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PowerPoint Lectures for
Biology: Concepts & Connections, Sixth Edition
Campbell, Reece, Taylor, Simon, and Dickey
Chapter 14 The Origin of Species
Lecture by Joan Sharp
Until recently, over 500 species of cichlid fishes lived in East Africa’s Lake Victoria
– Where did these species come from?
– Why are they disappearing?
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Introduction: The Rise and Fall of Cichlids
Lake Victoria’s cichlids diversified 100,000 years ago
– Different species of cichlid have specialized mouthparts that allow them to specialize on different food sources
– The bright colors of the males vary with species, as females chose males with specific colors
Groups isolated by diet or female mate choice may have lost the ability to interbreed
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Introduction: The Rise and Fall of Cichlids
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A species can be defined as a group of organisms whose members can breed and produce fertile offspring, but who do not produce fertile offspring with members of other groups
Introduction: The Rise and Fall of Cichlids
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Two closely related species of cichlid, Pundamilia nyererei and Pundamilia pundamilia, feed at different depths
The males of the two species differ in color
Introduction: The Rise and Fall of Cichlids
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Females of each species prefer brightly colored males with the ―right‖ color
How do females benefit from this choice?
Introduction: The Rise and Fall of Cichlids
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Over the last 30 years, 200 species of cichlids have disappeared from Lake Victoria
– Some were eaten by the Nile perch, an introduced predator
In the polluted waters of Lake Victoria, it is more difficult for females to choose brightly colored males of the right species
– As a result, the gene pools of separate species are mixing, as two species fuse back into one
Introduction: The Rise and Fall of Cichlids
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14.1 The origin of species is the source of biological diversity
Speciation is the emergence of new species
Every time speciation occurs, the diversity of life increases
The many millions of species on Earth have all arisen from an ancestral life form that lived around 3.6 billion years ago
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14.2 There are several ways to define a species
Taxonomy is the branch of biology that names and classifies species and groups them into broader categories
Carolus Linnaeus developed the binomial system of naming organisms using physical characteristics to distinguish over 11,000 species
Similarities between some species and variation within species can make defining species difficult
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The biological species concept defines a species as a population or group of populations whose members have the potential to interbreed in nature and produce fertile offspring
Reproductive isolation prevents gene flow and maintains separate species
14.2 There are several ways to define a species
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Can the biological species concept always distinguish species from each other?
14.2 There are several ways to define a species
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The morphological species concept classifies organisms based on observable phenotypic traits
It can be applied to asexual organisms, fossils, and in cases when we donít know about possible interbreeding
There is some subjectivity in deciding which traits to use
14.2 There are several ways to define a species
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The ecological species concept defines a species by its ecological role or niche
– Consider the cichlids, which are similar in appearance but feed at different depths in the lake
14.2 There are several ways to define a species
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The phylogenetic species concept defines a species as a set of organisms representing a specific evolutionary lineage
– Morphological or DNA similarities or differences can be used to define a species
– Defining the amount of difference required to distinguish separate species is a problem
14.2 There are several ways to define a species
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14.3 Reproductive barriers keep species separate
Reproductive barriers serve to isolate a species gene pool and prevent interbreeding
Reproductive barriers are categorized as prezygotic or postzygotic, depending on whether they function before or after zygotes form
Prezygotic Barriers
– Prezygotic barriers prevent mating or fertilization between species
– In temporal isolation, two species breed at different times (seasons, times of day, years)
– In habitat isolation, two species live in the same general area but not in the same kind of place
14.3 Reproductive barriers keep species separate
Video: Blue-footed Boobies Courtship Ritual
Video: Albatross Courtship Ritual
Video: Giraffe Courtship Ritual
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Prezygotic Barriers
– In behavioral isolation, there is little or no sexual attraction between species, due to specific behaviors
– In mechanical isolation, female and male sex organs are not compatible
– In gametic isolation, female and male gametes are not compatible
14.3 Reproductive barriers keep species separate
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Postzygotic Barriers
– Postzygotic barriers operate after hybrid zygotes are formed
– In reduced hybrid viability, most hybrid offspring do not survive
– In reduced hybrid fertility, hybrid offspring are vigorous but sterile
– In hybrid breakdown, the first-generation hybrids are viable and fertile, but the offspring of the hybrids are feeble or sterile
– The process of speciation depends on whether reproductive barriers prevent gene flow between populations
14.3 Reproductive barriers keep species separate
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If two related species live in the same area, would natural selection favor the evolution of prezygotic or postzygotic reproductive isolating mechanisms?
