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BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
CHAPTER 13
How Populations Evolve
Modules 13.4 – 13.12
From PowerPoint® Lectures for Biology: Concepts & Connections
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Natural Selection
• Darwin observed that
– organisms produce more offspring than the
environment can support
– organisms vary in many characteristics
– these variations can be inherited
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Natural Selection
• Darwin concluded that individuals best
suited for a particular environment are
more likely to survive and reproduce
than those less well adapted
• Aka: survival of the fittest
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• Darwin saw natural selection as the basic
mechanism of evolution
– As a result, the proportion of individuals with
favorable characteristics increases
– Populations gradually change in response to the
environment
– Phenotypes that are better reproduce more,
eventually, better genotypes become more
common.
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• Darwin also saw that when
humans choose organisms
with specific characteristics as
breeding stock, they are
performing the role of the
environment
– This is called artificial
selection
– Example of artificial
selection in plants: five
vegetables derived from
wild mustard
Figure 13.4A
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– Example of artificial selection in animals: dog
breeding
German shepherd
Yorkshire terrier
English springer
spaniel
Mini-dachshund
Golden retriever
Hundreds to
thousands of years
of breeding
(artificial selection)
Ancestral dog
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Figure 13.4B
• These five canine species evolved from a
common ancestor through natural selection
African wild
dog
Coyote
Fox
Wolf
Jackal
Thousands to
millions of years
of natural selection
Ancestral canine
Figure 13.4C
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13.6 Populations are the units of evolution
• A species is a group of populations whose
individuals can interbreed and produce fertile
offspring
– People (and animals) are more likely to choose
mates locally.
Figure 13.6
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13.7 Microevolution
• A gene pool is the total collection of genes in a
population at any one time
• Microevolution is a change in the relative
frequencies of alleles in a gene pool
• New mutations are constantly being generated
in a gene pool, by accident or as a response to
environmental changes.
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What causes evolution?
• Genetic drift
• Bottleneck Effect
• Founder Effect
• Gene Flow
• Mutation
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Genetic Drift
• Genetic drift is a
change in a gene pool
due to chance
– Genetic drift can
cause the bottleneck
effect: an event that
drastically reduces
population size
(fire, flood,
earthquake)
Original
population
Bottlenecking
event
Surviving
population
Figure 13.11A
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Genetic drift…
• If a population is very diverse and something
bad happens, at least a few individuals will
survive.
• These individuals will then reproduce and the
species will evolve, or change.
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– The founder effect is when some individuals
leave a population and start living somewhere
new.
– Only a few people or animals leave, and the new
population will be closely related to due lack of
genetic diversity.
Figure 13.11B, C
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• Gene flow can change a gene pool due to the
movement of genes into or out of a population
(new organisms move in or leave)
• Mutation changes alleles, these are random
changes in DNA that can create new proteins or
new characteristics.
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Natural Selection
• Natural selection results in the accumulation of
traits that adapt a population to its environment
– If the environment should change, natural
selection would favor traits adapted to the new
conditions
– Organisms with helpful traits survive and
reproduce, and these traits are passed on to
offspring.
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13.8 Hardy-Weinberg Equilibrium
• The Hardy-Weinberg equation shows that in
a changing environment, evolution always
happens.
• Hardy-Weinberg equilibrium states that the
shuffling of genes during sexual reproduction
does not alter the proportions of different
alleles in a gene pool
• Populations are always evolving and not
usually in equilibrium.
Figure 13.8A
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13.10 Five conditions are required for HardyWeinberg equilibrium
• If all 5 of these conditions are met, evolution
will not happen in nature:
• 1. The population is very large
• 2. The population is isolated
• 3. Mutations do not alter the gene pool
• 4. Mating is random
• 5. All individuals are equal in reproductive success
• ***This does not happen in nature!
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The Equation (yes you have to do math)
• p2 + 2pq + q2 = 1
• p+q=1
• p = frequency of the dominant allele in the population (A)
q = frequency of the recessive allele in the population (a)
p2 = percentage of homozygous dominant individuals (AA)
q2 = percentage of homozygous recessive individuals (aa)
2pq = percentage of heterozygous individuals (Aa)
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Practice Problem: p2 + 2pq + q2 = 1
p+q=1
• You have sampled a population in which you know that the percentage
of the homozygous recessive genotype (aa) is 36%. Using that 36%,
calculate the following:
• The frequency of the "a" allele.
• The frequency of aa is 36%, which means that q2 = 0.36. If q2
= 0.36, then q = 0.6. Since q equals the frequency of the a
allele, then the frequency is 60%
• The frequency of the "A" allele.
• Since q = 0.6, and p + q = 1, then p = 0.4; the frequency of A is
by definition equal to p, so the answer is 40%
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• The frequencies of the genotypes "AA" and "Aa."
• The frequency of AA is equal to p2, and the
frequency of Aa is equal to 2pq. So, using the
information above, the frequency of AA is 16% (i.e.
p2 is 0.4 x 0.4 = 0.16) and Aa is 48% (2pq = 2 x 0.4 x
0.6 = 0.48).
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Speciation
• Speciation: is the evolutionary process by
which species arise.
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How new species form
• Stabilizing selection: average individuals are
favored. Ex: average sized spiders
• Directional selection: one trait is more likely to
reproduce than another. Ex: long beaked
woodpeckers
• Disruptive selection: extreme traits are more
likely to survive and reproduce. Ex: black
moths
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How new species form
• Geographic isolation: physical barriers divide a
population (Ex. Grand Canyon)
• Reproductive isolation: can no longer mate and
produce fertile offspring (ex. CA salamanders)
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Gradualism vs. Punctuated
• How fast does evolution go?
• Gradualism says that it happens slowly and
steadily over time
• Punctuated equilibrium says that it happens
quickly in rapid bursts
• There are examples of both!
• Gradualism: dinosaurs into modern birds
• Punctuated: finches in the Galapagos and
extreme weather (Grant study)
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Speciation
• 2 animals cannot produce viable, fertile
offspring if they are from different species.
• This is because of either pre-zygotic barriers
which prevent the animals from ever meeting
and reproducing, or post-zygotic barriers that
prevent a zygote from developing into a fertile
adult.
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Pre-Zygotic Barriers
• 1. Habitat isolation: 2 species live in different
habitats and don’t see each other
• 2. Temporal isolation: 2 species breed at different
times of the day or year
• 3. Behavioral isolation: courtship rituals are
species specific, and don’t attract other species
• 4. Mechanical isolation: the sexual pieces don’t fit
together
• 5. Gametic isolation: even if sex occurs, the egg
and sperm can’t meet
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Post-zygotic barriers
• 1. Reduced hybrid viability: hybrid babies die
before reaching reproductive age
• 2. Reduced hybrid fertility: hybrid babies are
sterile and can’t reproduce
• 3. Hybrid breakdown: first generation babies
can breed, but their children are infertile (F2)
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Speciation
• There are a few exceptions, called hybrids:
– Lion + tiger = liger (the grandchildren can’t
reproduce)
– Horse + donkey = mule (all sterile)
– Polar bear + grizzly bear = pizzly (too soon to
tell if they are fertile or not)
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The Results of Evolution
• Earth began 4.6 billion years ago.
• First, there were only bacteria cells.
• These evolved into eukaryotic multi celled simple animals.
• The first animals lived in the ocean and evolved into fish.
• Lungfish were able to survive on land and evolved into
amphibians and reptiles.
• Reptiles evolved into birds and small mammals.
• Small rodent like mammals evolved into lemurs, which then
evolved into apes, monkeys, and humans.
• Apes and monkeys are our close cousins, and we share a
common ancestor.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings