chapter 16: evolution of populations. 1.when darwin developed his theory of evolution, he did not...
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Chapter 16:Chapter 16: Evolution Evolution
of of PopulationsPopulations
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1.1. When Darwin developed his theory of When Darwin developed his theory of evolution, he did not understand:evolution, he did not understand:
• how heredity worked.how heredity worked.
This left him unable to explain two This left him unable to explain two things:things:
a.a. source of variationsource of variation
b. how inheritable traits pass b. how inheritable traits pass from one generation to from one generation to
the nextthe next
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In the 1940’s, Mendel’s work on genetics In the 1940’s, Mendel’s work on genetics was “rediscovered” and scientists began was “rediscovered” and scientists began to combine the ideas of many branches of to combine the ideas of many branches of biology to develop a modern theory of biology to develop a modern theory of evolution. When studying evolution evolution. When studying evolution today, biologists often focus on a today, biologists often focus on a particular particular populationpopulation.. This evolution of This evolution of populations is called populations is called microevolutionmicroevolution. .
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2. Vocabulary: 2. Vocabulary:
population: population: group of individuals of the group of individuals of the same species living in the same area that same species living in the same area that breed with each other. breed with each other.
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2. gene pool: 2. gene pool: combined genetic info. combined genetic info. for all members of a populationfor all members of a population
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2. allele: 2. allele: one form of a geneone form of a gene
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2. relative frequency of an allele: 2. relative frequency of an allele: # # times an allele occurs in the gene pool times an allele occurs in the gene pool compared to other alleles (percent)compared to other alleles (percent)
Example
Relative Frequency:
70% Allele B
30% Allele b
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3. 3. Sources of VariationSources of Variation: :
a. a. mutationsmutations: : any change in DNA sequenceany change in DNA sequence
♦ Can occur because of: Can occur because of:
♦mistakes in replicationmistakes in replication
♦ environmental chemicalsenvironmental chemicals
♦ May or may not affect an organism’s May or may not affect an organism’s phenotypephenotype
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3. Sources of Variation3. Sources of Variationb. b. Gene ShufflingGene Shuffling: : recombination of genes recombination of genes that occurs during production of gametesthat occurs during production of gametes♦ Cause most Cause most inheritableinheritable differences between differences between
relativesrelatives♦ Occurs during Occurs during meiosismeiosis♦ As a result, As a result, sexualsexual reproduction is a major reproduction is a major
source of variation in organisms.source of variation in organisms.♦ Despite gene shuffling, the frequency of alleles Despite gene shuffling, the frequency of alleles
does not change in a population. does not change in a population. Explain why Explain why this is truethis is true..
Similar to a deck of cards – no matter how Similar to a deck of cards – no matter how many times you shuffle, same cards (alleles) many times you shuffle, same cards (alleles) are always there.are always there.
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4. Gene Traits: 4. Gene Traits:
A) Single gene trait:A) Single gene trait: controlled by single controlled by single gene with two allelesgene with two alleles
♦ ExamplesExamples: widow’s peak, hitchhiker’s thumb, : widow’s peak, hitchhiker’s thumb, tongue rollingtongue rolling
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(4. Gene Traits:)(4. Gene Traits:)
B) Polygenic trait: B) Polygenic trait: controlled by 2 or more controlled by 2 or more genes, each with 2 or more allelesgenes, each with 2 or more alleles
♦ Examples: Examples: height, hair color, skin color, eye colorheight, hair color, skin color, eye color
Most human traits are Most human traits are polygenic.polygenic.
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Do the following graphs show the distribution of Do the following graphs show the distribution of phenotypes for single-gene or polygenic traits? Explain.phenotypes for single-gene or polygenic traits? Explain.
type: single gene
why? Only two phenotypes possible
Example: tongue roller or non-tongue roller
type: polygenic
why? Multiple (many) phenotypes possible
Example: height range 4feet to 9 feet all
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5. Natural selection acts on 5. Natural selection acts on phenotypesphenotypes, not , not genotypesgenotypes. .
Example: in a forest covered in brown leaves, Example: in a forest covered in brown leaves, dirt and rocks which mouse will survive better dirt and rocks which mouse will survive better brown or white? brown or white?
Brown, more hidden. Brown, more hidden.
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5. If brown is dominant can the a predator tell the difference 5. If brown is dominant can the a predator tell the difference between: between:
Mouse with highest fitness will have the most Mouse with highest fitness will have the most allelesalleles passed on to passed on to the next generation.the next generation.
