How Populations Evolve
Chapter 7
Natural Selection?
• On the following three slides, you will
read statements based on student ideas
about now natural selection works. Use
your clickers to vote for whether you
think that the statement IS or is NOT
what scientists mean by “natural
selection.”
“ Our little toes aren’t necessary for walking. Because of this,
some people think that they will get smaller each generation
and eventually disappear from lack of use over time.”
50%
50%
1. This IS what
scientists mean by
natural selection
2. This is NOT what
scientists mean by
natural selection
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2
“Dark-colored grasshoppers resting on pale, dry grass stalks
are more easily seen by predators than light-colored
grasshoppers of the same species. Because the dark-colored
grasshoppers have higher mortality, there will be fewer darkcolored grasshoppers each generation”
50%
50%
1. This IS what
scientists mean by
natural selection
2. This is NOT what
scientists mean by
natural selection
1
2
“The flowering shrub called Hydrangea has flowers that are
blue if the shrub is growing in acidic soil. If the same shrub is
transplanted into alkaline soil, its blossoms will turn pink.”
50%
50%
1. This IS what
scientists mean by
natural selection
2. This is NOT what
scientists mean by
natural selection
1
2
How Selection Works
1. Variation Exists
• All populations vary
as the result of the
accumulation of
small, random
mutations over
many generations.
2. Inheritance of traits
• Inheritable traits
(those coded for by
genes) are passed
directly to the
offspring from the
parents through
genetic
information.
3. Differential Survival
• More offspring are
born than can
survive. Many
offspring die young.
Those with traits best
suited to the
environment are
more likely, though
not guaranteed, to
survive.
4. Differential Reproduction
• Some survivors fail
to reproduce.
Some have traits
that better insure
reproduction than
others.
5. Differential Inheritance
• Survivors that
reproduce pass
some of their traits
on to their offspring.
Those with
favorable traits may
pass those
favorable traits on
— or not.
• People who work with feral cats have
noticed that when cats are abandoned to
the wild and have kittens, over several
generations the kittens are more and
more feral and more difficult to tame.
Use what you have learned about natural
selection to develop a brief explanation
for this. Talk to your neighbor about your
ideas.
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Natural Selection in Cats
• Traits that ensure a
domestic cat’s
survival among
humans:
• Endearing
qualities: purring,
snuggling, being
playful, cuteness.
• Helpful qualities:
rodent control.
• When domestic cats are
abandoned:
• Most die within two weeks
from starvation, accident, or
predation. The traits that
help them in a domestic
setting are useless in the
wild.
• The few that survive have
the strongest feral instincts.
• Av. feral lifespan: 2-4 years
Selection’s Effects
Generation 1
Generation 2
Generation 3
As time passes...
• With each generation,
domestic traits are
selected against, while
feral traits are favored.
• After several
generations, even
kittens that are
captured young can be
challenging to tame.
Which of these is true?
1. Nature gives
abandoned cats the
traits that they need
to live in the wild.
2. Only those few cats
with traits that help
them avoid
predators and catch
prey will survive and
have offspring.
50%
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50%
2
Genes and Evolution
• Genes are the units of
heredity.
• Genes are segments of DNA
that code for proteins, which
result in our set of traits.
• Genes are passed from
parent to offspring through the
sex cells.
• Different “versions”
of genes are alleles.
• Dominant alleles are
expressed in the
phenotype
(expressed trait)
even if only one
copy is inherited.
• Recessive alleles
are expressed only if
two copies are
inherited.
“Genotype” is a description of the
alleles for a given trait in an
individual: BB, Bb, or bb
Genetic basis of inheritance
• Genotype - the genetic
composition of an organism
Allele - the
specific form of the gene
• Phenotype -
the outward expression of
genotype
• Gene pool -
the total of all alleles for
all genes in a population
BEY2 gene – controls
pigmentation in the eye
You’re probably familiar with
genetics in terms of following
traits passed from parent to
offspring and predicting the
outcome.
Blackfin shark genotypes in the Gulf of Mexico.
http://www.zoology.siu.edu/heist/blacktip.htm
But how do we track traits
passed from generation to
generation across an entire
population? How does
population genetics work?
The Gene Pool Concept
• The “gene pool” of a population is the
entire collection of alleles for a given trait
throughout a given population.
• The word for all genes for all traits in an
individual or population is genome.
Genetic change
Mutation, genetic
drift, and natural
selection will, over
time, change the
genetics of both
populations.
Genetic change
Causes of Genetic Change in a
Population
Hardy-Weinberg Equilibrium
• Allele ratios in a gene pool will NOT change
from generation to generation (that is, no
evolution) ONLY if ALL of these things are
true:
• No mutation
• Large population
• No migration
• No selection
• Random mating
Mutations
• Small mutations appear randomly in
populations.
• The appearance of mutations changes
allele ratios by
• “breaking” functional alleles (as in genetic
disorders)
• adding new alleles
• Copy errors are rare,
but given that genes
are copied millions of
times in a lifetime,
errors can occur.
• Mutagens in the
environment can
increase the error rate.
• Small errors can create
new alleles.
Population size
• In large populations, random events have a
very small effect.
• In small populations, because fewer
individuals have any given trait, random
events can have a larger effect. Changes in
gene ratios caused by random events are
called “genetic drift.”
A population bottleneck is genetic drift.
Founder effect is genetic drift.
What does Founders Effect do to the
gene pool of the new population?
1. Improves it by
weeding out bad
genes.
2. Creates new traits
that the new
population will need.
3. Makes it much
smaller with less
genetic variation.
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33%
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3
Why is a genetic bottleneck of concern to
wildlife biologists bringing an endangered
species back from near extinction?
1. Small populations
have more
mutations.
2. The small gene pool
increases inbreeding
and preservation of
bad traits.
3. Much harder to find
survivors for
breeding programs.
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33%
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Migration
• Migration into and out of a population can
change gene ratios.
• Immigrants can bring in new mutations, or
a different ratio of alleles.
• Emigrants may take away a high
proportion of a certain allele.
• Small population are more affected than
large populations.
Migration can lead to
Founders Effect:
Surtsey, Iceland
Surfaced 14 Nov 1963
Moss: 1967
Puffin nests:
2004
Selection
• Selection may increase or
decrease the frequency of
certain alleles:
• Directional selection:
favors one end of a
range over another.
• Disruptive selection:
disfavors the midrange.
• Stabilizing selection:
favors the mid-range.
Natural selection:
Differential survival.
Peppered moth
Changes allele frequency.
Which of these is true?
1. Predation caused the
Peppered Moths to
develop new genes
that they needed to
survive.
2. Predation changed
the gene frequencies
already existing in
the population.
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2
The phenotype on which selection
acted in the Moth example was:
1.
2.
3.
4.
5.
Predation
Level of pollution
Size of trees
Number of trees
Coloration pattern of
moths
80%
18%
2%
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0%
3
0%
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5
The abiotic variable that influenced
natural selection in moths was:
65%
1.
2.
3.
4.
5.
Predation
Pollution level
Size of trees
Number of trees
Coloration pattern on
moths.
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2%
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2
3
0%
4
2%
5
The biotic variable that influenced
selection in moths was:
1.
2.
3.
4.
5.
Predation
Level of pollution
Size of trees
Number of trees
Coloration pattern on
moths.
83%
4%
1
2
8%
6%
0%
3
4
5
Mating Behavior
• Mate choice among most organisms is
selective, not random.
• Sexual selection may favor traits that
are in conflict with natural selection. For
example, bright-colored male guppies
attract more females, but are also more
visible to predators.
Sexual selection: competition to mate
Widow bird (text E1-2 pg 13)
We can measure phenotype
Number of males with different tail lengths in a population of redcollared widowbirds. Data based on Pryke and Andersson 2002.
Selection acts directly on…
38%
1.
2.
3.
4.
Genotype
Phenotype
Environment
Depends on if it is
natural or sexual
selection.
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Antibiotic Resistance
• Antibiotic resistance has been an
increasing problem since the 1970s.
• How does natural selection contribute to
the rise in antibiotic-resistant bacteria?
(Note it is bacteria that become resistant,
not people. Bacteria do not become
“immune” — they do not have immune
systems.)
Which of these is true?
1. Antibiotics cause
bacteria to develop
immunity.
2. Antibiotics kill the leastresistant bacteria.
3. Antibiotics cause people
to resist their effects.
4. Antibiotics create new
mutations in bacteria
that cause resistance.
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• Use the principles of natural selection to
explain antibiotic resistance. Be sure to
include these in your answers:
• Variation in the original population.
• Differential survival.
• Differential reproduction
• “Need,” “purpose,” and “immune” should
not be in your answer!
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Helicobacter pylori is a bacteria that causes
stomach ulcers.
Most H. pylori bacteria,
when encountering an
antibiotic, metabolize it
with an enzyme that
turns it to a toxin.
A few H. pylori bacteria
have a mutation that
interferes with the
production of the
enzyme. The mutant
bacteria aren’t affected
by antibiotics.
• Which of these explanations correctly uses the
Theory of Natural Selection to explain
antibiotic resistance in H. pylori?
• The antibiotic caused the mutation that
made the bacteria resistant.
• The bacteria needed to resist the antibiotic,
so they adapted by changing their enzyme.
• Those bacteria already in the population
with the mutation were able to survive the
antibiotic, so those were the bacteria that
reproduced.
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Using antibacterial hand soap
many times each day is:
1. A good idea because it kills
all bacteria.
2. A good idea because it kills
both bacteria and viruses.
3. A bad idea because it
causes mutations in
bacteria.
4. A bad idea because it
serves as a selective factor
for resistant bacteria.
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Evolution Happens
• Because perfect Hardy-Weinberg
equilibrium is never met with in nature,
all populations experience small shifts in
gene ratios with each generation.
• Gene ratio shifts may fluctuate with
cyclical changes in climate. Long-term
changes in habitat (such as global
climate change) can shift the gene ratios
far enough to bring about speciation.
• Choose ONE of these and explain how a
knowledge of mechanisms of evolution could
be useful to:
• A public health official dealing with an
outbreak of pneumonia in a homeless
population.
• A zoo that wants to start an endangered
species breeding program.
• A Fish and Wildlife zoologist who is
concerned about the decline in size and
health of bull elk over the last 50 years.
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Recap
• Natural Selection is a phenomenon that
can be studied directly.
• Natural Selection causes change in the
genetics of a population over
generations (evolution).
• Other factors that can change genetics
of a population include migration, sexual
selection, mutations, and effects of
random events in small populations.