Transcript Chapter_13_HB_How_Populations_Evolve

Chapter 13
How Populations Evolve
PowerPoint Lectures for
Biology: Concepts and Connections, Fifth Edition
– Campbell, Reece, Taylor, and Simon
Lectures by Chris Romero
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Clown, Fool, or Simply Well Adapted?
• The blue-footed booby has many specialized
characteristics that are very functional in water
but less useful on land
• Such evolutionary adaptations are inherited
traits that enhance an organism's ability to
survive and reproduce in its particular
environment
• Evolution is the changes in organisms over
time
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First comes love
Then comes baby!
DARWIN'S THEORY OF EVOLUTION
13.1 A sea voyage helped Darwin frame his
theory of evolution
• Pre-Darwinian ideas about the origin of
species
– Early Greek philosophers: Simpler life forms
preceded more complex ones
– Aristotle: Species are fixed and do not
evolve; had a great impact on Western
thinking
– Judeo-Christian biblical view: All species
were individually designed by a divine
creator
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• In the century prior to Darwin, only a few
scientists questioned the belief that species
are fixed
– Buffon: The study of fossils suggested that
Earth is older than 6,000 years, and fossil
forms might be early versions of modern
forms
– Lamarck: Fossils are related to modern
forms because life evolves; acquired
characteristics are inherited
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•Charles Darwin made a round-the-world sea voyage as
a naturalist on HMS Beagle in the 1830s
Great
Britain
Europe
Asia
North
America
PACIFIC
OCEAN
The
Galápagos
Islands
PACIFIC
OCEAN
Pinta
Marchena
Pinzón
Isabela
0
0
40 km
Equator
Daphne
Islands
Santa Santa
Cruz Fe
Florenza
40 miles
Africa
San
Cristobal
Española
PACIFIC
OCEAN
Equator
South
America
Genovesa
Santiago
Fernandina
ATLANTIC
OCEAN
Australia
Cape of
Good Hope
Cape Horn
Tierra del Fuego
Tasmania
New
Zealand
– Darwin observed similarities between living
and fossil organisms and the diversity of life
on the Galápagos Islands
– Darwin's experiences during the voyage
helped him frame his ideas about evolution
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Frigate bird
and
giant tortoise
Marine iguana
Lyell's Principles of Geology led him to realize that
still-operating natural forces gradually change Earth
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• After his return, Darwin began to document his
observations and his new theory of evolution
– Alfred Wallace conceived a theory almost
identical to Darwin's; both works were
presented to the scientific community
– Darwin's On the Origin of Species by Means
of Natural Selection was published in 1859
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• "Descent with modification" summarizes
Darwin's view of life
– All organisms are related through descent
from a remote common ancestor
– Descendants spread into diverse habitats
over millions of years and acquired
adaptations to their environments
– The history of life resembles a tree with
multiple branchings from a common trunk
– Species that are closely related share
characteristics
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13.2 Darwin proposed natural selection as the
mechanism of evolution
• The essence of Darwin's theory of natural
selection is differential success in reproduction
– Organisms produce more offspring than the
environment can support
– Organisms vary in many characteristics that
can be inherited
– Excessive numbers of organisms lead to a
struggle for survival
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– Individuals whose characteristics are best
adapted to their environment are more likely
to survive and reproduce
– The unequal ability of individuals to survive
and reproduce leads to a gradual change in
the characteristics of a population over
generations
• Natural selection is supported by evidence
from artificial selection
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Controlled Breeding
• Artificial selection is selective breeding to
produce plants and animals that possess
desirable traits
• Modern dogs descended from wolves
• In only a few thousand years, humans
artificially selected for all breeds of modern
dogs
LE 13-2c
African wild dog
Coyote
Wolf
Thousands to
millions of years
of natural selection
Ancestral canine
Fox
Jackal
Controlled Breeding
• Humans have created tremendous
variation in several species over relatively
short periods of time through artificial
selection
• Isn’t it plausible that much larger changes
could result from hundreds of millions of
years of natural selection?
