Download Chapter 15 Darwin`s Theory of Evolution

• All
living things share a
common ancestor.
• We can draw a Tree of Life to
show how every species is
related.
• Evolution is the process by
which one species gives rise to
another and the Tree of Life
grows
• The theory of Evolution
deals with how evolution
happens. Our
understanding of this
process is always changing.
• Evolution is also a fact as
there is a huge amount of
indisputable evidence for its
occurrence.
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1809 - 1882
Most influential
contributor to thoughts
about evolution
The Origin of Species
 1859
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Presented evidence for
changes in species through
Natural Selection
 Galapagos Islands
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Most scientists in Darwin’s day thought the
Earth was only a couple thousand years old
 They believe that Earth had not changed, nor had
any of its species
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Helped scientists recognize that Earth is
many millions of years old and the processes
that changed Earth in the past are the same
ones operating now
– hypothesis regarding geological
forces that have shaped Earth
– past events that affected the earth
are still happening
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One of the first scientists to
recognize that living things
have changed over time
Species were descended from
other species
Also realized that organisms
had adapted to their
environment
 By selective use or disuse of
organs, organisms acquired or
lost certain traits during their
lifetime
The idea that organism’s alter
their body by using it in new
ways
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Believe acquired traits could be inherited
If an organism’s body was altered, it would
pass that trait to its offspring
Example?
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5 year round-the-world voyage
 1831 - at age 22
H.M.S. Beagle
 Ship’s naturalist
 At beginning of trip - believed
species were immutable
Collected and examined the
species that inhabited the
regions the ship visited
 Fossils, coral ,plants, animals
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Volcanic islands - 3.5 mya
Isolated, west of Ecuador
All inhabitants are descended
from species that arrived on
islands from elsewhere
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Finches
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Tortoises
Iguanas

Blue-footed Booby
13 species of finches
Share many
morphological features
 Differ in several ways
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 Beak size
 Beak shape
 Food eaten
Evolved from a single
species
 He attempted to correlate
variations in their traits
with environmental
challenges
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WET ISLANDS
Short neck
Domed shell
Long neck
Flared shell
DRY ISLANDS
Marine Iguana
Algae eater
Land Iguana
Terrestrial vegetation
 The characteristics of many
animals and plants varied
noticeably among the
different islands of the
Galapagos
Darwin’s Evolutionary Ways of Thinking
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Darwin publishes Origin Of Species
Prompted after Wallace publishes essay
Words/phrases we are introduced to
 Artificial selection
 Struggle for existence
 Fitness
 Adaptation
 Survival of the fittest

Nature provides variation, humans select
variation that they found useful
 Livestock
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Members of the same species compete
 What are they competing for?
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What Darwin called the ability of an individual
to survive and reproduce
Fitness is a result of adaptation
Adaptation  any inherited characteristic
that increases an organism’s chance of
survival
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Individuals with
characteristics that are
better suited to their
environment –
adaptations that enable
their fitness – survive and
reproduce most
successfully
 Over time, natural selection
results in changes in the
inherited characteristics of a
population.
 These changes increase a
species’ fitness in its
environment
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Principle proposed by Darwin
Over long periods of time, natural selection
produces organisms that have different
structures, establish different niches, or
occupy different habitats
RESULT  species today look different
from their ancestors
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All species, living and extinct, are derived
from a common ancestor
A single “tree of life” links all living things!

Darwin argued living things have been
evolving on Earth for millions of years
 Evidence for this process can be found:
▪ Fossil record
▪ Geographical distribution of living species
▪ Homologous structures of living organisms
▪ Similarities in embryology
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Fossils  remains of ancient life
Darwin compared fossils from older rock
layers to those in younger rock layers to
document the change of life on Earth
© World Health Org.
en.wikipedia.org/wiki/Image:Eopraptor_sketch5.png
© NASA
origins
bacteria
complex cells
dinosaurs
humans
The fossil record shows a sequence from simple bacteria to more
complicated organisms through time and provides the most
compelling evidence for evolution.
Species living on different geographical areas had each
descended from different ancestors
 However, because of similar ecological conditions they
were exposed to similar pressures of natural selection 
similar features
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Structures that have
different mature
forms in different
organisms, but
develop from the
same embryonic
tissue.
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Organs of many species are so reduced in size
that they are just traces of homologous
organs in other species
Examples: miniature legs, tails
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Appendix
Wisdom Teeth
Tailbone
Particular Ear Muscles
VNO
Plica semilunaris
Embryology of many animals with backbones are
very similar
 WHAT DOES THIS MEAN?
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 Certain embryonic cells develop in the same order and in
similar patterns to produce the tissues and organs of all
vertebrates
At one point… you
looked like this.
Thought you should
know…
 Individuals organisms differ, and some of this
variation is heritable
 Organisms compete for resources
 Each organism has different
advantages/disadvantages  organisms best
suited survive and pass their traits to offspring
 Species today are descended with modification
from ancestral species; common ancestor, single
tree of life
Evolution of Populations
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When Darwin developed
this theory of evolution he
had no idea how heredity
worked
This left him unable to
explain:
 Source of variation
 How inheritable traits are
passed through generations
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Evolutionary biologists
connect Mendel and
Darwin’s work in the 1930s
Gene pool: consists of all
genes, including all the
different alleles, that are
present in a population
 Typically contain traits with
two or more alleles
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When studying evolution today, biologists
often focus on a particular
Evolution of populations is called
In genetic terms, evolution is any change in
the relative frequency of alleles in a
population

