Chapter 13 Notes - Great Neck Public Schools

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Charles Darwin
Charles Darwin

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Alfred Russel Wallace
Alfred Russel Wallace

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Chapter 13 – How Populations Evolve
I. The Blue-Footed Booby
A. Found on Galapagos islands
B. “booby” – slang for stupid
1. Easily approachable by humans (easily killed)
C. Why do they have the physical features they do? blue
feet, etc…
1. Helps them succeed in environment of course
a) large webbed feet –
(1) great for swimming (they hunt in water)
(2) terrible on land for walking or taking off (flight)
b) body and bill streamlined like torpedo
(1) minimize friction on 80 foot high dives into
shallow water
c) Large tail
(1) to steer them out of dive before hitting ocean floor
d) nostrils that close
(1) prevent water intake
e) many glands (specialized groups of cells that
secrete substance into blood (endocrine), body cavity
or surface of body (exocrine))
(1) preen gland - base of tail secretes oil (fatty acid)
for waterproofing
(2) supraorbital gland – found in bird eye socket –
converts salt water to fresh water
(a) removes excess salt from blood, concentrates it,
and puts it into nasal cavity and expelled when bird
shakes head from side to side.
D. Are all adaptations positive all the time?
1. Adaptations usually represent a trade-off
a) webbed feet – water vs. dry land
(1) net advantage - improves chance of surviving long
enough to reproduce.
E. Evolutionary adaptations – inherited traits that enhance
an organism’s ability to survive and reproduce in a
particular environment
1. We will examine some of the processes by which
species develop evolutionary adaptations – EVOLUTION
(inherited change in organisms over time).
F. How do organisms change?
II. A sea voyage helped Darwin frame his theory of
A. The main idea advanced by Darwin
1. Species change over time and that living species have
arisen from earlier life-forms
B. This idea can be traced back to the ancient Greeks
1. 2500 years ago Anaximander (greek philosopher)
a) life arose in water and simpler forms preceeded
more complicated forms
C. The road from Anaximander to Darwin – long, torturous
1. Aristotle – very influential – species are fixed and do
not evolve
2. Judeo-Christian culture – literal interpretation of the
Book of Genesis – all species individually designed by a
divine creator
3. Static species on a 6,000 year old Earth dominated
Western world for centuries
4. In the century before Darwin, only a few scientists
questioned the biblical story of creation and fixed
a) mid 1700’s – study of Fossils by French Naturalist
Georges Buffon suggested that the Earth might be
older than 6,000 yrs.
(1) Fossils – imprints or remnants of organisms that
once lived
b) Similarities observed between living organisms
and fossils
c) 1766 – Buffon proposed that certain fossil forms
might be ancient versions of similar living species
d) Early 1800’s – French Naturalist Jean Baptiste
Lamarck suggested that this is best explained by
organisms evolving.
(1) Lamarck is today remembered most for his
erroneous view of how species evolve.
CHARACTERISTICS - Proposed that by using or
not using its body parts and individual may develop
certain traits which it passes on to offspring
(i) Ex – Giraffes inherited long neck from
ancestors who stretched their necks higher
and higher to reach leaves in trees
(2) Lamarck set the stage for Darwin!!
D. Darwin – Dec. 1831, age of 22, embarked on a round-the
world sea voyage as a crewmember of the HMS Beagle.
1. HMS Beagle – Surveying ship to chart the poorly
known stretches of South American coastline.
2. He collected thousands of living and fossilized
specimens along the south American shoreline
3. He noted that the fossils resembled the living species
in the region which the fossils were found!
4. Organism distribution on the Galapagos islands also
made strong impressions on Darwin.
5. He was influenced by geologist Charles Lyell’s
Principles of Geology – Earth undergoes continual,
gradual, persistent change – explaining why he found
fossils of marine snails in the Andes.
6. The beagle returned to England in 1836 and Darwin
began work on an essay to document his observations,
which was completed in the early 1840’s, but did not
publish it fearing rightfully that it would cause a social
7. In the mid-1850’s, Alfred Wallace, a British Naturalist,
conceived a theory similar to Darwins – Darwin was sent
the manuscript (unpublished)
8. In 1858, Wallace’s manuscript along with excerpts of
Darwin’s essay were presented together.