13.3 The study of fossils provides strong
evidence for evolution
• Fossils are the hard parts of organisms that
remain after organic materials decay
– Rarely, an entire organism is fossilized
• The fossil record strongly supports the theory
of evolution
– Changes in sea level and drying and
refilling of lakes over time result in rock
strata that trap organisms
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– Fossils appear in an ordered array within
layers of sedimentary rocks
– The fossil record reveals that organisms
have evolved in a historical sequence
• Many fossils link early extinct species with
species living today
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13.4 A mass of other evidence reinforces the
evolutionary view of life
• Biogeography
– The geographic distribution of species
suggested to Darwin that organisms evolve
from common ancestors
– Isolated organisms resemble each other
more than organisms in similar but distant
places
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• Comparative anatomy
– Homologous structures are features that
often have different functions but are
structurally similar because of common
ancestry
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LE 13-4a
Human
Cat
Whale
Bat
– Vestigial structures are remnants of structures that
served important functions in an organism's
ancestors
• Analogous structures are structures that are
outwardly similar in appearance, but differ in
their evolutionary origin
• Analogous structures result from convergent
evolution rather than descent from a common
ancestor
• Convergent evolution occurs when similar
environmental pressures and natural selection
give rise to similar (analogous) structures in
distantly related organisms
• Comparative embryology
– Common embryonic structures in all
vertebrates are evidence for common
descent
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Lemur
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pig
human
Embryology
• All vertebrate embryos possess genes that
direct development of gill slits and a tail
• These genes were inherited from a
common ancestor
Embryology
• Adult fish retain gills and tail because the
genes are active throughout their
embryonic development
• Humans are born without gills and a tail
because the genes are active only during
early embryonic development
LE 13-4b
Pharyngeal
pouches
Post-anal
tail
Chick embryo
Human embryo
• Molecular biology
– Comparisons of DNA and amino acid
sequences between different organisms
reveal evolutionary relationships
– Molecular biology provides strong evidence
that all life forms are related
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Molecular biology
• All organisms share related biochemical
processes:
– All cells use DNA as genetic blueprint
– All use RNA, ribosomes, and approximately
the same genetic code for translation
– All use roughly the same set of 20 amino acids
to build proteins
– All use ATP to transfer energy
Molecular biology
• Striking genetic similarities between
organisms imply evolutionary relatedness
• e.g. the DNA nucleotide sequence of the
human and mouse cytochrome c gene is
very similar, suggesting shared ancestry
Cytochrome C gene-function in ETC
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CONNECTION
13.5 Scientists can observe natural selection in
action
• Examples of evolutionary adaptation observed
over a short time
 Camouflage
 Coloration in Trinidadian guppies
 Pesticide resistance
 Experimental introductions of Anolis sagrei
lizards
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Camouflage
Coloration
• Female guppies prefer to mate with brightly
colored males; however, brightly colored
males are more likely to be eaten by
predators
– Males found in areas lacking predators were
brightly colored
– Males found in areas with predators were duller
by comparison (predators eliminated brightly
colored males before they could reproduce)
Coloration
• Conclusion: When fewer predators are
present, brighter coloration can evolve
• Conclusion was confirmed
– Predators were introduced to previously
predator-free areas (males were brightly
colored)
– Within a few generations male guppies in
those areas evolved to become less colorful
Pesticide Resistance
• Numerous insect pests have evolved
resistance to pesticides
– Roaches developed resistance to Combat®, an
insecticide bait that acted as an agent of
natural selection
– Resistant roaches possessed a rare mutation
that caused them to dislike glucose, the main
attractant in Combat®
LE 13-5b
Chromosome with gene
conferring resistance
to pesticide
Additional
applications of the
same pesticide will
be less effective, and
the frequency of
resistant insects in
the population
will grow
Pesticide application
Survivor
Pesticide Resistance
• At least one insect species is resistant to
every pesticide in existence
Experiments
• Small groups of Anolis sagrei lizards were
introduced onto 14 small Bahamian islands
with thinly-branched bushes and no trees
– Lizards were originally from Staniel Cay, an
island with thickly-branched trees
– Their long legs were adaptive for maneuvering
in these trees
• The introduced lizards thrived and
reproduced
Experiments
• After 14 years, comparisons were made
between lizards on the Bahamian islands
and those of Staniel Cay
• Lizards on all 14 Bahamian islands had
shorter, thinner legs
Experiments
• Conclusion: Individuals with shorter,
thinner legs evolved because they were
able to escape predators better than their
longer-legged ancestors in the new
environment
• Examples of evolutionary adaptation reveal
three key points about natural selection
– Natural selection is more of an editing
process than a creative mechanism
– Natural selection is contingent on time and
place
– Significant evolutionary change can occur in
a short time
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POPULATION GENETICS AND THE MODERN
SYNTHESIS
• 13.6 Populations are the units of evolution
– Population
• A group of individuals of the same species
living in the same place at the same time
• May be isolated from other groups or
concentrated
• The smallest unit that can evolve
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• Population genetics
– Combines Darwin's and Mendel's ideas in
studying how populations change
genetically over time
– The modern synthesis
• Connects population genetics with other
sciences
• Focuses on population as the unit of
evolution and central role of natural
selection
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• Studying evolution at the population level
– Evolution: change in the prevalence of
certain heritable characteristics in a
population over a span of generations
– Gene pool: the total collection of genes in a
population at any one time
– Microevolution: a change in the relative
frequencies of alleles in a gene pool
– Species: a group of populations capable of
interbreeding and producing fertile offspring
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13.7 The gene pool of a nonevolving population
remains constant over the generations
• In a nonevolving population, the shuffling of
alleles that accompanies sexual reproduction
does not alter the genetic makeup of the
population
• In Hardy-Weinberg equilibrium, the frequency
of each allele in the gene pool will remain
constant unless acted upon by other agents
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• For a population to be in Hardy-Weinberg
equilibrium, it must satisfy five main conditions
– The population is very large
– The population is isolated
– Mutations do not alter the gene pool
– Mating is random
– All individuals are equal in reproductive
success
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• The Hardy-Weinberg conditions are rarely met
in nature
– We can follow alleles in a population to
observe if Hardy-Weinberg equilibrium
exists
– Hardy-Weinberg equilibrium provides a
basis for understanding how populations
evolve
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LE 13-7a
Webbing
No webbing
LE 13-7b
Phenotypes
Genotypes
WW
Ww
ww
Number of animals
(total = 500)
320
160
20
Genotype frequencies
320
500
160
500
= 0.64
Number of alleles
in gene pool
(total = 1,000)
640 W
Allele frequencies
800
1,000
= 0.32
160 W + 160 w
= 0.8 W
200
1,000
20
500
40 w
= 0.2 w
= 0.04
LE 13-7c
Recombination
of alleles from
parent generation
Sperm
W sperm
p = 0.8
w sperm
q = 0.2
WW
= 0.64
Ww
pq = 0.16
p2
W egg
p = 0.8
Eggs
w egg
q = 0.2
wW
qp = 0.16
q2
ww
= 0.04
Next generation:
Genotype frequencies
Allele frequencies
0.64 WW
0.8 W
0.32 Ww
0.04 ww
0.2 w
CONNECTION
13.8 The Hardy-Weinberg equation is useful in
public health science
• Public health scientists use the HardyWeinberg equation to estimate frequencies of
disease-causing alleles in the human
population
– Example: phenylketonuria (PKU)
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13.9 In addition to natural selection, genetic drift
and gene flow can contribute to evolution
• Genetic drift: change in the gene pool of a
population due to chance
– Can alter allele frequencies in a population
– The smaller the population, the greater the
impact
• Bottleneck effect: an event that drastically
reduces population size
• Founder effect: colonization of a new
location by a small number of individuals
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LE 13-9a
Original
population
Bottlenecking
event
Surviving
population
• Gene flow: the movement of individuals or
gametes between populations
– Can alter allele frequencies in a population
– Tends to reduce differences between
populations
• Natural selection
– Best-adapted individuals have the most
reproductive success
– Results in accumulation of traits that adapt
a population to its environment
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CONNECTION
13.10 Endangered species often have reduced
variation
• Loss of genetic variability due to bottlenecking
may reduce a population's ability to adapt to
environmental change
– Particularly threatening to endangered
species such as the cheetah
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VARIATION AND NATURAL SELECTION
13.11 Variation is extensive in most populations
• Individual variation exists in all sexually
reproducing populations
• Heritable variation results from a combination
of genotype and environmental influences
– Polymorphism: two or more forms of
phenotypic characteristics
– Geographic variation: variation of an
inherited characteristic from place to place
• May occur along a geographic continuum (a
cline)
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13.12 Mutation and sexual recombination
generate variation
• Mutations-changes in the nucleotide sequence
of DNA-can create new alleles
– Only mutations in cells that produce
gametes can affect a population's gene pool
– A mutation may rarely improve adaptation
to the environment and thus contribute to
evolution
• Sexual recombination generates variation by
shuffling alleles during meiosis
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LE 13-12a
A1
Parents
A2
A1
A3

Meiosis
A1
Gametes
A2
A3
LE 13-12b
A1
A2
A3
Gametes
Fertilization
Offspring,
with new
combinations
of alleles
A1
A1
A2
and
A3
CONNECTION
13.13 The evolution of antibiotic resistance in
bacteria is a serious public health concern
• Natural selection has led to the evolution of
antibiotic-resistant bacteria
• Overuse and misuse of antibiotics has
contributed to the proliferation of antibioticresistant strains
– Example: tuberculosis
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13.14 Diploidy and balancing selection preserve
variation
• Diploidy (two sets of chromosomes) helps to
prevent populations from becoming genetically
uniform
– Recessive alleles are "hidden" from natural
selection and remain in the population
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• Balancing selection allows two or more
phenotypic forms in a population
– Balanced polymorphism may result from
• Heterozygote advantage; example: sicklecell disease
• Frequency-dependent selection
• Neutral variation provides no apparent
advantage or disadvantage
– Example: fingerprints
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13.15 The perpetuation of genes defines
evolutionary fitness
• Evolutionary fitness is the relative contribution
an individual makes to the gene pool of the
next generation
• Survival of genes depends on production of
fertile offspring
• Selection indirectly adapts a population to its
environment by acting on phenotype
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13.16 Natural selection can alter variation in a
population in three ways
• Stabilizing selection: favors intermediate
phenotypes
• Directional selection: acts against individuals
at one of the phenotypic extremes
• Disruptive selection: favors individuals at both
extremes of the phenotypic range
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Frequency of individuals
LE 13-16
Original
population
Phenotypes (fur color)
Original
population
Evolved
population
Stabilizing selection
Directional selection
Disruptive selection
13.17 Sexual selection may produce sexual
dimorphism
• Sexual dimorphism
– The distinction in appearance between
males and females of a species
• Sexual selection
– The determining of "who mates with whom"
– Leads to the evolution of secondary sexual
characteristics that may give individuals an
advantage in mating
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13.18 Natural selection cannot fashion perfect
organisms
• There are at least four reasons why natural
selection cannot produce perfection
– Organisms are limited by historical
constraints
– Adaptations are often compromises
– Chance and natural selection interact
– Selection can only edit existing variations
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