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Unit 5: Evolutionary Biology
Chapter 12: The Theory of Evolution
DARWIN IN HISTORICAL CONTEXT
Evolution
• One theory that is still debated today is if and
how organisms change over time
• Early in history people realized that organisms
display a huge variation in complexity
• People thought (and some still think) that
every organism was created for a purpose,
was permanent and unchanging
New information…
• New information was made available
suggesting that over time species change in
response to their environment
• Building on ideas and observations made by
himself and other people, Darwin proposed
his theory of evolution in 1859
Evolution
• Evolution: The changes that have
transformed life from single-celled organisms
of the past to complex organisms seen today
• Darwin proposed that populations of
organisms change over time in response to
environmental pressures
– These changes occur within a population due to
differences of reproductive success
– i.e. “Survival of the fittest”
Survival of the Fittest
• Organisms that most fit to survive in their
environment are more likely to have offspring
and pass on their genes
Why the controversy?
• Darwin’s Theory of Evolution is widely
accepted by the scientific community
• However, acceptance of his theory took a
while and many people still do not agree with
his ideas
• Darwinian evolution takes place over very long
periods of time, and is difficult to test in a lab
Why the controversy?
• The theory contradicts the religious beliefs of
many people
• Perhaps one of the most difficult ideas for
some people to accept is that humans
descended from ape-like ancestors
Pre-Darwin
• The conventional belief was that the Earth
was only a few thousand years old
• It was thought that each type of organism was
created for a specific purpose, was
unchanging and permanent
Aristotle
• In 380 BC, the Greek philosopher Aristotle
began classifying organisms by complexity
• Each type of organism was a rung on the
ladder of life called “The Scale of Nature”
– On this ladder of life there are no vacancies and
no mobility along the ladder
Species classification
• For over 200 years, scientists classified
organisms by arbitrary criteria
– For example, all domestic animals were grouped
together
• In 1735 the Swedish botanist Linnaeus
developed a new way to name organisms
based on their physical characteristics
Linnaeus’ classification
• Species: A specific kind of organism
• Each species was given a two-part name made
up of a generic name and a specific name
– Similar organisms could have the same generic
name, but each had a unique specific name
– Example: lions, tigers and panthers have the
names Panthera leo, Panthera tigris and Panthera
pardus, respectively.
Hutton and Lyell
• Geologists that agreed that the Earth is very
old and constantly changing
• In 1795 Hutton suggested that geological
changes occur over time due to mechanisms
such as volcanic eruptions and erosion
– Felt that the Earth was changing slowly, but
continually
• In 1830, Lyell agreed with Hutton
• Both believed that life stayed the same
Cuvier
• A big step in understanding evolution was the
discovery and study of fossils
• Fossil: An organism or trace of an organism
preserved in the Earth’s crust
• In 1804, the French biologist George Cuvier
proposed that the history of life was recorded in
the layers of the Earth’s sedimentary rock
– He noticed that each layer was characterized by
unique species
– Proposed that the boundary between two layers
indicated a catastrophic event that caused the
extinction of some of the species
Charles Darwin
• Born in England in 1809
• Attended Cambridge University (studying to
become a clergyman)
• Became a student of botanist Rev. John
Henslow
• Henslow suggested that Darwin go on the
World-wide science expedition aboard the
steamship the HMS Beagle
Darwin
• From 1831 to 1836, Darwin travelled on the
Beagle and studied the plants and animals
that he found
Darwin
• Darwin’s most important findings were made
along the coast of South America and during
his time on the Galapagos Islands
• The Galapagos Islands are located about 500
miles from the West coast of South America
• Darwin noticed that while many of the
organisms were unique to the islands, they
resembled the organisms of South America
• Many organisms were specific to a single
island, but were still similar to mainland
organisms
Darwin’s finches
• Darwin collected information about the 13
species of finches on the islands
• Found that a finches beak was adapted for
its food supply on its home island
– Some islands had large, hard-shelled seeds
that fell to the ground – finches had large
powerful beaks
– Other islands had small seeds that had to be
picked out of cacti – finches had small thin
beaks
– On other islands, finches ate insects or insect
larvae – beaks adapted to prey capture
Darwin’s finches
Evolution of Darwin’s ideas
• Darwin catalogued the different beak shapes
and how they related to the food sources
• While on the Beagle, Darwin read Lyell’s paper
with the idea that the world is very old and
constantly changing
• Darwin speculated that constant change,
driven by adaptation to different