9. 1859 – Darwin published his complete text, On the
Origin of Species by Means of Natural Selection –
contained a ton of evidence and presented a strong,
logical argument for evolution and how he thought it
worked – Natural Selection
10. Descent with modification – the term used by
Darwin in the first edition of his book until the last
paragraph where it switched to evolution.
a) All organisms related through descent from an
unknown species in the distant past
b) As descendents spread into various habitats over
millions of yrs, they accumulated diverse
modifications (adaptations), that acommidated them
to diverse ways of life
c) Resembled a tree with the common ancestor at the
E. Darwins idea of Evolution has been greatly extended
since the 1800’s, but his idea remains the core of evolution
to this date. It has stood the test of time. There has not
been one shred of evidence to disprove what Darwin
suggested! There have been few contributions in science
that have explained as much and stimulated as much
research as those of Charles Darwin.
III. Evidence for Evolution
A. Fossils – evidence of past life – preserved remains or
traces of organisms
1. critical in the understanding of evolution (Darwin)
a) they document change over time
2. Mostly hard parts of organisms found – soft, organic
parts decay quickly
3. Only a tiny fraction of organisms have been fossilized.
4. 2 main forms of fossils
a) true form fossils – fossils of animal or remnant
b) trace fossils (ichnofossils) – fossilized nest,
burrows, footprints…
5. Types of fossils
a) petrified fossils – hard or soft parts (usually hard)
are replaced with minerals
b) unaltered fossils (rare) – insect stuck in amber
-protected from bacteria and fungi
c) mold fossils – a fossilized impression made by
substrate (negative image)
d) cast fossils – formed when a mold is filled
6. Paleontologists – the scientists who study fossils
7. Fossil Record – ordered array in which fossils appear
within layers of sedimentary rock
a) Sedimentary rock – forms from layers of minerals
that settle out of water – younger strata on type and
older on bottom
8. Oldest known fossil – 3.5 billion years – prokaryotes
9. Vertebrate fossil record – fishlike fossils are first
observed, then amphibians, then reptiles, then mammals
and birds
If evolution is correct, what should we find?
a) transitional fossils (missing links) – fossils that link
two modern day species to each other
-Whales – hind leg bones, fossils of extince
whales with hind limbs
-Feathered dinosaurs
-Change in skull size in humans
B. Biogeography – geographic distribution of species
1. Darwin noticed that the animals on the Galapagos were
more similar to animals on the near mainland of South
America as compared to animals on similar islands
elsewhere in the world – Galapagos species evolved from
South American immigrants.
C. Comparative Anatomy – comparison of body structures
in different species
1. anatomical similarity gives sign of common descent
2. Also sited by Darwin
3. Homologous structures – structurally similar, different
a) If the structures (a bats wing and whales forelimb)
had been uniquely engineered, we would expect
different designs
b) Evolution builds on what is already present. It
remodels organisms, it doesn’t create anew – what
served as a flipper can be tweaked to serve a new
c) Human spine and knee – modified from ancestral
structures that supported four-legged animals – knee
and back problems.
D. Comparative Embryology – study of structures
appearing during development of different organisms
1. Closely related organisms have similar stages in
embryonic development
a) Pharyngeal slits on sides of throat present in all
vertebrates (fishes, frogs, snakes, birds, apes,
- Most become gills in fish
-Become bones of skull, bones supporting
tongue and voice box of mammals
E. Molecular Biology – the study of the molecular basis of
genes and gene expression
1. Universal genetic code
2. Within species - Related individuals of the same
species have greater similarity in DNA and proteins
compared to unrelated individuals
3. Between species - Closely related species have a
greater similarity in DNA and proteins compared to more
distantly related species
4. Hemoglobin polypeptide comparison (146 amino acids)
5. Master Control Genes – homeotic genes (code for
transcription factors) – regular groups of other genes
during embryonic development. Similar nucleotide
sequences in these genes especially of related organisms
F. So how does evolution operate?
IV. Darwin proposed natural selection as the
mechanism of evolution
A. Darwin - Influenced by British economist Thomas
Malthus – much of human suffering (disease, famine,
homelessness, war) was the consequence of our ability to
grow in population faster than we could produce what we
need to live.
B. Darwin recognized that:
1. All species tend to produce excessive numbers of
offspring (Will all humans born today get to survive and
2. Only a percentage of offspring will survive in each
generation – limited by natural resources (Who is most
likely to survive and reproduce?)
3. Individuals of a population vary extensively and their
survival depends on their traits.
4. If an organism gets the chance to reproduce, these
traits will be passed on (inherited).
a) Natural Selection - Individuals whose
characteristics adapt them best to their environment
are more likely to survive and reproduce, leaving
more offspring
b) Essence of natural selection = differential
(unequal) reproduction
c) environment filters variations, favoring some
characteristics over others.
d) Favorable characteristics will be present more and
more and unfavorable ones less and less over
generations leading to a gradual change in the
population over time.
