Transcript Genetic drift

BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
Evolution
Lesson I
Modules 13.1 – 13.3
From PowerPoint® Lectures for Biology: Concepts & Connections
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My intro
• Feelings/beliefs/AP Bio
• Timeline of Earth
• How it went (in general)
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Evolution
• Evolution is genetic change in a population
over time.
• Charles Darwin was the first scientist to
propose the theory of evolution, in 1859.
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EVIDENCE OF EVOLUTION
13.1 The Voyage of the Beagle
• Darwin was born in 1809.
• In 1831, he was on a boat that
was mapping coastlines, the
HMS Beagle.
• He studied plants and animals
on the Galapagos Islands.
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• Darwin observed:
– similarities between living and fossil organisms
– the diversity of life on the Galápagos Islands,
such as finches (birds) and giant tortoises
Figure 13.1A
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• Darwin concluded that:
– The Earth was old and constantly changing (4.6
billion years old)
– Living things also change (evolve) over
generations.
– Living things are related to animals and plants
that used to exist but are now extinct.
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Evidence for Evolution
• Fossils
• Biogeography
• Comparative Anatomy
• Comparative Embryology
• Molecular Biology
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13.2 Fossils
• Fossils are the preserved
remains of dead organisms.
• They show how life has
changed over time.
• Examples:
– Hominid skull: an early
relative
– Petrified trees: trees turned to
Figure 13.2A, B
stone
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– Ammonite casts: 375
million year old
aquatic organisms
– Fossilized organic
matter in a leaf:
molecular and
cellular structures
are preserved.
Figure 13.2C, D
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– Scorpion in amber:
30 million years old,
intact DNA
– “Ice Man”: 5,000
years old, cells and
DNA preserved.
Figure 13.2E, F
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• The fossil record shows that
organisms have appeared in a
historical sequence
• Many fossils link
early extinct species
with species living
today
Figure 13.2G, H
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Biogeography
• Biogeography is the
geographic distribution
of species (where
animals live).
• Plants and animals in
different parts of the
world are related
because they share
common ancestors.
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Comparative Anatomy
• Anatomical
similarities among
many species show
signs of common
descent.
• Humans, cats, whales,
and bats have the
same skeletal
Human
elements because we
all evolved from a
common ancestor.
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Cat
Whale
Bat
Comparative Embryology
• Closely related
organisms often have
similar stages in their
embryonic
development.
• Fish, frogs, snakes,
birds, apes, and people
all have pharyngeal
slits as embryos which
develop into either gills
or lungs.
• We are all related!
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Molecular Biology
• Scientists can compare DNA sequences and
amino acid sequences between species to see
how closely related we are.
• Humans and chimps share 98.5% of their DNA.
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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 (giraffe example)
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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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13.8 Hardy-Weinberg Equilibrium
• Showing that evolution has to happen by
showing that characteristics in nature are
always changing...
• 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
• 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
• ***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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