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A history of life and natural
selection
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Evidence for
Evolution
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Fossils
 A fossil is the remains or traces of an organism that died
long ago
 Many of the oldest fossils we find are of extinct species
 Most fossils are found in sedimentary rocks that settle at
the bottom of seas, lakes and marshes
 All the fossils together have created a geologic record of
Earth’s history
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Fossils
 Fossils show us different
organisms lived at different times
 For example, rock strata from
about 2-3 bya would show
fossils of only single celled
organism
 However, rock strata from 150
mya would show fossils of
dinosaurs, the first birds, and a
wide variety of plant life
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Fossils
 If evolution has occurred, we would see
different species throughout history
 Fossils show us that there has been!!
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Fossils
 Fossils also give us clues to
transitional species.
 Transitional species show
how organisms gradually
change over time
 For example, scientists
believe the whale ancestor’s
were once land dwelling
 We have found several
transitional fossils to
support this idea
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Anatomy
 Anatomy is the study of
the body structure
 If organisms have
evolved from common
ancestors, then they
would have similar
anatomical
features…RIGHT?!?!?!
 Well, THEY DO!
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Anatomy
 Some organisms have homologous structures,
which are anatomical structures that occur in
different species and that originated by heredity
from a common ancestor
 The function of that structure may differ in
related organisms
 Finding homologous structures in different
species indicates they have a common ancestor
 Ex: Limb bones in mammals
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Homologous Structures
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Anatomy
 Some organisms have analogous structures, which are
anatomical structures that have closely related functions,
but were not derived from the same ancestor
 Analogous structures evolve independently, but have the
same function
 EX: wings in bats, birds, and bugs
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Anatomy
 Many organisms display vestigial
structures, which are anatomical structures
that seem to have no function but resemble
structures used in ancestors
 Remember, that just because something
becomes useless, DOES NOT mean it goes
away…it must become detrimental to
survival
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Vestigial structures
Eyes on a mole
Hip bones on a whale or snake
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Embryology
 Embryology studies the development
of embryos
 The early stages of vertebrate
development are incredibly similar
 The explanation for this is that
vertebrates share a common ancestor
and inherited common stages of
development
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Fish, reptile, bird, and mammal
embryos all have a tail and gill slits.
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Biological Molecules
 Biologists now have the
technology to compare
DNA, RNA, proteins, and
other biological molecules in
different organisms
(molecular homology)
 Organisms that have the
least amount of differences
in these molecules are
closely related by a common
ancestor
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Biogeography
 Closely related species live in the same geographic
regions
 This results from having similar needs so that they need
similar habitats
 Furthermore, in isolated areas (islands), you will find
species that are unique in the world (endemic)
 Additionally, similar environments will give rise to
different species that have similar traits
 Example: Flying squirrel (mammal)of North America is
very similar to the Sugar glider (marsupial) of Australia
Meiosis KM
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Can we see evolution today?
 Organisms on Earth ARE currently evolving
 One familiar example is bacteria- we have
to keep coming up with different
antibiotics to fight the rapid evolution of
bacteria. They evolve to become resistant
to the antibiotics
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Can we see evolution today?
 It is very hard to see evolution in higher
organisms- as the process takes
hundreds-thousands of years
 But we CAN observe natural selection
in many species in a relatively short
period of time (such as with the
peppered moths)
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The goal of population genetics is to
understand the genetic composition of
a population and the forces that
determine and change that
composition.
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Variation in a Population
 Population genetics is the study of evolution
from a genetic point of view
 Evolution at the genetic level is microevolution
 Population genetics looks at the alleles
(variations) in a population and how they
change over time (evolve)
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Variation in a Population
 Population genetics also looks at populations as a
whole, because population is the smallest unit in
which evolution can occur
 A population is defined as a group of individuals
of the same species that routinely interbreed
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What can cause traits to
vary in a population?
 Mutations-which are random
changes in DNA sequences
 Recombination-genes are
reshuffled in meiosis-due to
independent assortment and
crossing over
 Random pairing of gametesorganisms produce many gametesany one can be involved in
fertilization
 The environment also influences
the outcome of many traits
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The Gene Pool
 The gene pool is
the total genetic
information
available in a
population
 So, ALL alleles for
EVERY gene in a
population
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Genetic Equilibrium
 Hardy-Weinberg Genetic Equilibrium is a principle that
states genotype frequencies tend to remain the same over
generations unless acted upon by an outside force
 Hardy-Weinberg equation(s):
 p2 +2pq + q2 = 1
 p+q=1
 p = dominant allele
 q = recessive allele
 p2 = homozygous dominant
 pq = heterozygous
 q2 = homozygous recessive
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Example
 A population of bunnies has the phenotype of 36%
white bunnies(the recessive gene) and the rest are
black. Based on this data, what are the frequency
of each genotype?
