Download What is Evolution?

What is Evolution?
A change in the genetic make-up
of a population over time.
How Does Evolution Occur?
There were two hypotheses
Evolution by acquired traits
by Jean Baptiste de Lamarck
Do we acquire traits?
Climbing
Eating
Show
affection
Act nutty
Defend yourself
These traits are not genetic so
evolution does not occur
How Does Evolution Occur?
• NATURAL SELECTION !!! (major mechanism of
evolution)
– Darwin’s hypothesis (now a theory):
• Survival of the fittest…..how is fitness measured?
– By reproductive success
• Inheritable variations occur in individuals in a population. Due
to competition for limited resources, individuals with more
favorable variations or phenotypes are more likely to survive
& produce more offspring, thus passing traits to future
generations.
Darwin’s View of History
• Like a Tree with
multiple branches.
– Common trunk
– Tips of twigs =
diversity of organisms
living in the present.
– Forks = most recent
common ancestor
Natural Selection Summary
• A process in which individuals that have certain
heritable traits survive & reproduce at a higher rate
than others because of those traits.
• Over time, natural selection can increase the match
between organisms & their environment.
• If an environment changes, or if individuals move to
a new environment, natural selection may result in
adaptation to these new conditions, sometimes
giving rise to new species.
Natural Selection acts on phenotypic
variations in populations
• Environments
change and act as
selective
mechanism on
populations.
• Phenotypic variations are not directed
by the environment but occur through
random changes in the DNA and
through new gene combinations.
Some phenotypic variations significantly
increase or decrease fitness of the
organism and the population.
Examples of Natural Selection
• Antibiotic resistance in bacteria
– Develops in several steps:
•
•
•
•
•
•
•
•
Person is sick from a bacterial infection
Takes antibiotics
Gets better
One bacteria is not killed by the antibiotics, due to
genetic modification & reproduces
Person gets sick again & goes to the doctor again
Same antibiotic is prescribed but there’s no effect
Doctor prescribes a different antibiotic which hopefully
works.
If bacterium continues to change it could become
resistant to more antibiotics
Examples of Natural Selection
• Pesticide resistance in rats
– Apply pesticide…most rats killed
– Due to natural variations, a few rats are not
affected by the poison.
– They reproduce passing on the trait to some if
not all of their offspring
– Seeing the rats, you apply the pesticide again
with worse results.
– A new pesticide must be used.
Artificial Selection
Darwin used
these examples
to help him
derive another
piece of his
theory.
Humans impact the
gene variation.
What Evidence is there that Evolution
by Natural Selection occurs?
• Direct observations
– Peppered moths
– Drug resistant bacteria
• Analysis of similarities among different organisms.
– Homologous structures
– Vestigial structures
• Fossil Record
– Documents the pattern of evolution
– Shed light on origin of new groups of organisms
• Biogeography
– Geographical distribution of species
• Continental drift (slow movement of the continents over time)
– Help make predictions of where fossils of groups of organisms can be found
Direct observations
Analysis of similarities among
different organisms
Homologous structures:
represent variations on a
structural theme that was
present in their common
ancestor.
Vestigial structures:
remnants of features
that served important
functions in the
organism’s ancestors.
Convergent Evolution: independent
evolution of similar features in different
lineages.
Species that share features because of convergent evolution are said to be
analogous. Analogous features share similar function but not common
ancestry.
Fossil
Record
Fossils can be dated, this helps us
provide evidence of evolution by
dating rocks where fossils are
found, it helps indicate the
relationships within phylogenetic
trees, as well as chemical properties
&/or geographical data.
Allow us to take a look at how
new groups came about & when
characteristics showed up.
EX: Cetaceans are
closely related to
even-toed ungulates.
Biogeography
We can use our understanding of
evolution and continental drift to predict
where fossils of different groups of
organisms might be found.
EX: Horses
Fossil dating indicates horses originated
in North America 5 million years ago.
Since North & South America were not
yet connected it was predicted that the
oldest horse fossils would be located in
North America.
So far the prediction has been upheld
The environment does not
direct the changes in DNA,
but acts upon phenotypes that
occur through random
changes in DNA
These changes can involve alterations in
DNA sequences, changes in gene
combinations, and/or the formation of new
gene combinations.
Environments are always
changing
• No “perfect” genome (entirety of an organism's hereditary information).
• Diverse gene pool = long term survival in a changing
environment.
• Mutations contribute to diversity
– Some are silent
– Some result in phenotypic difference
• Interaction of the environment & phenotype
determines fitness.
What does “survival of the
fittest” mean?
The species that are most adapted in
their environment by reproducing
more successfully will survive.
Mutation




Change in the
nucleotide sequence
(are rare; change from
gen. to gen. is very
small.