Darwin saw evidence in:
C. Artificial Selection – if nature can do it, we should be
able to do it too – selective breeding of domesticated
plants and animals.
1. Darwin found convincing evidence in artificial
2. Humans play role of environment
a) Dogs
b) Vegetables
3. Achieved over relatively short span of time – compare
to evolutionary time scale
V. Scientists can observe natural selection in action
1. Over 100 cases have been identified
a) Changes in population of ground finch beak size in
20 years on Galapagos observed
(1) Depends on wet or dry season
(2) Wet years = many small seeds = birds with smaller
beaks have advantage (vice versa for dry years)
b) Insecticide resistance
(1) Poisons might kill 99% of the insects, but 1%
remains – resistant
(2) Could be considered artificial selection for
resistant insects
B. Natural selection is an EDITING PROCESS – it does not
create –
C. Natural selection depends on time and place – favors
characteristics that fit current, local environment.
D. Significant changes can occur in a short time.
A. Darwin (published 1859) did not understand the genetic
basis of evolution - Mendel’s work did not come to light
until 20 years after Darwin’s death (1902) – published 1866.
1. He could not explain the cause of variation among
individuals (law of segregation and independent
assortment, crossing over, mutation, etc…) within a
population or the perpetuation of parents traits in their
2. He knew nothing of genes
B. Modern Synthesis (1940’s) – theory of evolution that
includes genetics in combination with Darwin’s ideas
(natural selection).
1. What kind of genetics? The genetics of an individual?
What evolves? An individual?
2. A Population is the smallest unit that can evolve
a) Population – group of individuals of the same
species, living in the same place at the same time.
b) Populations can be isolated from each other
-Galapagos finches
c) Partially isolated
-New York City and Mexico City population
(partially isolated)
d) Population centers – not isolated, but more likely
to mate locally.
e) Result – individuals in one population are more
closely related on average to one another than to
members of other populations
3. Population genetics – focuses on two ideas
a) gene pool – total collection of alleles at any given
time (Evolution can’t make new alleles, it can only
edit alleles already present. How do new alleles arise?)
-Reservoir for the next generation, most loci
have two or more alleles
b) allele frequency – how often a particular allele
appears in a population
4. Examples of shifts in allele frequency
a) Example : Insecticide Resistance
b) Two alleles for an enzyme – one gives insecticide
resistance (breaks down insecticide), the other does
c) Initially the one that does not give resistance has
higher frequency in population.
d) Spray fields with insecticides.
e) Allele that confers resistance increases in
frequency while the other decreases.
f) Example: House Sparrows
5. Microevolution – change in relative frequencies of
alleles over time – evolution on its smallest scale
VII. Let’s Consider a hypothetical, non-evolving
population of blue footed boobies.
A. Two varieties - differ in foot webbing controlled by a
single gene
1. Allele for non-webbed feet - dominant (W)
2. Allele for webbed feet - recessive (w)
3. After many generations, which allele will win?
4. Neither, sexual reproduction simply shuffles the
genes, it does not make one appear more or less than the
other – shuffle a deck of cards, do you end up with more
of one card than another??
5. Hardy-Weinberg Equilibrium - Gene frequency will
remain the same (equilibrium) in a hypothetical nonevolving population unless acted on by OTHER agents.
B. Calculating allele frequencies
1. p + q = 1
p = frequency of dominant allele
q = frequency of recessive allele
2. Hardy-Weinberg Equation:
a) p2 + 2pq + q2 = 1
frequencies homozygotes and
VIII. Five conditions are required for Hardy-Weinberg
A. The Five:
1. The population is very large
2. The population is isolated; that is, there is no
migration of individuals or gametes into or out of the
3. Mutations (changes in genes) do not alter the gene
4. Mating is random
5. All individuals are equal in reproductive success; that
is natural selection does not occur
B. Can you think of a population where all 5 are met???
The causes of microevolution
A. Deviations in Hardy-Weinberg equilibrium causes
changes in gene pools = microevolution
B. 4 causes of microevolution
1. Genetic drift (major)
2. Natural selection (major)
3. gene flow
4. mutation
C. Genetic Drift – change in gene pool of a small
population due to chance
1. The smaller the sample the greater the chance of
deviation from an ideal result
a) Ex – compare flipping coin 10 times getting 3
heads and 7 tails vs. 1000 times getting 300 heads and
700 tails!
