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EVOLUTION
“Nothing in Biology Makes Sense
Except in the Light of Evolution.”
Theodosius Dobzhansky (Geneticist, 1900-1975)
“Lamarckism”
• Before Darwin published his theory of
evolution, Jean-Baptiste Lamarck’s idea
of evolution was widely accepted.
• Lamarck proposed that acquired traits
could be passed on to offspring. Ex.:
“Giraffes got their long necks by
striving to reach the leaves on the
tops of trees, so their babies were
born with long necks.” (NOT!)
• After Darwin published his theory, and
after inheritance was explained through
genetics, Lamarckism was abandoned.
If you grow big muscles by lifting weights,
will you pass this acquired trait on to
your offspring?
Charles Darwin
• Charles Darwin was an
English naturalist (1800’s).
• During the voyage of the
Beagle, Darwin observed
many species in their natural
environments, especially in
South America / the
Galapagos Islands.
• While studying the Galapagos
finches, he developed the
theory of natural selection.
• Darwin published “The Origin of Species” in 1859,
containing his theory of evolution by natural selection to
explain how the variety of living things existing today
developed through time.
Darwin was not the only one!
• Alfred Russel Wallace also
inferred evolution by natural
selection.
• Darwin had the idea first, but
only published his book once he
knew Wallace had also arrived at
the same conclusion – Darwin
had been working on it for over
20 years!! 
• Darwin explained his theory
more completely and with more
supporting evidence, and
established his priority.
Evolution of Species
• Evolution = change or
development through time
• Species = interbreeding
population of organisms that
can produce healthy, fertile
offspring
• Adaptation = inherited trait
that allows organisms to
survive in a particular
environment
• Species are adapted to their
niches.
• Speciation = evolution of a
NOT evolved from each other:
new species
share a common ancestor 5-6
m.y.a.
Adaptations are… Shape, size, color, hormones,
ability to make eggs, and other genetic traits.
Leaves that capture
sunlight, flowers
containing gametes for
reproduction
Poison to deter predators, enzymes
that digest organic matter, coloration
Small size, wings, feathers, beak size
Behavioral traits, including mating
behavior that optimizes reproduction
and parental care
Artificial Selection / Selective
Breeding of Cattle
(Videos at pbs.org – 5-6 min.)
Artificial Selection of Crop Plants
Artificial Selection /
Selective Breeding
Domestication
and selective
breeding by
people have
generated many
breeds/varieties
of animals and
plants.
Why and How do
Species Evolve?
Current biodiversity ~ 5-30 million species
99% species that ever lived on the earth
are now extinct!
Environments change  advantage for
some, disadvantage for others
Natural Selection
• Mechanism of evolution suggested by evidence
• Four factors:
–
–
–
–
Variation within populations  some favorable
Struggle for survival  limited resources, competition
Overproduction of offspring  not all young survive
Differential survival & reproduction  individuals w/
favorable variations survive & reproduce more, traits
become more prevalent in each generation

Cummulative effect over time results in changes in a
species (evolution)
Variation
• Genetic Variation = differences between
individuals in a population  fuel for evolution
• Environmental changes  variation increases the
chances of survival for a species
Ex.: new disease  some individuals may survive
• Sources of genetic variation:
– Recombination of existing alleles (forms of a gene) by
sexual reproduction;
– Mutation – change in the sequence of a gene; most are
neutral or harmful.
Selective Pressure
• Selective pressure = environmental factor
that creates struggle to survive/compete
• Individuals have to compete with other
individuals of the same species and other
species for limited resources:
– Space/territory
– Food, water, sunlight, soil nutrients
– Mates
Sexual selection – selection by member of the
opposite sex (mate)
Evolution is a Compromise Between
Different Selective Pressures
Illustration:
Guppy studies by John Endler at pbs.org
Peppered Moths & The Industrial
Revolution – A Natural Selection Tale
Before factories…
After factories…
“industrial melanism”
Tell me the story using the words
variation, reproduction, genes, and natural selection.