In multicellular organisms,
only mutations occurring in
gametes will be passed on
to offspring
Most occur in somatic cells.
Some are “silent” changes
Point
mutation
Genes duplicating
• Due to errors in meiosis
• During DNA replication
• Activities of transposable elements
(wandering DNA segments)
• Large chromosome segment duplications are
usually harmful
• Smaller pieces of DNA may not be.
– Gene duplications that don’t have severe effects can
persist & accumulate = potential new genes with new
functions.
Example of Beneficial Increases
in Gene Numbers
A remote ancestor had a
single gene for detecting
odors & have duplicated
over time. Today we have
about 1,000 olfactory
receptor genes.
Evolving Populations
What is a population?
A group of individuals of the same species that live in the same
area & interbreed, producing fertile offspring.
We can characterize a population’s genetic
make-up by describing its gene pool.
What is a gene
pool?
• The total number
of genes of every
individual in an
interbreeding
population.
The size of the gene pool affects the rate of mutation
Why is it vital to have diversity in
the gene pool?
• Environmental conditions change
– What kills one would kill them all.
Clearing Potential
Misunderstandings
PHENOTYPE
• Natural selection acts on ___________
– Ex: pepper moths
POPULATIONS
• _____________not
individuals evolve.
REPRODUCTIVELY fit
• Organisms that are _______________
GENES
will pass on their _________.
G.H. Hardy & W. Weinberg
(1908)
• Both men independently suggested a
scheme whereby evolution could be
viewed as changes in the frequency of
alleles in a population of organisms.
Calculating changes in allele
frequency
• Hardy-Weinberg
– For populations to be at equilibrium (remain
constant) the following conditions must be
met.
•
•
•
•
•
Population must be large
Absence of migration
No net mutations
Random mating
Absence of selection
– These conditions are seldom if ever met.
Of what value is this?
Provides a yardstick by which
changes in allele frequency, &
therefore evolution, can be
measured.
The Hardy-Weinberg Equation
Tests whether a population is
evolving.
The Hardy-Weinberg Principle
• The equation determines what the genetic make-up of a
population would be if it were not evolving at that locus.
• Those results are compared to data from an actual
population
• If there are no differences, it can be concluded that the
“real” population is not evolving
• It is also used in medical applications:
– Est. the % of a population that carries the allele for an inherited
disease.
•Knowing the frequency of the
phenotype, you can calculate the
frequency of the genotypes and alleles
in the population.
•Because there are only two alleles, the
sum of p and q must always equal 1.
p + q = 1
= frequency of
the dominant
allele
This equation finds
the allele frequency
=frequency of
the recessive
allele
The Hardy-Weinberg equilibrium
can be written as an equation
p2
+
% of Individuals
homozygous for allele
BB written as a decimal
By convention
2pq
+
q2
=
1
% of Individuals
homozygous for allele
bb written as a decimal
% of Individuals
heterozygous for alleles
Bb written as a decimal
The more common allele (B) is designated p
The less common allele (b) is designated q
This equation lets us calculate genotypic/phenotypic
frequencies in a very simple way.
Let’s say we have a population of 546 frogs. 142 of these
frogs have dark spots on them. The plain green frogs are
completely dominant to the spotted frogs. Determine the
genotypic frequencies within this population.
142/546 = .26 which represents q2 or gg
In order to get the homozygous dominant & heterozygous
we need to use the p + q = 1 equation.
q2 = .26  take the square root of each side to get q which is .51
2 = .24
p
p + q = 1 p= 1- q p= 1- .51  p = .49 
2pq = 2(.51 x .49) = .50
So, 26% of the frogs are recessive; 50% are heterozygous;
24% are homozygous dominant.
To determine the frequency of gametes carrying the dominant
allele. Meaning the percent of the population carrying at least one
dominant allele:
You would take half of the 2pq frequency and add it to
the p2 frequency.
.25 + .24 = .49
Which means, 49% of the frog population
carry at least one dominant allele.
To determine the recessive frequency take half of the 2pq & add to q2
.25 + .26 = .51
Which means 51% of the population
carries at least one g allele.
CALCULATING THE FREQUENCY OF CYSTIC FIBROSIS
Cystic fibrosis is caused by the recessive allele b.
Calculate the frequency of the recessive allele
If q2, the frequency of recessive homozygotes, is
0.00048, then q is √ 0.00048 , or 0.022.
Calculate the frequency for the dominant allele B:
p + q = 1 , p = 1-q
SO
p = 1 – 0.022,
OR
0.978
Determine the frequency of heterozygotes
2pq = 2 x 0.978 x 0.022 = 0.043
Meaning: 43 of every 1,000 Caucasian North Americans are
predicted to carry the cystic fibrosis allele unexpressed.
What percentage of the class are carriers for the
tongue curling trait? (Tongue curling is a dominant
trait.)