2. If population is large, then its more likely that the new
generation will be representative of the gene pool of the
3. If its small, it may not be
4. Example
a) 10 Ww x Ww crosses in a population. Each cross
produces a single offspring.
b) 1000 Ww x Ww crosses in a population – more
likely to be closer to a 1:2:1 ratio
5. This is genetic drift, an allele becomes more frequent
by chance alone having nothing to do with other agents
like the environment.
a) It’s most likely to play a major role in evolution
when populations have 100 or fewer individuals.
b) What can shrink populations down to a small
(1) Bottle neck effect – events that drastically reduce
population like earthquakes, floods, or fires that do
so unselectively (randomly) – shaking marbles out of
a bottle.
(a) Ex. Human hunters (1890’s) – reduced
population of northern elephant seals to ~20!
(b) Protected now and back to 30,000, but upon
examination of 24 gene loci, scientists found only 1
(c) Southern elephant seal has great variation (not
(2) Founder effect – colonization of a new location by
a small number of individuals
(a) Extreme case – single seed or single pregnant
animal populates a new place.
(b) Contributed to evolutionary divergence of finches
that initially arrived on the Galapagos.
(c) Explains relatively high frequencies of certain
inherited human disorders in human populations
established by small numbers of colonists.
(i) 1814 – 15 people founded a British
colony on Tristan da Cunha- small group of
islands in middle of Atlantic.
(ii) One colonist carried recessive allele for
retinitis pigmentosa (RP - progressive form
of blindness) 1 in 30 genes
(iii)Result – of 240 descendants still living
on the island in the 1960’s, 4 had RP, 9
others were carriers – frequency is much
higher than frequency from which the allele
came! 1 in 14
D. Gene Flow – fertile individuals move into or out of a
population or when gametes are transferred between
populations (ex. Sperm of plant pollen).
1. Tends to reduce genetic differences b/w populations
a) The isolation of human populations has reduced
gene flow over history resulting in phenotypic
variations such as skin color and facial
b) Things like migration and war work against
reproductive isolation – tend to increase
interbreeding (gene flow)
c) Today there is more gene flow between
geographically different populations than ever before.
E. Mutation – Random change in an organism’s DNA that
may create a new allele
1. Rare event – once per gene locus per 105 or 106
a) Does not have much of an effect in a single
generation of a large population
b) Over the long term, it is the only force that
generates new alleles!!! – creates – ultimate source of
genetic variation!!
F. Natural Selection
1. Only cause of microevolution that is likely to result in
adaptive change in a gene pool.
2. Blue-footed booby example – webbed feet variation
(ww) might product more offspring because they find
food more efficiently than birds with non-webbed feet
(Ww or WW).
3. The frequency of the w allele would increase as the
frequency of the W allele decreases.
4. If environment changes, so might the favored traits for
that new environement (the W gene may make a
5. What limits the degree of adaptation? Amount and kind
of genetic variation.
X. Variation
A. extensive in most populations
B. Individual vary in sexually reproducing populations
C. Obvious differences at the anatomical level
D. Some ONLY observed at the molecular level
E. Not all variation is heritable
1. Can’t pass on muscle mass from working out
2. Only the genetic component is subject to natural
F. POLYMORPHISM - two or more morphs (different forms
of a phenotypic trait) for a characteristic are present in
noticeable numbers
1. Humans - Presence or absence of freckles, ABO blood
2. California King Snakes
G. Most species exhibit geographic variation between
1. Cline – a graded change in an inherited characteristic
along a geographic continuum.
a) Ex. Body size of birds and mammals tends to
increase with increasing latitude in North America –
large size in colder regions to conserve body heat
H. How do we measure genetic variation
1. Look at gene diversity and nucleotide diversity
a) Gene diversity – average percent of gene loci that
are heterozygous in a population
b) Nucleotide diversity – compare nucleotide
sequences of DNA samples
2. Humans have LESS genetic diversity than most other
a) Gene diversity = 14% and nucleotide diversity is
b) Why do you think this is?
XI. Mutation and sexual recombination generate
A. Most mutations in DNA will be harmless or harmful
B. On rare occasions, a mutant allele might actually
improve the adaptation of its bearer to the environment
and enhance reproductive success.
1. Likely to come to light when the environment changes
in such a way that mutations that were once
disadvantageous are now favorable.
a) Example
(1) Mutations occur that make HIV resistant to
antiviral drugs.