White mouse will have low fitnessWhite mouse will have low fitness
BB Bb
?
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5. Which mouse will have the lowest fitness? 5. Which mouse will have the lowest fitness?
White, bb (recessive)White, bb (recessive)
Will the fitness of BB and Bb differ? Why?Will the fitness of BB and Bb differ? Why?
No, Both BB and Bb have the same fitness advantage of being No, Both BB and Bb have the same fitness advantage of being brownbrown
BB Bb
?
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6. Three ways in which 6. Three ways in which natural selection affects natural selection affects
polygenic traits.polygenic traits.
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a. a. Directional SelectionDirectional Selection: : individuals at one individuals at one end of the curve have higher fitness so end of the curve have higher fitness so evolution causes increase in individuals evolution causes increase in individuals with that traitwith that trait
♦ Individuals with highest fitnessIndividuals with highest fitness: : those at one those at one end of the curveend of the curve
♦ Example: Example: Galapagos finches – beak sizeGalapagos finches – beak size
Food becomes scarce.
KeyLow mortality, high fitness
High mortality, low fitness
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Directional Selection (page 398)Directional Selection (page 398)
Food becomes scarce.
Key
Low mortality, high fitness
High mortality, low fitness
Directional Selection
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b. Stabilizing Selection: b. Stabilizing Selection: individuals at the individuals at the center of the curve have highest fitness; center of the curve have highest fitness; evolution keeps center in the same position evolution keeps center in the same position but narrows the curvebut narrows the curve
Key
Per
cen
tag
e o
f P
op
ula
tio
n
Birth Weight
Selection against both
extremes keep curve narrow and
in same place.
Low mortality, high fitnessHigh mortality, low fitness
Stabilizing Selection
Individuals Individuals with highest with highest fitness: fitness: near near the center of the center of the curve the curve (average (average phenotype)phenotype)
Example: Example: human birth human birth weightweight
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Key
Per
cen
tag
e o
f P
op
ula
tio
n
Birth Weight
Selection against both
extremes keep curve narrow and in same
place.
Low mortality, high fitness
High mortality, low fitness
Stabilizing Selection
Stabilizing Selection
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c. c. Disruptive SelectionDisruptive Selection: : individuals at individuals at both ends of the curve survive better both ends of the curve survive better than the middle of the curve. than the middle of the curve.
♦ Individuals with highest fitness: Individuals with highest fitness: both ends both ends of curveof curve
♦ Example: Example: birds where seeds are either birds where seeds are either large or smalllarge or small
Disruptive Selection
Largest and smallest seeds become more common.
Nu
mb
er o
f B
ird
sin
Po
pu
lati
on
Beak Size
Population splits into two subgroups specializing in different seeds.
Beak Size
Nu
mb
er o
f B
ird
sin
Po
pu
lati
onKey
Low mortality, high fitness
High mortality, low fitness
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Disruptive Selection (pg 399)Disruptive Selection (pg 399)
Disruptive Selection
Largest and smallest seeds become more common.
Nu
mb
er o
f B
ird
sin
Po
pu
lati
on
Beak Size
Population splits into two subgroups specializing in different seeds.
Beak Size
Nu
mb
er o
f B
ird
sin
Po
pu
lati
onKey
Low mortality, high fitness
High mortality, low fitness
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Quiz Monday!!Quiz Monday!!
•Evolution review ½ sheet. Evolution review ½ sheet. (yes some questions are (yes some questions are missing)missing)
•Thursday and Friday’s Thursday and Friday’s concepts will be on the quiz:concepts will be on the quiz:–Directional, Stabilizing and Directional, Stabilizing and Disruptive selection.Disruptive selection.
–Geographic, Behavioral, Geographic, Behavioral, Temporal IsolationTemporal Isolation
–Small populations caused by Small populations caused by bottleneck and founder effectbottleneck and founder effect
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The Process of The Process of SpeciationSpeciation
•The formation of new The formation of new biological species, usually biological species, usually by the division of a single by the division of a single species into two or more species into two or more genetically distinct one. genetically distinct one.
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Three Isolating Three Isolating MechanismsMechanisms: :
Isolate species forming Isolate species forming subspecies and perhaps subspecies and perhaps
causing speciation. causing speciation.
1.1.Geographic Geographic IsolationIsolation
2.2.Behavioral IsolationBehavioral Isolation
3.3.Temporal IsolationTemporal Isolation
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1. Geographic 1. Geographic IsolationIsolation•Two populations Two populations
separated by a separated by a geographic geographic barrierbarrier; ; river, lake, canyon, river, lake, canyon, mountain range.mountain range.