environments, could cause be occurring in
organisms
Darwin’s ideas
• Darwin thought that the process of adaption
was related to the formation of new species
• When two populations of the same species
were isolated from one another, they would
adapt to their new environments and become
increasingly dissimilar
• Eventually, the populations would diverge into
different species
Darwin
• In 1844, Darwin wrote an essay on his theory
of evolution
• He called the theory “Descent with
modification”
• He believed that all organisms were
descended from a single unknown prototype
• Over time, organisms have acquired
modifications that make each species unique
Natural Selection
• Darwin proposed that evolution occurs
through a process called “natural selection”
– There is variation within a population
– Individuals with advantageous traits produce
more offspring
– The unequal ability of organisms to survive and
reproduce leads to gradual changes in a
population
Constraints
• There are constraints to the process of natural
selection
– Only works on variations present in a population
– Only affects traits that are passed on to offspring
– Causes changes in a population, not an individual
Evidence to support evolution
• Evolution leaves observable signs as clues to
the past
– Fossils support the theory of evolution
• Help to establish the order of when organisms
appeared, even if now extinct
• Scientists can see how similar organisms have changed
over time
Evidence to support evolution
• Anatomical similarities between species also
supports evolution
– Example: Humans, whales, bats and all other
mammals have similar forelimbs
– Structures are similar, even though they perform
very different functions
– Some organisms possess vestigial structures
• An ancestral structure that has lost its use
• For example, some snakes have the remnants of a
pelvis and legs, suggesting that they evolved from
lizards
Evidence to support evolution
• Molecular evidence supports evolution
– Today scientists can compare the DNA and protein
sequences of organisms
– Closely related organisms often have similar
amino acid sequences between certain proteins
– Certain fundamental processes, like cell division,
have been conserved throughout evolution from
yeast, to plants to mammals
Evidence to support evolution
• While evolution is usually difficult to observe,
there are some examples that are easy to see
– Example: English peppered moth population
before and after the Industrial Revolution
– These moths spend much of their time on Birch
tree bark (normally have light colored bark)
– Before the Industrial Revolution, 99% of the
moths were light colored and were difficult for
predators to see and catch
Evolution of the English Pepper Moth
• The Industrial Revolution introduced many sootproducing factories
• The soot coated the birch trees, making them black
• Light colored moths became easy to see
• After the Industrial Revolution, 99% of the moth
population was dark colored
Artificial Selection
• Artificial selection is used in the selective
breeding of domesticated plants and animals
• Examples:
– Farmers breed cows to increase milk production and
generate leaner beef
– Crops are selected for higher yields and for taste
– Flowering plants are selected for their large, showy
flowers
– Often, characteristics of organisms can be changed in
just a few generations
• This suggests that the same process could occur
via natural selection
Unit 5: Evolutionary Biology
Chapter 12: The Theory of Evolution
MECHANISMS OF EVOLUTION
Evolution
• When viewed macroscopically, organisms on
Earth are incredibly diverse
• Example: Compare bread mold and a spider
– Their differences are obvious
– However, they have similarities, too
– Both are eukaryotic organisms
– Their cells are composed of proteins, lipids,
carbohydrates and nucleic acids
– At this level, the spider and the mold are very
much alike!
Common origin
• The similarities between bread mold and
spiders reflects their common origin
• Both the mold and spider arose from a
common ancestor
• Their divergence reflects the divergent
evolutionary pressures that acted on their
ancestors
Definitions
• Macroevolution: Major change over very long
periods
• Microevolution: A change in the gene pool of
a population over a few generations.
– Smaller change
Population evolution
• Evolution: The change in the genetic makeup
of a population over time
• In order for a population to evolve:
– The population must have genetic variation
– Genetic variation: Genetic dissimilarity among
members of a population
– Mutations are a major source of genetic variation
– Also, sexual reproduction contributes to genetic
variation (independent assortment and crossing
over)
Population evolution
Sexual Reproduction
Independent assortment
Sexual Reproduction
Recombination
(aka crossing over)
Mutation
Genetic Diversity
Evolution
Individuals do not evolve
• Populations, not individuals, evolve
• Selection occurs at the level of the individual
• The reproductive success of individuals alters
the frequencies of certain traits within a
population, and the population evolves
Allele frequency
• How is the genetic makeup if a population
determined?