(2) These same mutations also slow viral
(3) The mutant alleles can be positive or negative for
the success of the organisms depending on the
presence or absence of antivirals.
C. Chromosomal mutations – larger scale (8.23A and
1. A single chromosomal mutation affects many genes
and likely will be harmful
2. Very rarely, a duplication of a chromosomal segment
might bring benefits
a) May provide a larger genome with extra genes that
take on new functions – show protein examples.
D. Microorganisms with very short generation spans –
mutation generates genetic variation rapidly!
1. HIV – generation span of 2 days
a) Produces >1010 new viruses a day in an AIDS
b) Each replication is a chance for mutation!
c) HIV also uses RNA = greater mutation rate (no
d) Result – single drug treatments will not be very
effective against HIV – double drug treatments don’t
work well either – need drug “cocktails”, greater than
2 drugs at once.
2. Bacteria – also multiply rapidly
a) Like viruses, they are generally haploid (one gene
per trait) – newly created allele can have an instant
3. Plants and Animals – typically long generation time
and diploid architecture
a) Most mutations prevented from significantly
affecting genetic variation from one generation to the
b) So how do get our genetic diversity? shuffling
up existing alleles - sexual recombination (Fig. 13.14):
(1) Crossing over
(2) Independent assortment of homologous
(3) Random fertilization
XII. Overview: How natural selection affects variation
A. Why aren’t the less adaptive alleles eliminated
completely by natural selection as the best are passed
B. Natural selection typically strives to reduce genetic
variation in a population – what counters natural selection?
1. Two sets of chromosomes (diploid) – recessive alleles
can hide behind dominant ones (heterozygotes) – if they
do not influence phenotype and thus are not subject to
natural selection.
a) When are recessive alleles subject to natural
b) Individuals who are homozygous recessive may be
selected against in one environment, but selected for
in another.
2. Balanced polymorphism – ability of natural selection to
maintain stable frequencies of two or more phenontypic
forms in a population.
a) Sometime there is a heterozygote advantage –
heterzygotes have a greater reproductive success
compared to homozygotes –
(1) Two or more traits are thus maintained by natural
(2) Ex. Resistance to malaria conferred by the
recessive sickle-cell allele (heterozygotes resistant to
b) Frequency-dependent selection – survival and
reproduction of any one morph declines if that
phenotype form becomes too common.
(1) Ex. A butterfly population has many different
morphs. Birds may more easily locate one morph
over others if it becomes too common allowing other
morphs to catch up.
XIII. Not all genetic variation may be subject to natural
A. Neutral variation – variation in a heritable characteristic
that provides no apparent selective advantage
1. Ex. Fingerprints
2. Some neutral alleles will increase frequency in gene
pools and others decrease due to genetic drift, but are not
touched by natural selection.
3. It is impossible to determine what degree a genetic
variation is neutral – what appears neutral may confer
some advantage or disadvantage that we don’t yet
XIV. Endangered species often have reduced variation
A. Many species are in danger of becoming extinct thank to
our species.
B. Population declines = gene pool declines
1. Ex. Cheetah – fastest land animal (70mph top speed)
2. Thought to have suffered a severe bottleneck from
drought, human hunting, disease – 10,000 yrs ago
3. Only three small cheetah populations exist – East
Africa, South Africa, Northern Iran (number less than 50)
4. African cheetah – 0.04% of gene loci heterozygous
5. Second bottleneck during 19th century – South African
farmers hunted them to near extinction
6. Little genetic variation = reduced capacity to adapt to
ever increasing environmental challenges – read pg 275 –
13.17 last paragraph.
XV. The perpetuation of genes defines evolutionary
A. “survival of the fittest” is highly misleading implying
direct competitive contests b/w individuals in a population.
B. Reproductive success is more subtle and passive
1. Ex. Wing color in moths to hide from predators, plants
and their ability to attract pollinators due to flower color,
shape, fragrance, etc…
C. Darwinian Fitness – the contribution and individual
makes to the gene pool of the next generation relative to
the contributions of other individuals.
1. Thus, fittest individuals are those that pass on the
greatest number of genes to the next generation.
2. Survival alone is not enough. Biggest, fastest,
strongest of any organism has a fitness of ZERO if it is
3. Production of fertile offspring is the only score that
counts in natural selection!
D. Evolutionary fitness has to do with genes, but it is the
phenotype that is exposed to the environment
1. The fitness of any one allele depends on the entire
genetic content –
a) Ex. A gene that enhance tree trunk growth would
be useless in the absence of alleles to enhance root
growth as well – Homerun slugger on a last place
b) Ex. Alleles that are neutral of maladaptive might
get to hang around if they are perpetuated in
individuals with high fitness – whole baseball team
wins world series!
