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Chapter 15.1 Darwin’s Theory of
Evolution by Natural Selection
Section 1 Main Idea: Charles Darwin developed a theory of evolution based on
natural selection.
Developing the Theory of Evolution:
Darwin on the HMS Beagle & The Galápagos Islands
• When Darwin began his journey, most people thought the world was ~6000
years old, and that animals and plants were unchanging.
• Darwin’s job on the Beagle was to be a naturalist, and to collect biological
and geological specimens during the travels.
• Darwin noticed that the different islands all seemed to have their own, slightly
different varieties of animals.
• Almost every specimen Darwin collected on the Galápagos was new to
European scientists, though they were similar to species on mainland South
America.
• Island and mainland species should not have been so similar, unless mainland
species had changed since arriving on the islands.
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Essential Questions
Answer these essential questions to
better understand the concepts in this
chapter.
What evidence
convinced Darwin that
species could change
over time? _________
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What are the four
principles of natural
selection?__________
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How can natural
selection change a
population? ________
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Vocabulary
Developing the Theory of Evolution: Darwin continued his studies
• Darwin hypothesized that new species could appear gradually through small
changes in ancestral species.
• Darwin inferred that if humans could change species by artificial selection –
directed breeding to produce offspring with desired traits, then perhaps the
same process could work in nature.
• Darwin described the process of natural selection: some competitors would be
better equipped for survival than others, those less equipped would die.
• Principles of natural selection:
• Individuals show variation
• Variations are heritable
• More offspring are born than can survive
• Variations that increase reproductive success will be more common in the
next generation
Selective Breeding
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Artificial Selection
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Natural Selection
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Evolution
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The Origin of Species
• Darwin published On the Origin of Species by Means of Natural Selection in
1859.
• Today, scientists use evolution to mean cumulative change in a group of
organisms through time.
• Natural selection is not synonymous with evolution – it is a mechanism by
which evolution occurs.
Chapter 15.2 Evidence of Evolution
Essential Questions
Section 1 Main Idea: Multiple lines of evidence support the theory of evolution.
How do fossils
provide evidence of
evolution? _________
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Support for Evolution
• The theory of evolution states that all organisms on Earth have descended
from a single ancestor.
• Recall that theories provide explanations for natural phenomena based on
observation.
• Darwin’s On the Origin of Species demonstrated how evolution might
happen, and evidence of its occurrence.
• Evidence for evolution comes from: the fossil record, comparative anatomy,
comparative embryology, comparative biochemistry, and geographic
distribution.
Support for Evolution: The fossil record
• Fossils show modern species resemble ancient species.
• They also reveal that some species have changed very little.
• The fossil record is an important source of information for determining the
ancestry of organisms and the patterns of evolution.
• Darwin predicted the existence of fossils intermediate in form between
species, such as Archaeopteryx.
• Researchers consider two major classes of traits when studying transitional
fossils:
• Derived traits are newly evolved features, such as feathers, that do
not appear in the fossils of common ancestors.
• Ancestral traits are more primitive features, such as teeth and tails,
that do appear in ancestral forms.
How does morphology
provide evidence of
evolution? _________
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How does biochemistry provide
evidence of evolution?
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Vocabulary
Fossil_____________
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Derived Trait_______
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Support for Evolution: Comparative anatomy
• Anatomically similar structures inherited from a common ancestor are called
homologous structures.
• Evolution predicts that an organism’s body parts are more likely to be
modifications of ancestral body parts than entirely new structures.
• Vestigial structures are structures that are the reduced forms of functional
structures in other organisms.
• Evolutionary theory predicts that features of ancestors that no longer have a
function for that species will become smaller over time until they are lost.
Ancestral Trait______
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Homologous Structure
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Support for Evolution: Comparative anatomy
• Analogous structures can be used for the same purpose and be superficially
similar in construction, but are not inherited from a common ancestor.
• Analogous structures show that functionally similar features can evolve
independently under similar conditions.
• An embryo is an early, pre-birth stage of an organism’s development.
• Vertebrate embryos exhibit homologous structures during phases of
development that become totally different structures in the adult forms.
Support for Evolution:
Comparative biochemistry
• Common ancestry can be seen
in the complex metabolic
molecules that many different
organisms share.
• The more closely related
species are to each
other, the greater the
biochemical similarity.
Support for Evolution:
Geographic distribution
• The distribution of
plants and animals
were what first
suggested evolution
to Darwin.
• The distribution of
plants and animals
around the world is studied in the field of biogeography.
• Evolution is linked to migration patterns, climate,
and geological forces (such as plate tectonics).
Adaptation: Types of adaptations
• An adaptation is a trait shaped by natural selection that increases an
organism’s reproductive success.
• Fitness is a measure of the relative contribution an individual trait makes to
the next generation.
• The better an organism is adapted to its environment, the greater its chances
of survival and reproductive success.
• Camouflage is a suite of morphological adaptations that allow an organism to
blend into its environment.
• Mimicry is a type of morphological adaptation where a species evolves to
resemble another species.
• Antimicrobial resistance develops in some bacteria in response to sub-lethal
exposure to antibiotics.
• Not all features of an organism are necessarily adaptive.
• Some features are consequences of other evolved characteristics.
• Helplessness of human babies: humans give birth at a much early
developmental stage than other primates.
• May be a consequence of larger brain size and upright posture
Vestigial Structure
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Analogous Structure
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Embryo ___________
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Biogeography_______
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Fitness ____________
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Camouflage ________
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Mimicry___________
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Chapter 15.3 Shaping
Evolutionary Theory
Section 1 Main Idea: The theory of evolution continues to be refined as
scientists learn new information.
