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EVOLUTION
Evolution
• process that transformed life on Earth
from earliest beginnings to diversity
seen today
• unifying force for biology
• encompasses small & large scale
evolution
–changes in gene frequency in a
population from one generation to next
Evolution
• Responsible for pattern of relationships
between species
• as lineages evolve & split & modifications are
inherited, evolutionary paths diverge
• produces branching pattern of relationships
• by studying inherited characteristics &
historical evidence can reconstruct
evolutionary relationships & represent them
on a family tree
• phylogeny
Phylogeny of Animalia lineage
Understanding Phylogeny
• root of tree represents ancestral lineage
• tips of branches represent descendents of
that ancestor
• as move from root to tipsmove forward in
time
Pre-Darwin Theories
• supernatural intervention
• naturalistic models
– based on logic & philosophy
• 5th century BC-Greece
– Plato (428-348 BC) & Aristotle (384-322 BC)
– naturalistic view of origins
– Anaximander
• believed life arose in water & simpler forms preceded more complex ones
• 1766 Georges Buffon
– species represented by fossils could be an ancient version of groups of similar
living species
• Jean Baptiste Lamarck-1800s
– relationship of fossils & current due to evolution
– proposed that by using or not using a body part individual
develops specific characteristics which get passed to offspring
– inheritance of acquired characteristics
Charles Darwin
• formalized theory of evolution-1859
• The Origin of the Species
– species living today descended from
ancestral species
• Diversity is based on different modifications
of common ancestors from which varieties of a
species descended
• theory of how evolution works is natural
selection
Theory Development
• gathered evidence
during around-theworld voyage
• HMS Beagle-18311836
• southern islands,
South American
coast & Australia
• collected specimens
of fossils & living
plants & animals
Theory Development
• began to notice
organism made
adaptations to live
in particular area
• Galapagos Islands
• noted presence of
unique organisms
that were like but
different from
organisms on
mainland
Theory Development
• while on the Beagle,
reading Principles
of Geology by
Charles Lyell (17971875)
– suggested gradual
changes in Earth’s
surface
• proposed slow &
subtle processes
over vast amounts
of time led to
enormous changes
Theory Development
• finches interested Darwin
• normally seed-eating birds
adapted insect-eating habits
• subtle form & habit changes of
these birds entrenched ideas of
change over time
• seemed reasonable that
organisms had been transformed
over time
• new structures & habits had
developed over time
• began to think species descended
from ancestral species
Theory Development
• 1840’s Darwin wrote essay on
evolutionary theory
• knew would create social furor
• delayed publishing
• middle 1850’s Alfred Wallace came up
with similar theory
• Wallace asked Darwin to take a look
at his essay to see if it should be
published
• 1859 Darwin published On the Origin
of Species by Means of Natural
Selection
• Darwin proposed viable mechanism
on how species evolve
• natural selection
Natural Selection
• best theory on how evolution
occurs
• based on two observations
• Observation 1: species
produce excessive numbers
of offspring with limited
natural resources
• too many individuals for
natural resources to
support struggle for
existence ensues
• Observation 2: variation
exists among individuals of a
population
• no 2 are exactly the same
Natural Selection
• Darwin inferred:
• "As many more individuals of each species
are born than can possibly survive, and as,
consequently, there is a frequently recurring
struggle for existence, it follows that any
being, if it vary, however slightly, in any
manner profitable to itself under the complex
and sometimes varying conditions of life, will
have a better chance of surviving and thus
be naturally selected”
Natural Selection
• differential reproductive
success
• individuals with inherited
traits better suited to local
environment are more
likely than lesser fit
individuals to survive &
reproduce
• individuals who function
best tend to leave more
offspring
Evolution
• Earth with liquid water has
existed for more than 3.6 billion
years
• cellular life has been around for
at least half that time
• organized multicellular life has