XVI. There are three general outcomes of natural
A. Starting with a hypothetical, bell-shaped, polygenic
population –
1. Stabilizing selection –
a) Favors intermediate variants – reduce phenotypic
b) Typically occurs in a relatively stable environment
c) Likely prevails in most populations most of the
(1) Ex. Human birth weight 6.5 to 9lbs – larger or
smaller increases infant mortality.
2. Directional Selection
a) Shifts overall makeup of population by acting
against individuals at ONE of the phenotypic
b) Common during periods of environmental change
or when members of a species migrate to a new
(1) Ex. Insects exposed to insecticide or HIV exposed
to antivirals.
3. Diversifying selection
a) Occurs when the environment favors individuals at
BOTH extremes
b) Can lead to balanced polymorphism with two or
more contrasting morphs
(1) Ex. California King Snakes
XVII. Sexual selection may produce sexual dimorphism
A. Males and females obviously have different reproductive
B. Secondary Sexual Characteristics - they may also differ
in other aspects not directly related to reproduction
C. Sexual Dimorphism – the resulting distinction in
1. Often results in a size difference
2. Also evident in the form of male adornment (males are
usually the showier sex in vertebrates)
a) Colorful plumage in birds
b) Manes on lions
c) Antlers on deer
D. Sexual Selection – determining of who mates with whom
1. Intrasexual selection – common in species where the
winning male garners a harem of females
a) Secondary sex structures might be used to compete
with members of same sex for mates
(1) Usually ritualized displays
(2) Can by physical combat
2. Intersexual selection – mate choice, more common
a) Individuals of one sex (usually female) select a
(1) Males with the most impressive feathers are most
attractive apparently – ex. Peacock/peahen
(2) Darwin was intrigued because some of the mateattracting features do not seem adaptive otherwise
and in some cases pose risks – make male birds more
visible to predators or less maneuverable.
(3) Regardless, they enhance reproductive success
(Darwinian Fitness)
(4) When a female chooses a mate based on
appearance or behavior, the alleles that caused her to
make those decisions as well as the alleles she found
attractive in the male are perpetuated, and it goes on
and on and on….
Natural selection cannot fashion perfect
A. Why??
1. Organisms are locked into historical constraints –
evolution does not scrap ancestral anatomy and build
new structures from scratch. It modifies existing
structures for new situations.
2. Adaptations are often compromises – organisms must
do many different things. A foot good in water will not be
great on land and vice versa – blue footed booby.
3. Not all evolution is adaptive – chance likely affects
evolution more than we realize. Sometimes good genes
are lost and poor genes are kept.
4. Selection can only edit existing variations – Natural
selection can only work with the variations that it has.
New alleles arise by chance. You can’t just order up some
new alleles for the occasion.
B. Natural selection operates on a “better than” basis…
C. We can see evidence of evolution in an organism’s
subtle imperfections? What are ours?
XIX. The evolution of antibiotic resistance in bacteria is
a serious public health concern
A. Most antibiotics are naturally occurring chemicals found
derived from other microorganisms than the ones they
1. Ex. Penicillin is from a mold (prescribed since the
1940’s) – cured many once fatal infections – strep throat
B. 1950’s some doctors predicted the end of human
infectious diseases!! Why has this not happened?
C. Antibiotics select for resistant bacteria
1. Genes conferring resistance are carried on R plasmids,
which are transferred to offspring and to OTHER bacteria.
D. For almost every antibiotic ever developed, a resist
strain of bacteria has arisen within a few decades.
1. Ex. Multi-drug resistant tuberculosis – resistant to all
three commonly used antibiotics.
E. How do we contribute to this problem??
1. Livestock producers add antibiotics to animal feed as
a growth promoter! – much of the packaged meat in
supermarkets contain antibiotic resistant bacteria.
2. Doctors overprescribe antibiotics
a) To patients with viral infections!!
3. Patients misuse antibiotics
a) Stop taking them early because they feel better –
mutant bacteria that are slower to die by the drug
now multiply and subsequent mutations may lead to
antibiotic resistance.
F. Penicillin was effective against nearly all bacterial
infections in the 1940’s and today is essentially useless in
its original form!!!
G. New drugs have been developed, but they continue to
be rendered ineffective
H. There is a silent war between man and bacterial
evolution with the medical community and pharmaceutical
companies on the front line…

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