• Consider the allele frequency of a single gene
• Diploid organisms have two copies of each
gene (may or may not be identical)
• Allele: alternative form of the same gene
• In a population, the relative number of copies
of each allele may be different
• Allele frequency indicates the amount of
genetic diversity in a population
Eye color
• In humans is determined by 3 genes and
multiple alleles
• To simplify, let’s consider just 1 gene
– bey 2 gene
– B allele is dominant (brown eyes)
– b allele is recessive (blue eyes)
Allele frequency
BB
Bb
Bb
Bb
Bb
bb
• How many B alleles?
9
• How many b alleles?
11
• How many total?
20
bb
Bb
BB
bb
Allele frequency
•
•
•
•
9 B alleles
11 b alleles
20 total alleles (= B + b)
Let p = the frequency of the dominant allele
B
= 9/20
= 0.45
• Let q = the frequency of the recessive allele b
= 11/20
= 0.55
Allele frequency
•
•
•
•
The sum of allele frequencies always equals 1
p+q=1
0.45 + 0.55 = 1
This is possible because we know the
genotypes
• However, we don’t always know genotypes
• Can be difficult to distinguish between
homozygous dominant individuals and
heterozygous individuals
Hardy-Weinberg Equation
• Possible to estimate alleles frequencies using
the Hardy-Weinberg equation
• p2+2pq+q2 = 1.0
• This equation can only be used when there
are no selective pressures acting on a
population causing it to change
• i.e. this equation describes a population at
equilibrium
• Population that is not changing is at
equilibrium
Equilibrium
• Conditions for equilibrium (not changing):
– Population must be very large
– Mating must be random
– No mutations occurring nor evolutionary
pressures acting on the population
Back to eye color…
• p2 = Frequency of BB genotype (homozygous
individuals)
• q2 = Frequency of bb genotype (homozygous
individuals)
• 2pq = Frequency of Bb genotype
(heterozygous individuals)
Is this population at equilibrium?
•
•
•
•
•
BB
Bb
Bb
Bb
Bb
bb
bb
Bb
BB
bb
p2+2pq+q2 = 1.0 (at equilibrium)
p = 0.45
q = 0.55
(0.45)2 + 2(0.45)(.055) + (0.55)2 = ?
? = 1; therefore, the population is at equilibrium
Evolutionary change
• Populations are rarely at equilibrium
• Four pressures that can cause populations to
change
– Genetic drift
– Gene flow
– Non-random mating
– Natural Selection
Genetic drift
• Genetic drift: Changes in gene frequencies of
a small population due to chance
– In small populations, chance events can
permanently change the populations gene pool or
allele frequencies
Bottle Neck Effect
• The bottle neck effect can cause genetic drift
• Due to an event that leads to a significant
reduction in the population
• Only a few individuals survive to pass on their
genes – alters allele frequencies
Founder Effect
• The Founder Effect: Due to the migration of a
few individuals away from a population
• The new population is established in a new
location
• The allele frequency of the new population
may be very different from that of the old,
depending on the allele frequency of the
founding individuals
Gene Flow
• Gene flow: The change in a population’s allele
frequency resulting from migration
– Can be due to individuals entering or leaving a
population
– The frequencies of alleles change when individuals
enter or leave a population
Nonrandom mating
• Nonrandom mating: The
selection of mates on the
basis of a trait or group of
traits, not by chance
Nonrandom mating
• Assortative mating: Form of nonrandom
mating; partners resemble each other
– Although it does not change allele frequencies, it
does reduce the number of heterozygous
individuals
– Can lead to inbreeding: Mating between
genetically related individuals; increases the risk
that an individual will be born with a homozygous
damaging recessive trait
Nonrandom mating
• Sexual selection: another type of nonrandom
mating; selection based on the evolution of
traits among individuals of the same sex
– Results from either
• Selection by the opposite sex, or
• Competition among individuals of the same sex
Natural selection
• Natural selection
– Adapts organisms to their environments
– Facilitates evolution by increasing or decreasing
the odds that the organism successfully
reproduces
• Darwinian fitness: The contribution an
individual makes to the gene pool of the next
generation relative to others
– Difficult to measure
– Therefore, scientists look at “relative fitness”
Relative fitness
• Relative fitness: The contribution of a
genotype to the next generation relative to
other genotypes
• The reproductive success of an individual
ultimately depends on its genotype
• Differences in genotype arise from mutation
– Some mutations are harmful, some have no effect
and some are beneficial
– When a beneficial mutation occurs, the individual
has a reproductive advantage
Natural selection
• When an individual has a reproductive
advantage, the population is not at
equilibrium (Hardy-Weinberg equation does
not apply)
• The population evolves
• Favorable mutations accumulate and persist in
a population
Natural selection
• Natural selection changes allele frequency in
three ways
– Stabilizing selection: Minimizes extreme
phenotypes (example – average birth weights –
higher mortality rates in very small or very large
babies)
– Directional selection: Shifts phenotype to one
extreme (often caused by environmental shifts –
example – moth color)
– Diversifying selection: favors both extremes
(example – beak sizes of finches in different
habitats)
How is diversity maintained?
