principle of Population
(1798/1803)
• Population growth is exponential
• Food growth is limited/linear
• Human demand for food will
outstrip supply.
Population growth
Malthus
• 1798: 950 million
• Double every 25 years
• 2000: 242 billion (actual: 6.1 billion)
Food production
Charles Darwin

Charles Darwin was born at
Shrewsbury. His father was a doctor
and his mother was the daughter of
Josiah Wedgwood.

Darwin first studied medicine at
Edinburgh. It soon became clear to
the family, and particularly to Charles,
that he was not cut out for a medical
career; he was transferred to
Cambridge) there to train for the
ministry.

A Cambridge, befriended a biology
professor (John Stevens Henslow) and
his interest in zoology and geography
grew.
Charles Darwin

In 1831, Charles
accepted an invitation
to join an expedition
on the Beagle.
Charles Darwin
Charles’ ideas of
descent with
modification began to
when he noticed that
the finches from the
Galapagos were so
different from island to
island.

Darwin’s Finches
Beaks of Darwin’s Finches

Darwin collected a closely related group
of 14 finch species in the Galapagos
Islands.
• All were similar except for beak
characteristics.
• Beak structures suggested to Darwin that they
were shaped by evolution.
• Based on the nature of available food supply
Charles Darwin

He was additionally
inspired by artificial
selection and
breeding programs
in domestic animals.

In the fall of 1838,
formulated the
Theory of Natural
Selection
Charles Darwin


published his book The
Origin of Species,.
The book had two major
themes
1.All living species have
descended from a
common ancestor
2.Theory of natural
selection
Darwin’s Evolution



Common Descent –
radically different from
Lamarck.
Darwin was the first to
propose that species had
diverged from common
ancestors.
All of life could be viewed
as a family tree.
Darwin’s Evolution

Gradualness or Gradualism –
Differences between organisms are the result of a large
number of small “steps” or intermediate forms.

Puncuated Equlibria
- changes occur by large leaps without intermediates.
Somewhat controversial.
Natural Selection

Darwin claimed that the ENVIRONMENT
selected organisms with the best
adaptive traits to survive and reproduce

Traits were then passed on. Traits with
no advantage died with owners and
“died out”
Darwin’s Evolution


Natural selection –
original hypothesis,
independently determined
by Alfred Wallace.
Process by which the
proportions of variant
individuals within a
species change over
evolutionary time
(generations).
Beaks of Darwin’s Finches

Darwin collected a closely related group
of 14 finch species in the Galapagos
Islands.
• All were similar except for beak
characteristics.
• Beak correspondence suggested to Darwin that
they were shaped by evolution.
• nature of available food supply
Darwin’s Finches
Peppered Moths and
Industrialized Melanism

Until the mid-nineteenth century,
peppered moths, Biston betularia, had
predominately light-colored wings.
• then, dark individuals became predominant
• Industrial smog helped turn lichens on tree
trunks dark.
• Contrasting colors between trunk color and
moth color led to different hunting
(selection) by birds.
Peppered Moths and Industrial
Melanism
Peppered Moths and Industrial
Melanism

The second half of the twentieth century
saw widespread implementation of
pollution controls, thus trends reversed
and light colored moths again
dominated.
• pollution created generally darker environment
• Clean up created a generally lighter environment
Selection Against Melanism
Sources of New Species
Step 1 – Founding Fathers/Mothers

Organisms move into an area

These critters represent the founders
Sources of New Species
2. Physical Barriers to Mating (Gene
Flow)

Physical barrier
prevents gene flow
between
populations of a
species
•
Archipelago hotbed
of speciation
Figure 24.7 Allopatric speciation of squirrels in the Grand Canyon
3. Evolutionary Changes in the
Gene Pool

Adaptation to the local environment
exerts pressure on “traits”

Most adapted organisms are
SELECTED and so are their genes
4. Reproductive Isolation

Overtime, organisms develop new traits,
behaviors, mating preferences

Maybe that big-billed finch isn’t so attractive to
the slim-billed finch.

This is reproductive isolation
Consider the Chihuahua and the St. Bernard
as well

Reproductive Isolation
Mating really isn’t Possible
Sometimes …
Getting the idea here? … 
Adaptive Radiation

The idea that one
species gives rise to
MANY SPECIES
Divergent Evolution = Adaptive Radiation
Convergent Evolution

When “Mother
Nature” causes two
TOTALLY unrelated
critters to evolve in
similar ways,
showing similar
adaptations
Directional Selection
Directional Selection

Directional selection favors those individuals who have extreme
variations in traits within a population.

Ex breeding of the greyhound dog.

Early breeders were interested in dog with the greatest speed.
They carefully selected from a group of hounds those who ran
the fastest. From their offspring, the greyhound breeders again
selected those dogs who ran the fastest. By continuing this
selection for those dogs who ran faster than most of the hound
dog population, they gradually produced a dog who could run
up to 64km/h (40mph).

The greyhound was originally used to hunt the fastest of game,
fox and deer. Their bred dates to Egypt in 3BC.
Disruptive Selection
Disruptive Selection

Disruptive selection, like directional selection, favors the
extremes traits in a population.

Disruptive selection differs in that sudden changes in the
environment creates a sudden forces favoring that extreme.
Example – a meteor hits the earth and critters with traits to
survive are the only ones that do.

A population may split into two distinct species when extreme
forms have an advantages.

This happens with certain forms of fish
Surface dwellers (limnetic) – eat / live / reproduce at surface
Bottom (benthic) dwellers –eat /live/reproduce on bottom
Breeding doesn’t occur with sufficient frequency and the two
forms evolve into separate species.



Three-Spine Stickleback Fish
benthic (top) and limnetic (bottom) females
Stabilizing Selection

favors the norm, common, average traits in a population

Look at the Siberian Husky, a dog bred for working in the snow.
The Siberian Husky is a medium dog, males weighing 16-27kg
(35-60lbs). These dogs have strong pectoral and leg muscles,
allowing it to move through dense snow. The Siberian Husky is
well designed for working in the snow. If the Siberian Husky
had heavier muscles, it would sink deeper into the snow, so
they would move slower or would sink and get stuck in the
snow.
Stabilizing Selection – reduces extremes
The Case for Sickle Cell Anemia
Sickle cell trait helps carrier survive malarial infection –
an evolutionary advantage
Speciation

A species is a group of organisms that are
able to interbreed successfully, producing
offspring who themselves can reproduce.
When one group of organisms can no longer
breed successfully with another group, then
those two groups are considered to be
different species.
Speciation of Hawaiian Honeycreepers

New species
•
•
Poor habitats on an
isolated archipelago
Start of geographic
separation and, over
time, speciation