: # of times the allele occurs in
a gene pool
 Can occur because of:
▪ mistakes in replication
▪ environmental chemicals
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May or may not affect an
organism’s
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: recombination of genes that
occurs during production of gametes
 Causes most inheritable differences between relatives
 As a result, sexual reproduction is a major source of
variation in organisms
 Despite gene shuffling, the frequency of alleles does not
change in a population
Similar to a deck of cards – no matter
how many times you shuffle, same
cards (alleles) are always there.
controlled by single
gene with two alleles
 Ex.) widow’s peak, hitchhiker’s thumb,
tongue rolling
: controlled by 2 or more
genes, each with 2 or more alleles
 Examples: height, hair color, skin color, eye color
single gene
polygenic
Only two
phenotypes possible
Multiple (many)
phenotypes possible
tongue
roller or non-tongue
roller
height range
4feet to 9 feet all
Natural selection affects
polygenic traits through:
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 Single phenotype is favored,
causing the allele frequency to
continuously shift in one direction.
 Individuals with highest fitness:
those at one end of the curve
Key
Directional Selection
Food becomes scarce.
Low mortality,
high fitness
High mortality,
low fitness
 Individuals at the center of the curve have
highest fitness; evolution keeps center in the same
position but narrows the curve
Average phenotype
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: human birth weight
Stabilizing Selection
Key
Low mortality, high fitness
High mortality, low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Individuals at both ends of the curve
survive better than the middle of the
curve
 Natural selection favors extreme
phenotypes
 Individuals with highest fitness: both
ends of curve
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: birds where seeds are either
large or small
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Disruptive Selection
Largest and smallest seeds become more common.
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Low mortality,
high fitness
Number of Birds
in Population
Key
Beak Size
Evolution of Populations
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Natural selection is not the only source of
evolutionary change. In small populations, an
allele can become more or less common by
chance
Genetic drift: random change in allele
frequency that occurs in small populations
So, genetic drift and natural selection both involve
changes in allele frequencies
1. Founder Effect
2. Bottleneck Effect
 Allele frequencies change due
to migration of a small
subgroup of a population
Founder Effect: Beetles
on Hawaiian islands
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Major change in allele
frequencies when population
decreases dramatically due
to catastrophe
Hunted to near
extinction
♦ Population decreased to
20 individuals in 1800’s
♦ Those 20 repopulated so
today’s population is
roughly 30,000
♦ No genetic variation in
24 genes
♦
Catastrophe
Original
population
Surviving
population
The formation of new biological
species, usually by the division of a
single species into two or more
genetically distinct one.
1.
2.
3.
 Kaibab Squirrel
Abert Squirrel
 Kaibab Squirrel
Abert Squirrel
Two populations are capable of
interbreeding but do not
interbreed because they have
different ‘courtship rituals’ or
other lifestyle habits that differ
 Eastern and Western Meadowlark
populations overlap in the middle of
the US
Male birds sing a mating song that females
like
 East and West have different songs
 Females only respond to their subspecies
song.
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Populations reproduce at
different times
Mates in:
Mates in:
April
July
Geographic, Behavioral, and
Temporal Isolation are all
believed to lead to speciation.
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: large-scale evolutionary
patterns and processes that occur over long
periods of time
Six important topics
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Extinction
Adaptive radiation
Convergent evolution
Coevolution
Punctuated equilibrium
Changes in developmental genes
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More than
of all species that have ever lived
are now extinct
Several times in Earth’s history –
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Examples of mass extinctions and their causes?
What effects have extinctions had? Bursts of
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evolution!
Convergent Evolution
Process by which unrelated organisms
come to resemble one another
Pattern of long, stable periods interrupted by
brief periods of rapid change
 Why would rapid change occur?
 Isolation
 Migration
 Extinction
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Changes in genes for growth and
differentiation during embryological
development could produce transformations
in body shape and size
: insects
Anna Marie, Brandon - Baltra (South Seymour) Island
 Hunter, Shalom - Española (Hood) Island
 Joshua, Lindsey - Fernandina (Narborough) Island
 Gabe, Matt - Floreana (Charles or Santa María) Island
 Kerrington, Tiana - Genovesa (Tower) Island
 Aurora, Shannon - Pinzón (Duncan) Island
 Kyle, Taylor - San Cristóbal (Chatham) Island
 Alastair, Tenko - Santa Cruz (Indefatigable) Island
 Erin, Abby - Santiago (San Salvador, James) Island
 Diego, Curtis - Pinta (Abingdon) Island
 Susan, Ethan - Santa Fé (Barrington) Island
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Justin, John, Robert - Isabela (Albemarle) Island
Jordan, Colton, Lauren - Santa Cruz (Indefatigable) Island
Avelyn, Daniel, Freeman - Española (Hood) Island
Clarissa, Christina, Blake - Genovesa (Tower) Island
Stella, Grant, Ben Hagan - Santiago (San Salvador,
James) Island
Bruce, Ryan, Griggs - Fernandina (Narborough) Island
Jokima, Madison, Penny - San Cristóbal (Chatham) Island
Fahim, Ronnie, Hanna - Pinta (Abingdon) Island
Edgar, Olivia, Julien - Marchena (Bindloe) Island
Drew, Shayna, Dove - Pinzón (Duncan) Island