Try to curl your tongue upwards.
How would we determine q2?
Divide the total number of non-curler students by the
total number of students.
How do you determine q?
Calculate the square root of q2.
Since p + q= 1; determine p.
Now plug in numbers for 2pq.
Time for a simulation!!!
Let’s Get It On!!!!
Number of alleles present after
several generations
Number of offspring with AA _____X 2= _________ A alleles
Number of offspring with Aa _____X 1= __________A alleles
Total= __________A alleles
p=
TOTAL number of A alleles
TOTAL number of alleles in the population
(number of students X 2)
Altering Allele Frequencies in
Populations Directly & cause most
Evolutionary Change
• Natural Selection
• Genetic drift
• Gene Flow
Genetic Drift
Random fluctuation of alleles due to chance events
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Founder effect
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More likely to occur in
smaller population
Small group of individuals
establishes a population
in a new location
Bottleneck effect

A sudden decrease in
population size to
natural forces
Case Study:
Genetic Drift
The land was being converted
to farmland and other uses.
Led to significant loss of genetic
variation & an increase in the
frequency of harmful alleles.
A reduction of genetic variation
within a given population can
increase the differences between
populations of the same species.
Gene Flow
The transfer of alleles into or out of a
population due to the movement of fertile
individuals or their gametes.
If there is gene flow between two populations there is a
tendency for the amount of genetic variation between
the populations to decrease.
Natural Selection is the only mechanism that
consistently causes adaptive evolution
Stabilizing Selection
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Increase in the
frequency of the
intermediate phenotype
In humans, infants
with intermediate
weight at birth
have the highest
survival rate
In chicken, eggs of
intermediate weight
have the highest
hatching success
Fig. 13.13
Disruptive Selection
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In the African
seed-cracker
finch, large- and
small-beaked birds
predominate
Can open tough shells
of large seeds
Intermediatebeaked birds are
at a disadvantage
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Unable to open large
seeds
Too clumsy to open small
seeds
More adept at
handling small seeds
Directional Selection
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Drosophila flies that
flew toward light were
eliminated from the
population
The remaining flies
were mated and the
experiment repeated
for 20 generations
Common when
environment changes or
members of a
population migrate out.
Phototropic flies are far less
frequent in the population
Sexual Selection
Sexual selection acts on an organism's ability to
obtain (often by any means necessary!) or
successfully copulate with a mate.
Sexual dimorphisma difference in the 2
sexes
Sexual selection is often
powerful enough to
produce features that are
harmful to the individual’s
survival.
Australian Peacock Spiders
Operations of Sexual Selection
• Intrasexual selection:
Individuals of one sex compete for mates of the opposite sex. Mostly
males compete but there are some species where females compete
(ring-tailed lemurs and broad-nosed pipefish)
• Intersexual selection:
“mate choice” – individuals of one sex (usually females) are
choosy in selecting their mates of opposite sex. Many cases
it falls to the male’s showiness or behavior that wins the
female.
Why natural selection cannot
fashion perfect organisms
• Limited by species ancestors
– Birds ancestors are reptiles & only had 4 legs. Wings were opted
for flight so that left only 2 legs left.
• Adaptations are often compromises
– Seals spend a lot of time on rocks so legs would be a better trait
than flippers but then they would not swim as well.
• Chance & natural selection interact
– Snakes are carried to different islands on seaweed rafts but the
snakes that are carried are not necessarily the snakes best suited
to the environment.
• Selection can edit out only existing variations
– New alleles do not arise on demand
The biological species
concept
Emphasizes reproductive isolation
Belonging to the same
biological species
• Means you are reproductively compatible,
potentially.
But, it all hinges on…
• Reproductive barriers:
– Impede members of two species from
producing viable, hybrids.
– One barrier may not impede but a
combination of several can isolate a species
gene pool.
Prezygotic & Postzygotic
Barriers
• PREZYGOTIC:
– Impede mating or hinder the
fertilization.
• POSTZYGOTIC:
– If the sperm is able to
overcome the prezygotic
barriers then the zygote may
be prevented from developing.
Two main ways by which new
species form
Speciation can take place with or
without geographic separation
Geographic isolation
-reproductive isolation
may occur; preventing
interbreeding
Reproductive barrier must be
present.
-allopolyploidy
-reproductive isolation
-habitat, food source, or
other source not used
by parent pop.
-temporal or behavioral
isolation
Example of
sympatric
speciation
Adaptive Radiation
Tempo of Speciation
Phylogeny
The study of the evolutionary past
of a species.
Systematics
• An approach to looking at the diversity &
relationships between organisms living &
extinct.
– Morphology
– Biochemical resemblances
– Molecular comparisons (ex: DNA)
If similar with
any they are
likely to be
closely related
It’s also good to look at fossils- they reveal ancestral characteristics
that may have been lost over time.
