chapter 24 the origin of species. what you need to know: the difference between microevolution and...
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Chapter 24The Origin of Species
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What You Need to Know:• The difference between microevolution and
macroevolution.• The biological concept of a species.• Prezygotic and postzygotic barriers that
maintain reproductive isolation in natural populaitons.
• How allopatric and sympatric speciation are similar and different.
• How autopolyploid or an allopolyploid chromosomal change can lead to sympatric speciation.
• How punctuated equilibrium and gradualism describe two different tempos of speciation.
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Do Now # 8 Problem 1:• In humans, having the Rh blood antigen is
dominant over not having the Rh antigen. In the United States, 84% of the population is Rh positive. How many individuals are homozygous. (Assume it is Hardy-Weinberg)
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Homework 7 Problem 2:• One in every 1700 Caucasian babies that
are born have cystic fibrosis. Cystic fibrosis is an autosomal recessive disorder (c).
A. What is the frequency of the recessive allele?
B. How many of every 1700 babies are heterozygous?
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Speciation = origin of species
•Microevolution: changes within a single gene pool
•Macroevolution: evolutionary change above the species level▫cumulative effects of speciation over long
periods of time
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•Species = population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring▫Reproductively compatible
•Reproductive isolation = barriers that prevent members of 2 species from producing viable, fertile hybrids
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Prezygotic Barriers:▫Impede
mating/fertilizationTypes:
▫Habitat isolation▫Temporal isolation▫Behavioral isolation▫Mechanical
isolation▫Gametic isolation
Postzygotic Barriers:▫Prevent hybrid
zygote from developing into viable adult
Types:▫Reduced hybrid
viability▫Reduced hybrid
fertility▫Hybrid breakdown
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REDUCED HYBRIDVIABILITY
REDUCED HYBRIDFERTILITY
HYBRID BREAKDOWN
Types of Reproductive Barriers
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REDUCED HYBRIDVIABILITY
REDUCED HYBRIDFERTILITY
HYBRID BREAKDOWN
Types of Reproductive Barriers
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Other definitions of species:
•Morphological – by body shape, size, and other structural features
•Ecological – niche/role in community
•Phylogenetic – share common ancestry, branch on tree of life
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Two main modes of speciation
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Two main modes of speciation:Allopatric Speciation
“other” “homeland”
Geographically isolated populations
• Caused by geologic events or processes
• Evolves by natural selection & genetic drift
Eg. Squirrels on N/S rims of Grand Canyon
Sympatric Speciation
“together” “homeland”
Overlapping populations within home range
Gene flow between subpopulations blocked by:
• polyploidy• sexual selection• habitat differentiation
Eg. polyploidy in crops (oats, cotton, potatoes, wheat)
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Allopatric speciation of antelope squirrels on opposite rims of the Grand
Canyon
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2n = 6 4n = 12 4n2n
Autopolyploid Speciation
Sympatric Speciation by Polyploidy•Autopolyploid: extra sets of chromosomes
▫Failure of cell division (2n 4n)
▫Eg. Strawberries are 4n, 6n, 8n, 10n (decaploid)!
•Allopolyploid: 2 species produce a hybrid▫Species A (2n=6) + Species B (2n=4)
Hybrid (2n=10)
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Allopolyploidy
Allopolyploidy
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•Many new species arise from a single common ancestor
•Occurs when: A few organisms make way to new,
distant areas (allopatric speciation) Environmental change extinctions
new niches for survivors•Eg. Hawaiian archiepelago
Founding
Parents
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KAUAI5.1
millionyears OAHU
3.7millionyears
HAWAII0.4
millionyears
1.3millionyears
MAUIMOLOKAI
LANAI Argyroxiphium sandwicense
Dubautia linearisDubautia scabra
Dubautia waialealae
Dubautia laxa
N
Adaptive Radiation: Hawaiian plants descended from ancestral tarweed from North America 5 million years ago
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Hybrid Zones• Incomplete reproductive barriers•Possible outcomes: reinforcement, fusion, stability
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Hybrid zones
• Where divergent allopatric populations come back and interbreed
• Biologists look for patterns to study reproductive isolation
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Fig. 24-13
EUROPE
Fire-belliedtoad range
Hybrid zone
Yellow-belliedtoad rangeYellow-bellied toad,
Bombina variegata
Fire-bellied toad,Bombina bombina
Alle
le f
req
uen
cy (
log
sca
le)
Distance from hybrid zone center (km)
40 30 20 2010 100
0.01
0.1
0.5
0.9
0.99
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Reinforcement: Strengthening Reproductive Barriers
• The reinforcement of barriers occurs when hybrids are less fit than the parent species that is reproduce less successfully.
• Where reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric species
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Fig. 24-15
Sympatric malepied flycatcher
Allopatric malepied flycatcher
Pied flycatchers
Collared flycatchers
Nu
mb
er o
f fe
mal
es
(none)
Females matingwith males from:
Ownspecies
Otherspecies
Sympatric males
Ownspecies
Otherspecies
Allopatric males
0
4
8
12
16
20
24
28
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Fusion: Weakening Reproductive Barriers
• If hybrids are as fit as parents, there can be substantial gene flow between species
• If gene flow is great enough, the parent species can fuse into a single species
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Fig. 24-16
Pundamilia nyererei Pundamilia pundamilia
Pundamilia “turbid water,”hybrid offspring from a locationwith turbid water
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Stability: Continued Formation of Hybrid Individuals
• Hybrids continue to be produced between the two species in the area of their overlap, but the gene pools of both parent species remain distinct.
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Gradualism• Common ancestor• Slow, constant
change
Punctuated Equilibium• Eldridge & Gould• Long period of stasis
punctuated by short bursts of significant change
Tempo of Evolution
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