 Things to consider:



What are the possible genotypes?
Do you know any variables? 2pq, p2, or q2
How do you solve for p or q?
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Solution
 36 % of the bunnies are
ANSWER:
white
 Genotype = bb
 p2 = 0.16 or 16% (BB)
 Therefore, 36% of the
 2 pq = 0.48 or 48% (Bb)
q2






bunnies =
q2 = 0.36
q = 0.6
Since q = 0.6, then
p+q=1
1–q=p
p = 0.4 (40%)
 q2 = 0.36 or 36% (bb)
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Genetic Equilibrium
 Hardy-Weinberg Genetic Equilibrium
assumptions:
 No net mutations occur, so alleles remain the
same
 Individuals neither leave nor come into the
population (no gene flow)
 The population is large (ideally infinite)
 Natural Selection DOES NOT occur (so random
mating and no environmental pressures)
 Random mating (no sexual selection)
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Non-Equilibrium
 If the alleles and genes do not stay the same over
generations, we know populations are out of
equilibrium and evolution may occur
 Equilibrium is no change, evolution is change
 We can tell if frequencies stay the same by
calculating them using the Hardy-Weinberg
equation
 So, when genes and alleles change, natural
selection and evolution are occurring according
to Hardy-Weinberg
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Non-Equilibrium
 Many things can disrupt genetic equilibrium and in
fact populations RARELY stay at equilibrium for very
long
 What disrupts equilibrium and cause evolution?
 Mutations
 Gene flow
 Genetic drift
 Founder effect
 Nonrandom mating
 Natural Selection
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Mutations
 We have already discussed many avenues of
genetic mutation
 Mutation rates tend to be low in animals
and plants (about 1 mutation in every
100,000 genes per generation)
 In sexually reproducing organisms, sexual
recombination is a more important vector
for change
Meiosis KM
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Gene flow
 Immigration (individuals coming in) and
emigration (individuals leaving) naturally occurs
in many populations
 This causes gene flow- which is the movement
of genes from one population to another
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Genetic Drift
 In small populations,
chance events can
change allele
frequencies in a
population
 A change in allele
frequencies is called
Genetic Drift
 Frequency means
how often something
occurs
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Bottleneck effect
 A disaster in the environment can lead to a drastic
change in allele frequency
 Through random chance, certain alleles may be overrepresented in a population
 This gives us an avenue to alter the genotypic and
phenotypic expression of the population
 This bottleneck effect would change the new
population into something different from the original
population
Meiosis KM
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Genetic Drift Example
 In a population of 25 trees, where there are two alleles
for height- tall (T) and short (t) and the allele
frequency is 50:50
 A natural disaster- such as a fire, wipes out most if the
population
 Let’s say 2 trees survive, but they are both homozygous
tall (TT)- so now the allele frequency will be 100% for
the tall allele
 In a population of more (say 1,000), this is less likely to
happen- more trees and therefore more of the original
alleles would survive
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Genetic drift
 The
random
change of
allelic
frequency
in a
population
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Founder effect
 When a few individuals from a
population become isolated from the
source population, they may change to
fill new habitats (founder effect)
 Darwin observed this phenomena
when he observed the finches on the
Galapagos islands
Meiosis KM
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Nonrandom mating
 Many species do not mate randomly
 Some mate with individuals close to them
 Some mate with individuals that have similar
traits to them
 Both of these result in increasing certain allele
frequencies
 For example, very tall birds, may only mate with
other tall birds (not medium or small), this
would cause the tall allele to become more
prevalent
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Nonrandom mating
 Many species of birds, such as peacocks look for
specific characteristics when they mate, like
elaborate colors
 This is called sexual selection
 This leads to sexual dimorphism, a difference
between the physical
characteristics of males vs.
females
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Intersexual vs. Intrasexual
 Intrasexual selection:
 Intersexual selection:
 Occurs between the
 Occurs when one sex
same sex
 Direct competition
between individuals of
the same sex to
maintain the ability to
pass on their traits
 Example: Male lions
and control of the
pride
is able to select a mate
 Becomes a
competition to attract
a mate
 Examples: Birds –
showiness of plumage,
ability to build a nest,
birdsong
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Natural Selection
 Most significant factor in
evolution of populations
 Nature selects against
non-fit individuals
 Reduces harmful alleles
 Only acts on expressed
phenotypes
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Agents of evolutionary change
1. Mutation
2. Natural selection
3. Genetic drift
4. Gene flow
5. Nonrandom mating
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What is a species?
 The biological species concept says a species is a
population of organisms that can successfully
interbreed
 The morphological species concept says a species is
a population of organisms that have a similar
appearance
 Modern Species Concept- biologists use both of
these criteria to classify both living and extinct species
today
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Other definitions of species
 Paleontological species: focuses on the
morphological characteristics of organisms in the
fossil record.