existed for at least 800 million
years
• major life forms now on Earth
are not all represented in past
• major life forms of past-no
longer living on Earth
• all living forms come from
previous living forms that were
different
Methods & Evidence Used
to Date Evolutionary
Events
Radiometric Dating
half-life decay of radioactive
elements allows scientists to date
rocks & materials directly
allows scientists to estimate dates
Stratigraphy &
Fossil Record
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fossils-preserved remnants or
impressions left by organisms that
lived in the past
– mostdiscovered in sedimentary
rocks
– each layer contains fossils that
represent organisms that lived
during particular part of Earth’s
history
– position in layers reveals age
chronology of appearance in rock
layers leaves observable signs of
evolution
oldest are 3.5 billion years old
fossil record fits in with information
obtained with other evidences
fossil record is incomplete
in part because organisms with shells
or bony skeletons are more likely to be
preserved than those without hard
body parts
Indicators of Common Ancestry
• Comparative Anatomy
–Homologous Structures
–Analogous Structures
–Vestigial Structures
• Comparative Embryology
• Molecular Biology
Comparative Anatomy
• anatomical similarities among
species
– same skeletal elements
make up forelimbs of
humans, cats, whales &
bats
• structural similarities-not
surprising if all mammals
came from common ancestor
with prototype forelimb
• similarity in characteristics
that are the result of common
ancestry –homology
• Homologous structures
ANALOGOUS STRUCTURES
• insect & bat wings-analogous
– separate evolutionary
origins
• superficially similar
• have evolved to do same job
• convergent evolution
• though bird & bat wings are
analogous as wings, as
forelimbs they are homologous
• birds & bats did not inherit
wings from a common ancestor
with wings, but did inherit
forelimbs from common
ancestor with forelimbs
Vestigial Structures
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features that were adaptations for
organism's ancestorevolved to be
non-functional because organism's
environment changed
presence of such organs suggests
common ancestry
fish species that live in completely
dark caves have vestigial, nonfunctional eye
when sighted ancestors ended up
living in caves, there was no longer
any natural selection that maintained
function of the eyes
fish with better sight no longer outcompeted fish with worse sight
today, these fish still have eyes
not functional
by-products of fishes' evolutionary
history
Comparative Embryology
• closely related organisms
have similar embryologic
stages of development
• all vertebrates have
structures on sides of throat
during embryological
development-pharyngeal
pouches
• at this stage embryos of
fish, frogs, snakes, birds &
mammals look very similar
• in fish-pouches become
gills
• in humans-part of ears &
throat
Molecular Biology
• study of DNA &
proteins
• if two species have
genes & products
of genes-proteins
with similar
sequences of
nucleotides  must
have been copied
from common
ancestor
Molecular Biology
• 98% of our DNA sequences
match chimpanzees
• 95% of amino acids of
rhesus monkey hemoglobin
are identical to human
hemoglobin
• 14% of lamprey hemoglobin
is similar to human
hemoglobin
– conclude-rhesus
monkeys are more
closely related to
humans than lampreys
– chimpanzees are more
closely related to
humans than Rhesus
monkeys
Molecular Biology
• master control genes
– regulate groups of
other genes during
embryological
development
– very similar
nucleotide
sequences from fruit
flies & mammals
• conclusion-genes
arose in common
ancestor
Human Evolution
• did not evolve from
chimpanzees
• share recent common
ancestor
• neither chimpanzee nor
human
• humans are not higher
or more evolved than
other living lineages
Natural Selection in Action
• in some cases natural
selection can be
observed
• data shows shape of
finches' beaks on
Galapagos Islands has
tracked weather patterns
• after droughtsfinch
have deeper, stronger
beaks
• lets them eat tougher
seeds
Natural Selection in Action
• human activity has
led to environmental
changes that have
caused populations
to evolve through
natural selection
• population of dark
moths in 19th
century England,
rose & fell in parallel
to industrial
pollution
Natural Selection at Work
• How do antibiotics change the
phenotype of bacteria and make them
resistant?