Mechanisms of Evolution
• Natural selection is not the only mechanism of evolution.
• Evolution occurs at the population level, with genes as the raw material.
Mechanisms of Evolution: Population genetics
• Hardy and Weinberg showed that evolution will not occur in a population
unless allelic frequencies are acted upon by forces that cause change.
• Hardy-Weinberg principle states that when allelic frequencies remain
constant, a population is in genetic equilibrium.
• The Hardy-Weinberg principle can be numerically represented as:
• Where:
• p2 is the proportion of homozygous dominant
• 2pq is the proportion of heterozygous
• q2 is the proportion of homozygous recessive
• For a population to be in genetic equilibrium according to the HardyWeinberg principle, it must meet five conditions: 1) No genetic drift
2) No gene flow 3) No mutation 4) Mating must be random 5) No
natural selection
• These five conditions are
the mechanisms of
evolutionary change.
Mechanisms of Evolution:
Genetic drift
• Any change in the allelic
frequencies in a population
that results from chance
is called genetic drift.
• In smaller populations, the effects of genetic drift
become more pronounced, and the chance of losing an allele becomes greater.
• The founder effect results when a group of individuals with a different allele
frequency than the original population becomes isolated.
• Alleles that were infrequent in the original population may be common in the
new population.
• Happens when a subset of organisms settles in an area separated from their
original population
• Can result in large genetic variations in the separated population
Mechanisms of Evolution: Genetic drift
• A bottleneck results when population declines to a very low level and then
rebounds.
• The gene pool of the rebound population is similar to the low-level
population, which may have reduced diversity.
Essential Questions
What are the
conditions of the
Hardy-Weinberg
principle?__________
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What patterns can be
observed in evolution?
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What factors influence
speciation?_________
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Vocabulary
Allele_____________
Hardy-Weinberg
principle
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genetic drift
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founder effect
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bottleneck
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stabilizing selection
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directional selection
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Mechanisms of Evolution: Gene flow
• A population in genetic equilibrium experiences no gene flow – it is a
closed system.
• Uncommon in natural populations – organisms migrate/move between
populations
• Random movement increases genetic variation within a population and
decreases the genetic variation between populations.
Mechanisms of Evolution: Nonrandom mating & Mutation
• A population in genetic equilibrium must exhibit random mating.
• This rarely occurs in populations – mating occurs between individuals in close
proximity promotes inbreeding
• Changes allelic frequencies, favoring individuals that are homozygous
• A mutation is a random change in genetic material.
• The cumulative effect of mutations in a population might shift allelic
frequencies.
• Most mutations are harmful, but some may
be beneficial and become more common.
Mechanisms of Evolution: Natural selection
• Natural selection acts to select the
individuals that are best adapted
for survival and reproduction.
• Three types of natural selection:
1) Stabilizing
2) Directional
3) Disruptive
• Stabilizing selection operates to eliminate
extreme expressions of a trait when the
average expression leads to higher fitness.
• Directional selection increases the
expression of an extreme version of a trait and increases fitness.
• Disruptive selection removes individuals with average traits, but retains
individuals expressing extreme traits.
• Sexual selection drives change in the frequency of a trait based on the ability
to attract a mate.
• Common in populations where males and females look significantly different
• Some qualities that enhance mating success reduce odds of survival.
Reproductive Isolation
• Most scientists define speciation as the process whereby a population of
sexually reproducing organisms changes so much that it can no longer
produce fertile offspring with other populations.
• Two types of reproductive isolation mechanisms prevent gene flow:
Prezygotic and Postzygotic.
Reproductive Isolation: Prezygotic isolation & Postzygotic isolation
• Prezygotic isolating mechanisms operate before fertilization occurs.
• Prevent reproduction by making fertilization unlikely
• Often occur through geographic, ecological, or behavioral differences.
• Postzygotic isolating mechanisms operate after fertilization and ensure that
the resulting hybrid remains infertile.
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disruptive selection
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sexual selection
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prezygotic isolating
mechanism
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postzygotic isolating
mechanism
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allopatric speciation
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sympatric speciation
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adaptive radiation
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gradualism
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punctuated
equilibrium
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Reproductive Isolation: Prezygotic isolation & Postzygotic isolation
• Hybrid offspring either cannot develop or cannot reproduce.
Speciation: Allopatric and
Sympatric speciation
• For speciation to occur, populations
must diverge and become
reproductively isolated.
• Two types of speciation:
allopatric and sympatric.
• Allopatric speciation occurs when
populations are divided by a
physical barrier.
• Most scientists think allopatric speciation
is the most common type of speciation.
• Sympatric speciation occurs without
a physical barrier; populations
remain in close proximity.
• Common in plants due to polyploidy
Patterns of Evolution: Adaptive radiation
• Adaptive radiation takes place when
a large number of species arise from
a single common ancestor in response
to an ecological opportunity.
• Often follows large-scale extinction
events or creation of new habitat
Patterns of Evolution: Coevolution
• Coevolution occurs when
relationships between species
are so close that they influence
each other’s evolution.
• Mutualism – both species benefit
• Coevolutionary arms race –
parasitic or predatory relationship
Patterns of Evolution: Convergent evolution
• Unrelated species evolve similar traits even though they live in different parts
of the world.
• Occurs in environments that are geographically distinct but ecologically
similar
Patterns of Evolution: Rate of speciation
• Evolution proceeds in small, gradual steps according to a theory called
gradualism.
• Punctuated equilibrium states rapid spurts of genetic change causing rapid
speciation punctuate long periods of little change.