 Ecological species: looks at defining species
bases on their role in an ecosystem (niche)
 Phylogenetic species: how organisms develop
from a common ancestor (we will be examining
phylogenetic trees later on)
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Forming new species
 In order for new species to form, you must have
genetic variation (remember from meiosis,
mutation, etc)
 Yet not all variation is a result of genetics
 There are also numerous environmental factors
that affect phenotype
 Lack of nutrition/too much nutrition
 Key thing to remember: environmental
changes to phenotype are NOT heritable
Meiosis KM
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How does a species form?
 The process of species formation is called speciation
 Speciation begins with isolation
 In order to form a new species, you must begin with
some sort of reproductive isolation
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Scale of speciation
 When you talk about changes in a single
gene pool, this is described as
microevolution
 Example: Peppered moths of England
 When you discuss changes over vast tracts of
time, this is referred to as macroevolution
 Example: Going from the age of reptiles
to the age of mammals
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Fitness
 In order to determine which species will be successful
(or what traits are passed on to offspring), we must
examine adaptive advantages
 Fitness: contribution of an individual to the gene
pool for the next generation
 Relative fitness: contribution of a particular
genotype for the next generation
 Therefore, for speciation to occur, the new traits must
have some sort of fitness “advantage” for them to be
passed to a new generation
Meiosis KM
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How does a species form?
 Allopatric speciation is a result of
geographic isolation (some physical
barrier that separates populations)
 Allopatric means “different homelands”
 Once one species is separated into two (or
more), gene flow between them stops
 As each experiences different
environmental pressures, genetic drift
occurs in different ways
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How does a species form?
 Sometimes groups of organisms
become reproductively isolated
 This may or may not be due to a
physical barrier
 Reproductive isolation is when two
individuals cannot successfully mate
(this means mate and produce
healthy FERTILE offspring)
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How does a species form?
 Types of reproductive isolation
 Prezygotic isolation- occurs before
fertilization
 Different species do not breed at the same
time
 Different species have different mating rituals
(such as a mating call or “dance”)
 Basically this type of isolation means the
different species WILL JUST NOT MATE in
nature
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Types of Pre-zygotic Isolation
1. Geographic isolation
2. Ecological isolation
4. Behavioral isolation
3. Temporal isolation
5. Mechanical isolation
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How does a species form?
 Postzygotic isolation- occurs after fertilization
 Gametes are not be compatible and do not
produce healthy offspring
 If offspring is healthy it may be infertile
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How does a species form?
 Sympatric
Speciation- occurs
when 2
subpopulation
become
reproductively
isolated, but have
no physical barriers
between them.
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Sympatric speciation
 Examples of sympatric speciation:
 Polyploidy is a mutation that often occurs in plants
(can change chromosome number – go from 2n to 4n,
autopolyploid)
 As a result the 4n plant can no longer breed with the 2n
plant . . . different chromosome number
 Animals taking advantage of different aspects of the
same resources (Darwin’s finches)
 An environment that repeatedly, and drastically, changes
 Lake or pond repeatedly drying out and refilling
Meiosis KM
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Rate of Speciation: Punctuated
Equilibrium vs. Gradualism
 Gradualism
 Evolution occurs
much more slowly
(gradually) and
consistently
 Speciation occurs
at intervals
further apart
 Punctuated
Equilibrium
 Evolution that
occurs at more
intervals and less
consistently
 Speciation occurs
more frequently
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Types of Evolution
 Convergent evolution-
the process by which
different species evolve
similar traits
 This often occurs due to
the different species
living in similar types of
environments
 Sugar Gliders and Flying
squirrels both adapted to
living in tall trees, but on
different continents
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Types of Evolution
 Divergent Evolution- a process in which
the descendents of a single ancestor
diversify into several different species that
fit a variety of habitats
 A great example is Darwin’s finches
 One species of finch came from South
America and evolved into 13 distinct
species-each of which has a different
habitat
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Types of Evolution
 Adaptive Radiation– when a new population in a
new environment undergoes divergent evolution
until it fills many parts of the environment
 The finches evolved in almost every part of the
Galapagos Islands
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Types of Evolution
 Coevolution– when two or more species
have evolved adaptations due to each other’s
influence
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Coevolution:
Pollinators help plants reproduce and
plants give food to pollinators
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Steps of Speciation in Darwin’s finches
1.
2.
3.
4.
5.
Founding Fathers & Mothers-finches made their way
from South America
Separation of Populations-finches crossed to
different islands
Changes in gene pool-Over time, populations
adapted to the needs of their environment
Reproductive isolation-birds prefer to mate with
birds that have same beak as they do-2 species have
evolved.
Share same island-co-existance, extinction, or
further evolution
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