Population & Evolution
• individuals do not evolve
• populations do
– group of individuals of
same species living in
same place at same
time
– smallest biological unit
that can evolve
• evolutionary impact of
natural selection can only
be seen in tracking how a
population changes over
time
Population Genetics
• tracks variations
in populations
over time
• only traits that
can be passed
on in genes can
affect evolution
Inheritable Traits
• not all traits are
heritable
• phenotype is result of
genotype &
environment
• only genetic
characteristics can be
inherited
• when population has
two or more forms of
one phenotype, they
are polymorphic
Genetic Variation
• basic mechanisms of
evolutionary change
cannot operate without
genetic variation
• two main sources
• Sexual recombination
• Mutation
Sexual Recombination
• during ovum & sperm formation-meisois
• genes are rearranged-prophase 1
• original chromosomes are different from
chromosomes of parents
Sexual Recombination
• Sex can introduce new gene
• when organisms reproduce
sexually, some genetic
"shuffling" occurs
• bringing together new
combinations of genes
• might have bushy eyebrows &
big nose since mom had genes
associated with bushy
eyebrows & dad had genes
associated with big nose
• combinations can be good, bad,
or neutral
• shuffling can introduce new
combinations of genes in every
generation
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Mutations
random changes in DNA
beneficial, harmful or neutral
do not supply what is needed
mutation is unrelated to how
useful it is
not all matter to evolution
somatic mutations occur in nonreproductive cells
– will not be passed to offspring
golden color on half of this
Red Delicious apple was caused
by somatic mutation
seeds do not carry mutation
Reproductive Cell Mutations
• occur in reproductive cells-eggs
& sperm
• No change in phenotype
– mutation might occur in stretch
of DNA with no function
– mutation occurs in proteincoding region, but ends up not
affecting amino acid sequence of
protein
• Small change in phenotype
– single mutation caused cat's
ears to curl backwards slightly
• Big change in phenotype
– some phenotypic changes are
caused by single mutation
– can also have strong negative
effects for organism or strong
benefits
– mutations causing death are
lethal
Causes of Mutations
• DNA fails to copy accurately
– most mutations that matter to
evolution are naturallyoccurring
– when cells divide-make copy
of DNA
• sometimes copy is not quite
perfect
• small difference from original
sequence is a mutation
• External influences
– exposure to specific chemicals
or radiation
– cause DNA to break down
– when cell repairs DNA, might
not do a perfect jobcell ends
up with DNA slightly different
than original DNA mutation
Sickle Cell Anemia
Analyzing Gene Pools
• to study evolution at
population level focus is on
gene pool
– all alleles in all individuals
making up a population at
a given time
• when frequencies of alleles
in population changes over
number of generations
• Conclude-evolution is
occurring
Hardy-Weinberg Equilibrium
• gene frequencies & genotype
ratios in a randomly-breeding
population remain constant from
generation to generation
• tells us recessive alleles do
not tend to disappear in a
population
• if this were to happen
population would soon become
homozygous
• therefore sexual reproduction
alone does not lead to evolution
• frequency of each allele in a
gene pool will remain constant
unless acted on by other agents
Gene Frequency
• measures frequency of
particular gene relative to
other genes at a particular
locus in a population
• if genotype contains two
genes
• total of 16 genes per locus
in population of eight
individuals:
• Aa AA aa aa AA Aa AA Aa
• frequency of A = 9/16 =
0.5625
frequency of a = 7/16 =
0.4375
Gene Frequency
• use p to indicate frequency of A
(dominant)
• use q to indicate frequency of a
(recessive)
• frequencies for all possible
genotypes = 1
•p+q=1
Hardy-Weinberg Equation Uses
• (p + q)2 = p2 + 2pq + q2 = 1
• formula allows us to
determine frequencies by
observing the population
• public health professionals
use it to calculate how
many people carry certain
alleles for inherited
diseases
• Example-used to calculate
percentage of population
carrying an allele for PKUphenylketonuria
When Hardy-Weinberg Law Fails to
Apply
• works in non evolving
populations
• genetic equilibrium
• law says-gene pools
remain constant over
time
• frequencies remain
unchanged
• if equation does not
equal one-evolutionary
change is occurring
Circumstances that lead
to evolutionary change
•gene flow
•genetic drift
•natural selection
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Gene Flow
occurs when individuals migrate &
mate outside original population
many species make up local
populations
members tend to breed within their