Sorting Analogy from Homology
• How can you tell if the structure is similar
because of a common ancestor or due to
convergent evolution?
Search for corroborating similarities between the species
Fossil evidence
Look at the complexity of the characters & note points of
similarities.
Molecular comparisons
of Nucleic Acids
Species 1 & 2
DNA are identical
Mutations shift the
matching sequences
Homologous regions (in
yellow) no longer align
Using a computer program
homologous regions now align
because appropriate gaps were
created.
Begin to diverge
Connecting Classification with
Evolutionary History
Binomial nomenclature
• Two part naming
system
– genus and specific
epithet
• Canis lupus
or
• Canis lupus
Hierarchy of
Classification
Fig. 14.3
Using & making a dichotomous
key
Phylogenetic trees &
cladograms are graphical
representations (models) of
evolutionary history that can
be tested
Phylogenetic
Tree
Each branch point
represents the
divergence of two
species from a
common ancestor
Cladogram – depicts patterns
of shared characteristics
Does not imply
evolutionary history.
If characteristics are homologous the
cladogram can serve as a basis for a
part of the phylogenetic tree.
Outgroup?
Ingroup?
Making cladograms
Organism Multicellular
Vertebral Hair
column
Placenta
Totals
Sponge
Yes
No
No
No
1
Sailfish
Yes
Yes
No
No
2
Wombat
Yes
Yes
Yes
No
3
Elephant
Yes
Yes
Yes
Yes
4
sponge
wombat
sailfish
elephant
placenta
hair
Vertebral column
multicellular
Constructing a cladogram
• Using the following animals, list as many characteristics
which each organism possesses .
• Create & fill in a chart with the animals & the
characteristics you have come up with.
• Build your cladogram
Rhesus monkey
Kangaroo
Bull frog
Tuna
Snapping Turtle
Human
Rhesus
Monkey
Tuna
Bull
Frog
Snapping
Turtle
Kangaroo
Short
canine
teeth
Placenta
Mammary
glands
Amniotic
sac
Paire
d
legs
Human
Structural evidence supports the
relatedness to all eukaryotes
•
•
•
•
Cytoskeleton
Membrane-bound organelles
Linear chromosomes
Endomembrane systems, including
nuclear envelope.
Which of these is
most likely in your
opinion?
Sometimes the most
obvious evidence isn’t
the best
Molecular & genetic evidence from existing and
extinct organisms indicates all organisms on Earth
share a common ancestral origin of life
Molecular building blocks are
common to all life forms
Common genetic code are
shared by all modern
organisms.
Metabolic pathways are conserved across all currently
recognized domains (bacteria, archea, & eukarya)
The Universal
Tree of Life
1. Last common ancestor of
all living things.
2. Possible fusion of
bacterium with archea
making eukaryotes
3. Symbiosis of
mitochondrial ancestor with
ancestor of eukaryotes
4. Symbiosis of chloroplast
ancestor with ancestor of green
plants
The History of Life on Earth
Earth formed approximately 4.6
billion years ago. The earliest
fossil is dated 3.5 billion years old.
Primitive Earth
• Provided inorganic precursors from which
organic molecules could have been synthesized
due to the presence of available free energy &
the absence of a significant quantity of oxygen.
– These molecules served as monomers of building
blocks for the formation of more complex molecules,
including amino acids & nucleotides.
– Joining of the monomers produced polymers with the
ability to replicate, store & transfer information.
– The RNA World hypothesis proposes that RNA could
have been the earliest genetic material.
Miller/Urey Experiment
This experiment showed it was possible to form
complex organic molecules from inorganic
molecules in the absence of life.
How are rocks & fossils
dated?
Radiometric dating


Fossils have been
found linking all
the major groups
The forms linking
mammals to
reptiles are
particularly well
known
Fig. 13.4 Whale
“missing links”
Radiometric Dating
• Certain atoms are known to decay (break down) at
a specific rate. Scientists can look at these atoms
to determine how old an organic object is.
–
Radioactive isotope 14C- gradually decays over time
back to 14N (known as Carbon Dating)
• It takes ~5600 years for half of the 14C present in a sample to
be converted to 14N.
• This length of time is called the half-life.
• Half life (t1/2): the time needed for half of the atoms
of the isotope to decay
• For fossils older than 50,000 yrs scientists use
other isotopes such as, potassium isotope
– t1/2 of 40K = 1.3 billion years to turn to argon (40Ar)
Mass Extinctions
• Are rapid during times of ecological stress.
• Fossil records chronicles mass extinctions
1st single-celled organism
• Prokaryotes
st
1
Eukaryotes
• About 2.1 billion years old.
• The endosymbiosis theory