• What prevents natural selection from
eliminating unfavorable traits?
• Are several mechanisms at work:
– The diploid character of most eukaryotes hides
recessive alleles in heterozygous individuals
– Polymorphism: The co-existence of different
alleles in the same population
• Balanced polymorphism: The coexistence of different
alleles without any change in their frequency
Balanced polymorphism
• How does a balanced polymorphism occur?
• Heterozygote advantage: The reproductive
advantage of heterozygous individuals over
homozygous individuals
– Example: the allele for sickle cell disease is
maintained at relatively high levels in some
African countries
– Heterozygous individuals have the selective
advantage of surviving Malaria
Balanced polymorphism
• In plants, inbreeding often results in reduced
yields and increased sensitivity to disease
• Crossing of two inbred varieties produces
hybrid offspring that are more vigorous than
either parent
• Hybrid vigor: The superiority of a hybrid
offspring
– Unfavorable recessive alleles are “hidden”
– Promotes heterozygote advantage
Balanced Polymorphism
• The environment plays a role
• Different habitats
• Drive divergent evolution
• Act to preserve different phenotypes
Balanced polymorphism
• Frequency-dependent selection: A type of
selection in which the frequency of a certain
phenotype determines the reproductive success
of the organism
– Reproductive success declines when a phenotype
becomes too common
– Example: HIV evolves rapidly to avoid immune system
detection and destruction and drug intervention
– When a variant becomes too common it is more easily
recognized by the host’s immune system, or targeted
by drugs
Unit 5: Evolutionary Biology
Chapter 12: The Theory of Evolution
POPULATION GENETICS AND
EVOLUTION
Consider Blood Rh Factor
• When a person receives foreign blood, their
immune system may recognize the blood as
being foreign
• The blood may be recognized because of
proteins and sugars present on the outer
surface of the blood cells
• The molecules are classified into blood groups
• Example: ABO group or the Rh factor group
Rh factor
• The Rh blood group is a very complex blood
group polymorphism
• The molecules in this group are collectively
referred to as the Rh factor
• About 85% of the population posses the factor
and are Rh+
• About 15% lack these molecules and are Rh– Serious complications can arise when Rh- mothers
carry Rh+ babies as the mother’s immune system
targets the babies blood as being foreign
For our example…
• We will simplify the Rh factor down to just
two alleles
– One dominant (A) and one recessive (a)
– AA
Rh+
– Aa
– aa Rh-
Remember…
• If population size (N) = 100, the total number
of alleles = 200
• The number of A alleles in a population of 100
people = 120
• How many a alleles?
• 80
• Allele frequency = the number of allele copies
total alleles
Remember…
• The frequency of A = 120/200 = 0.60
• The frequency of a = 80/200 = 0.40
• In this example, we are given the allele
frequencies
• It can be difficult to determine allele
frequencies and genotype frequencies based
on phenotype alone
• We can use the Hardy Weinberg formula to
calculate genotype and allele frequencies
Hardy-Weinberg Conditions
• Random mating
• Large breeding population
• No differential migration (when individuals with
specific traits leave the population)
• No mutation of the alleles
• No natural selection
• When these conditions are met, populations do
not evolve and allele frequencies do not change
– The Hardy-Weinberg formula can be used to predict
frequencies of future generation
Unit 5: Evolutionary Biology
Chapter 13: The Origin of Species
SPECIATION
The Grand Canyon
• The sides of the grand
canyon are about 10
miles apart
• Each side has very
different plants and
animals
• Example: Abert Squirrels
on the south side and
Kaibob squirrels on the
north side
Squirrels
Abert Squirrel
Kaibob Squirrel
5 million years ago
• Before the Grand Canyon existed, the
common ancestor of these squirrels was
found on both sides of the Colorado River
• Some squirrels could get across the river
• The gene pool from the north side was mixing
with the gene pool from the south side
• Gene pool: All the genes present in a
population
• Gene flow: Transfer of alleles between two
populations
Barrier was formed…
• Over time, the river cut into the ground and
made it impossible for the squirrels to cross
over this new boundary
• Prevented mixing the gene pools
• As the canyon became wider, the conditions
on each side became different, and the
populations on each side adapted to the
changing environments
The species evolved…
• Species: A specific kind of organism
• Speciation: The origin of a new kind of
species through evolution
Geographic isolation
• In order for speciation to occur, gene flow
between two populations must be blocked
• Geographic isolation occurs when two
populations are physically separated