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Example: 10,000 years Example: 10,000 years ago the ago the Colorado RiverColorado River separated two squirrel separated two squirrel populations. populations.
•Kaibab Squirrel Kaibab Squirrel Abert SquirrelAbert Squirrel
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This resulted in a This resulted in a subspecies, but subspecies, but did notdid not result in speciation result in speciation because the two can still because the two can still matemate if brought if brought togethertogether
•Kaibab Squirrel Kaibab Squirrel Abert SquirrelAbert Squirrel
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2. 2. Behavioral Behavioral IsolationIsolation
•Two populations are Two populations are capablecapable of interbreeding of interbreeding but do but do not interbreednot interbreed because they have different because they have different ‘courtship rituals’ or other ‘courtship rituals’ or other lifestyle habits that differ.lifestyle habits that differ.
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Example: Eastern and Example: Eastern and Western Meadowlark Western Meadowlark
•Eastern and Western Eastern and Western Meadowlark populations Meadowlark populations
overlapoverlap in the in the middle of the USmiddle of the US
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Example: Eastern and Western Example: Eastern and Western Meadowlark Meadowlark
•Male birds sing a Male birds sing a
matting songmatting song that that females like, East and females like, East and West have different songs. West have different songs. Females only respond to Females only respond to their subspecies song. their subspecies song.
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3. Temporal 3. Temporal IsolationIsolation
Populations reproduce Populations reproduce at different timesat different times
January
1 2 3 4 5 6
7 8 9 10 11 12 13
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Example: Northern Leopard Example: Northern Leopard Frog Frog
& North American Bullfrog & North American Bullfrog •Mates in: Mates in:
Mates in: Mates in:
AprilApril JulyJuly
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ConclusionConclusion::•Geographic, Behavioral Geographic, Behavioral and Temporal Isolation and Temporal Isolation are all believed to lead are all believed to lead
to to speciation.speciation.
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However:However:–No examples ever No examples ever
observed in observed in animalsanimals–A couple examples that A couple examples that may demonstrate may demonstrate speciation exist in speciation exist in plants plants and some insectsand some insects. .
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Genetic DriftGenetic Drift• random change in allele random change in allele
frequency that occurs in small frequency that occurs in small populationspopulations
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The results of genetic crosses can The results of genetic crosses can usually be predicted using the laws of usually be predicted using the laws of
probabilityprobability. In . In smallsmall populations, however, these populations, however, these predictions are not always accurate.predictions are not always accurate.a. a. Founder effect: Founder effect: allele frequencies change due to allele frequencies change due to migration of a small subgroup of a populationmigration of a small subgroup of a population
♦ Example: Example: fruit flies on Hawaiian islandsfruit flies on Hawaiian islands
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Two phenomena that Two phenomena that result result in small populations in small populations and cause genetic and cause genetic
driftdrift1.1.Founder EffectFounder Effect
2.2.Bottleneck Bottleneck EffectEffect
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Founder effectFounder effect
allele frequencies change due allele frequencies change due to migration of a small to migration of a small subgroup of a populationsubgroup of a population
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Sample of Original Population
Founding Population A
Founding Population B
Descendants
Founder Effect: : Fruit Fruit Flies on Hawaiian Flies on Hawaiian
islandsislands
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major change in allele frequencies major change in allele frequencies when population decreases when population decreases dramatically due to catastrophedramatically due to catastrophe
♦ Example: Example: northern elephant sealsnorthern elephant seals
decreased to 20 individuals in 1800’s, now 30,000decreased to 20 individuals in 1800’s, now 30,000
no genetic variation in 24 genesno genetic variation in 24 genes
2. Bottleneck effect2. Bottleneck effect
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Bottleneck Effect: Northern Bottleneck Effect: Northern Elephant Seal PopulationElephant Seal Population
♦ Hunted to near extintionHunted to near extintion
♦ Population decreased to Population decreased to 20 individuals in 1800’s, 20 individuals in 1800’s, those 20 repopulated so those 20 repopulated so today’s population is today’s population is ~30,000~30,000
♦ No genetic variation in 24 No genetic variation in 24 genesgenes
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Bottleneck EffectBottleneck Effect
Original population
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Bottleneck EffectBottleneck Effect
Original population
Catastrophe
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Bottleneck EffectBottleneck Effect
Original population
Catastrophe
Surviving population
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Another picture to Another picture to illustrate bottleneck effect illustrate bottleneck effect
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