group
each local population can develop
gene pools distinct from that of other
local populations
members of one population may breed
with occasional immigrant from
adjacent population of same species
introduces new genes or alters existing
gene frequencies in residents
gene flow reduces genetic
differences between populations
gene immigration increases variability
of gene pool
Genetic Drift
• change in gene pool due to chance
• more impact in small populations
Genetic Drift
• two situations can
shrink populations
down to size
where genetic drift
can take place
• bottle neck effect
• founder effect
Bottle-Neck Effect
• result of disaster
– flood, earthquake or fire
• drastically reduces
population size
• certain alleles may be
present at higher
frequency in surviving
population than in
original one
• effect usually reduces
overall genetic variability
of a population
Founder Effect
• few individuals
colonize isolated
area
• smaller colony
less genetic makeup
will represent gene
pool of larger
population from
which colony
migrated
Natural Selection
• process by which variation,
introduced by mutation, gene
flow, or genetic drift, is refined
• individuals with phenotypes not
appropriate for an environment
will not pass genes on
• selection produces adaptations
• if individuals having certain
genes are better able to produce
mature offspring than those
without a gene then frequency of
genes will increase
• expression of natural selection is
seen in alterations in gene pools
Fitness
• genotype's fitness depends on
environment in which organism
lives
• how good particular genotype is
at leaving offspring in next
generation
• not necessarily strongest,
fastest, or biggest
• includes ability to survive, find
mate, produce offspring
• ultimately leaving genes in next
generation
• if brown beetles consistently
leave more offspring than green
ones because of their color, you
would say brown beetles have
higher fitness
Fitness
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caring for offspring
producing thousands of young
sporting fancy feathers that attract females
these strategies increase fitness because
help parents get more of their offspring
into next generation
Natural Selection
• results from differential mortality
and/or differential fecundity
• differential mortality could be
termed mortality selection
meaning that certain genotypes
are less successful than others
in surviving to end of their
reproductive period
• evolutionary impact of mortality
selection can be felt anytime
from formation of new zygote to
end of organism's period of
fertility
• another way of describing
Darwin's criteria of fitness:
survival
Natural Selection
• Fecundity selection
• certain phenotypes (thus
genotypes) may make
disproportionate contributions to
gene pool of next generation by
producing disproportionate
number of young
• certain phenotypes are better
able than others to contribute
genes to next generation
• By Darwin's standards they are
more fit
Natural Selection
• population does not "want" or
"try" to evolve
• natural selection cannot try to
supply what organism needs
• selects among whatever
variations exist in population
• result is evolution
• natural selection is sometimes
interpreted as random
• misconception
• genetic variation that occurs in a
population because of mutation is
random
• selection acts on that variation in
a non-random way
• genetic variants that aid survival
& reproduction are much more
likely to become common than
variants that don't
Outcomes of Natural
Selection
• Directional
• Diversifying
• Stabilizing
Directional Changes
• shift phenotypic curve of
population by selecting in favor
of extreme phenotypes
• most common when local
environment changes or when
organisms migrate to new
environment
• when challenged species must
adapt or become extinct
• populations that survive often
change enough to be
considered a new species
• most common-extinction
Diversifying or
Disruptive Changes
• lead to balance
between 2 or
more
contrasting
phenotypic
forms in a
population
Stabilizing Changes
• maintain variations for
a particular trait within
a narrow range
• selects against
extreme phenotypes
• stabilizing outcomes
persist most of time
• lead to well adapted
populations
Evolution Exercise
• Beetles on a diet
• Beetles of a different color
Evolution Exercise Answer
• difference in weight in first situation occurred
due to environmental influences-low food
supply-not because of a change in frequency of
genes
• example 1 is not evolution
• because small body size was not genetically
determined, this generation of small-bodied
beetles will produce beetles that will grow to
normal size if they have a normal food supply
• changing color in example 2 is evolution
• these two generations of the same population
are genetically different
• How did it happen?