• Allopatric speciation: A type of speciation
caused by geographic isolation
• Types of geographic barriers can include:
– Formation of a mountain range
– Movement of a glacier
– Division of a large lake into smaller lakes
Reproductive isolation
• Reproductive isolation: genetic changes
cause two populations to become unable to
mate with each other, even though they are
not separated geographically
• Sympatric speciation: Speciation due to
reproductive isolation
• Blockage before fertilization (formation of a
zygote) (pre-zygotic barrier) or after
fertilization (post-zygotic barrier)
Pre-zygotic barrier
• Pre-zygotic barrier: A kind of reproductive
barrier that prevents organisms from mating
with each other and forming viable zygotes
– In order for reproduction to occur, individuals
must be
• In same location within a habitat
• Must mate at the same time of the day or season of the
year
• Must have same mating rituals
• Must have compatible anatomic parts
Post-zygotic barrier
• Post-zygotic barrier: A kind of reproductive
barrier that prevents a hybrid from developing
into a viable, fertile adult
• Hybrid: The product of breeding by organisms
of different species
– Hybrids tend to not completely develop
– Those that develop tend to not be healthy
– Hybrids tend to be not fertile
Speciation
• Allopatric and sympatric isolation stop gene
flow
• In the case of the Grand Canyon squirrels, two
populations were isolated from one another
• Their environments differed and each
population adapted to their environment
Speciation
• Allopatric speciation is most likely to occur when
a small population is separated from the parent
population
• Splinter population: A small group that is
isolated from the parent population
• Environment of the splinter population is often at
the extreme range of the parental population
• Genetic drift due to the Founder Effect will act on
the smaller population until the population size
increases
• The resulting population will have different allele
frequencies
Allopatric speciation
• Allopatric speciation is common on island
chains
• Adaptive radiation: The evolution of many
different species from a common ancestor
• Example: Darwin’s finches (13 different
species)
Sympatric speciation
• A new species can arise within a population,
even if there is no geographical isolation
• Sympatric speciation: due to reproductive
isolation
• Many plant species arose this way
• Polyploidy: An accident in cell division
resulting in an organism with more than two
sets of chromosomes
Polyploidy
• 2n = 14 -> 2n = 28 (polyploid)
• Polyploids cannot breed with individuals from
the parent population
• Wheat, oats, potatoes and tomatoes are all
polyploids!
Sympatric speciation in animals
• In animals, reproductive isolation usually does
not involve the formation of polyploids
• Reproductive isolation can occur in animals by
other means
• For example, a mutation in the ancestor
Drosophila heteroneura caused some males to
have a wider head, preferred by certain
females in mate selection
What criteria need to be met to be
considered a new species?
• Criteria considered differs, depending on the
organism being studied (and the scientist
making the call!):
– Ability to reproduce
– Anatomic differences
– Mate selection
– Physical behavioral components
What happens when geographic
barriers are removed?
• If the ranges of the Abert and Kaibab squirrels
overlapped, three possible outcomes could be
predicted
– The squirrels breed with one another and the
gene pools mix freely – speciation has not
occurred
Kaibab
Abert
Overlap of ranges…
• The second possibility is that the squirrels do
not breed with one another and speciation
has occurred (reproductive barrier may have
developed
Kaibab
Abert
Overlap of ranges…
• The third possibility is that the squirrels will
breed with one another, but gene flow will
only occur in the region where the
populations overlap
• Hybrid zone: A region where two related
populations come into contact with one
another after geographic isolation and
interbreed where their ranges overlap
Kaibab
Abert
Hybrid zone
• If species form a hybrid zone, gene flow will
only occur in the region where the two
populations overlap
• Hybrids make it difficult to define the term
“species”!
Rate of speciation
• Two theories
1. Speciation occurs gradually and the big
changes observed in a new species are the
result of many small changes over a long
period
2. Speciation occurs in rapid bursts followed by
long periods of little change (aka “Punctuated
equilibrium”)
Support for punctuated equilibrium
• In a small splinter population, genetic drift
and natural selection can cause significant
changes in a relatively short time (1000’s of
years)
• Only rarely is it possible to find gradual
transitions between fossil forms
– BUT, it is possible that the fossil record is
incomplete and does not record all information
(only skeletal…)