After environmental standards
improved / pollution control…
Species Evolve in Interaction with
Other Species
• Coevolution = two species evolving
interdependently
Ex.: predator and prey
• Each species is under selective pressure
(environmental limitations) for adaptations that
are favored in its habitat
• A species niche includes interactions with other
species
• When two species evolve in symbiosis, they
become adapted to each other
Coevolution
Examples
Symbiosis:
Parasitism
Symbiosis:
Mutualism
Predator & Prey
Reproductive Isolation Results in
Speciation
• Two populations of the same species may become
reproductively isolated  no more gene
exchange, evolve separately
• Geographic isolation causes reproductive
isolation: living in separate places  breeding in
separate groups
Ex.: islands colonized by mainland species
• Different environments next to each other may
favor different variations of the same trait
ex: forest hummingbirds, grassland hummingbirds
Reproductive Isolation
• Pre-Zygotic
before fertilization/mating;
no zygote/embryo forms
• Post-Zygotic
after fertilization/mating:
zygote/embryo not viable or not fertile
Ex.: Drosophilla melanogaster x D. pseudoobscura 
hybrids have atrophied testes (infertile)
– Hybrid is sterile, inviable, or has low fitness
Ex.: horse x donkey  mule (infertile)
Allopatric vs. Sympatric Speciation
• Allopatric speciation – populations diverge due
to geographic isolation (islands, areas
separated by a mountain, river, etc.)
• Sympatric speciation - populations diverge
without geographic isolation (usually due to
reproductive isolation in the same environment)
Allopatric
or
Sympatric
Speciation?
Galapagos Finches
Allopatric or Sympatric Speciation?
Allopatric or Sympatric Speciation?
Allopatric or Sympatric Speciation?
The European mosquito Anopheles consists
of six morphologically indistinguishable
species. They are isolated reproductively
as they breed in different areas of the
same habitat. Some breed in brackish
water, some in running fresh water and
some in stagnant fresh water. Therefore,
they never meet to breed. If this happens
for subpopulations of a species,
speciation may follow.
How Fast Does
Evolution Happen?
• Darwin originally proposed evolution happens at a
slow, gradual rate  Gradualism
• Gould & Eldredge (1972) proposed that
populations remain stable through long periods of
time, interrupted by rapid genetic changes 
Punctuated Equilibrium
 Fossil record lacks many intermediate forms.
Common
ancestor
Relatedness Between Species
• Depends on how long ago species shared a
common ancestor (diverged from ea. other)
• Scientists compare fossils, structures &
DNA sequences to establish relatedness
• A phylogenetic tree is a diagram showing
relationships between different species
Phylogenetic Tree of Vertebrate Animals
Analogous or homologous
structures?
Divergent
Evolution
Species evolve from
same ancestor by
differentiating from
each other.
Common
ancestor of
crocs & birds
fishapod-like
ancestor of land
vertebrates
Common ancestor of
all vertebrates (first
fish with a backbone)
Cladograms
are a type of
Phylogenetic
Tree
A clade is
a group
that
shares a
common
ancestor
 shared
characters
Shared characters:
quadruped body plan,
amniotic eggs, skin that
preserves moisture, etc.
Convergent Evolution
Unrelated or distantly
related species evolve
similar adaptations
due to similar
environment
Selective pressure for sustained rapid
swimming in a marine habitat
Analogous or homologous
structures?
Divergent
Evolution
CONVERGENT
EVOLUTION
CONVERGENT
EVOLUTION
Analogous or
homologous
structures?
No blind spot –
nerves behind
retina
Blind spot –
nerves & cell
layers in front
of retina
Convergent Evolution
East African cichlid fishes
Lake
Tanganyika
cichlids
Lake
Malawi
cichlids
Similar diets in
different lakes have
favored similar
jaw/mouth shapes.
Can Convergent Evolution happen in homologous structures?
Bones are homologous –
inherited from a
common quadruped
ancestor
Evidence suggests that wings evolved
separately in mammals and dinosaurs
 convergent evolution of flying
structures
Convergent Evolution:
Distantly related species,
similar environments
Adaptive
Radiation
Divergent evolution
in which several or
many species evolve
from one ancestor,
filling available
niches.
Ex.: Marsupials in
Australia
Adaptive
Radiation
Ex.:
Archaeoptherix, a
transitional form
between dinosaur
and bird, thought
to be the ancestor
of modern birds
Directional
Selection
Trait value
Trait value
Stabilizing
Selection
Trait value
Trait value
Directional vs. Stabilizing Selection
Disruptive Selection
What type of selection has happened?
Disruptive
Selection
Divergent
Evolution
Common
ancestor
Evolution Reminder: Keep your sense of humor… 