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Charles Darwin (1809-1882)
Sailed around the world 1831-1836
How did tortoises and birds differ
among the islands of the Galapagos?
• Each island
had its own
type of
tortoises and
birds that
were clearly
different from
other islands
Evolution is when organisms change over time. So,
modern organisms descended from ancient ones or
descended from a common ancestor.
Evolution is a Theory – Just like
Gravity!
• Evolution is a well
supported explanation
of phenomena that
have occurred in the
natural world
• A theory in science is a
well tested hypothesis,
not just a guess
Artificial Selection
• nature provides variation, humans select
variations that are useful.
• Example - a farmer breeds only his best
livestock
Selective Breeding
Brahman cattle:
Good resistance
to heat, but poor
beef.
English shorthorn
cattle: Good beef
but poor heat
resistance.
Santa Gertrudis
cattle
(cross of 2 breeds)
RESULT = good beef
and resistant to heat!
hot weather cow + beefy cow = supercow
Natural
Selection
• The traits that help
an organism survive
in a particular
environment are
“selected” in natural
selection
• Natural selection
acts on phenotypes
NOT genotypes
11. Natural Selection and Species
Fitness
• Overtime, natural selection results in changes
in the inherited characteristics of a
population.
• These changes increase a species fitness
(survival rate)
Evidence of
Evolution
Fossil Record provides evidence
that living things have
evolved
Fossils show the history of life on
earth and how different
groups of organisms have
changed over time
Recent Fossil Finds
– Modern whales retain
reduced pelvic bones
and, in some cases,
upper and lower limb
bones. However, these
structures no longer
play a role in
locomotion.
Homologous Structures
– Homologous bones, as shown by colorcoding, support the differently-shaped front
limbs of modern vertebrates.
Analogous Structure
Founder effect & Bottleneck effect
Primate Bone structure
Evidence of Evolution
2. Geographic Distribution of
Living Species
Similar animals in
different locations were
the product of different
lines of descent
What is an adaptation?
• An adaptation is a change in an animal’s physical
structure or behavior that helps an animal to survive in
their habitat.
– Examples: The shape of a bird’s beak, number of fingers and
toes, or the color of an animal’s fur.
• Physical adaptations do not develop during one lifetime,
but over many generations.
Consider how the amount of genetic
divergence (change) forms a continuum:
Microevolution
small changes
Macroevolution
large changes
Microevolution = adaptation
Macroevolution = speciation
The Evolution of Species
• Significant changes in the gene pool
could lead to the evolution of a new
species over time.
• The evolution of new species, a process called
speciation. It is a process typically caused by the
genetic isolation from a main population resulting in
a new genetically distinct species.
Evolution – change over time. The question is… How
much time?
Gradualism- a proposed
explanation stating that new
species arise from the
result of slight modification
over many generation. This
evolutionary change is slow,
gradual, and continuous.
Punctuated Equilibrium – A
proposed explanation stating that
species generally stable over long
periods of time. Occasionally there
are rapid changes that affect some
species which can quickly result in a
new species.
Why do organisms become extinct?
II. Extinction – occurs when the environment changes
and the adaptive characteristics of a species are
insufficient to allow its survival.
If our environment is constantly changing, why
doesn’t extinction occur more often?
Isolating
Mechanisms
Describe the mechanisms which
isolate individuals of a population
and allow speciation to occur.
Prezygotic barriers:
• Obstacle to mating or to fertilization if mating
occurs
geographic isolation
behavioral isolation
ecological isolation
temporal isolation
mechanical isolation
gametic isolation
Prezygotic Isolating Mechanisms
Prezygotic barriers:
Ecological isolation
• Species occur in same region, but occupy different
habitats so rarely encounter each other
– reproductively isolated
2 species of garter snake, Thamnophis,
occur in same area, but one lives in water &
other is terrestrial
lions & tigers could
hybridize, but they
live in different
habitats:
 lions in grasslands
 tigers in rainforest
Prezygotic Isolating Mechanisms
Prezygotic Isolating Mechanisms
Postzygotic Isolating Mechanisms
• Examples:
– Hybridization between sheep and goats produces embryos that die in the
earliest embryonic stages
– Leopard frogs of the United States are a group of similar species, and NOT
a single species as was long assumed
Postmating Isolation
• Postzygotic Isolation
• These isolations prevent successful
reproduction after fertilisation.
– Hybrid Inviability – a zygote is formed but does
not develop properly.
– Hybrid sterility – a hybrid forms but it is sterile
Postmating Isolation
– Hybrid Breakdown – the hybrid offspring are
fertile but produce many infertile or non-viable
offspring.
Polyploidy
• This is the abrupt formation of a new species.
It is an important sympatric speciation as it
does not involve geographical isolation.
• In one generation the parent and the offspring
can belong to a different species.
• More common in plants.
Isolation by Time
• A species that disappeared one million years
ago obviously can not interbreed with a
species living today.
Three kinds of natural selection
Natural selection acts on variations
• Stabilizing selection is a natural selection that
favors average individuals in a population.
Selection for
average size
spiders
Normal
variation
Natural selection acts on variations
• Directional selection occurs when natural
selection favors one of the extreme
variations of a trait.
Normal
variation
Natural selection acts on variations
• In disruptive selection, individuals with either
extreme of a trait’s variation are selected for.
Selection for
light limpets
Normal variation
Selection for
dark limpets