14.3 Reproductive barriers keep species separate
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14.4 In allopatric speciation, geographic isolation leads to speciation
In allopatric speciation, populations of the same species are geographically separated, separating their gene pools
Changes in the allele frequencies of each population may be caused by natural selection, genetic drift, and mutation, unaffected by gene flow from other populations
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Gene flow between populations is initially prevented by a geographic barrier
– The Grand Canyon and Colorado River separate two species of antelope squirrels
14.4 In allopatric speciation, geographic isolation leads to speciation
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Likelihood of allopatric speciation increases when a population is small and isolated
– A small population may have a different gene pool due to the founder effect
– Genetic drift and natural selection may have a greater effect in a small population in a new habitat
14.4 In allopatric speciation, geographic isolation leads to speciation
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14.5 In sympatric speciation, speciation takes place without geographic isolation
In sympatric speciation, new species may arise within the same geographic area as a parent species
Gene flow between populations may be reduced by factors such as polyploidy, habitat differentiation, or sexual selection
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Many plant species have evolved by polyploidy, the multiplication of the chromosome number due to errors in cell division
A tetraploid (4n) plant can arise from a diploid parent
14.5 In sympatric speciation, speciation takes place without geographic isolation
Parent species2n = 6
Tetraploidcells
4n = 12
1
Diploidgametes
2n = 6
2
Viable, fertiletetraploidspecies4n = 12
Self-fertilization
3
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Most polyploids arise from hybridization of two different species
Haploid gametes from two different species combine to produce a sterile hybrid
– Why is the hybrid sterile? How can it reproduce?
Chromosome duplications may produce a fertile polyploid species
– What is the chromosome number of the new species
14.5 In sympatric speciation, speciation takes place without geographic isolation
Species A2n = 4
Gameten = 2
1 2
Species B2n = 6
Gameten = 3
Sterile hybridn = 5
Chromosomes nothomologous(cannot pair)
Species A2n = 4
Gameten = 2
1 2
Species B2n = 6
Gameten = 3
Sterile hybridn = 5
Chromosomes nothomologous(cannot pair)
Viable, fertilehybrid species
2n = 10
3
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Sympatric speciation in animals more commonly occurs through habitat differentiation and sexual selection
– Remember the cichlids in Lake Victoria!
14.5 In sympatric speciation, speciation takes place without geographic isolation
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14.6 EVOLUTION CONNECTION: Most plant species trace their origin to polyploid speciation
80% of all living plant species are the descendants of ancestors that formed by polyploid speciation
Hybridization between two species accounts for most of these species
– What advantage might there be to hybridization?
Polyploid food plants include oats, potatoes, bananas, peanuts, barley, plums, apples, sugarcane, coffee, and bread wheat
Cotton is also polyploid
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Wheat has been domesticated for 11,000 years
– It is the most widely cultivated plant in the world
Bread wheat, Triticum aestivum, is a polyploid with 42 chromosomes
14.6 EVOLUTION CONNECTION: Most plant species trace their origin to polyploid speciation
Triticum monococcum
(14 chromosomes)Hybridization
AA BB
AB
Sterile hybrid
(14 chromosomes)
1
2
´
Cell division error
and self-fertilization
Wild
Triticum
(14 chromo-
somes)
AA BB DD´
Sterile hybrid
(21 chromosomes)
ABD
Hybridization3
4 Cell division error
and self-fertilization
T. tauschii
(wild)
(14 chromosomes)
T. turgidum
Emmer wheat
(28 chromosomes)
AA BB DD
T. aestivum
Bread wheat
(42 chromosomes)
Triticum monococcum(14 chromosomes)
Hybridization
AA BB
AB
Sterile hybrid
(14 chromosomes)
1
´
Cell division errorand self-fertilization
WildTriticum(14 chromo-somes)
AA BB
T. turgidumEmmer wheat(28 chromosomes)
2
AA BB DD´
Sterile hybrid(21 chromosomes)
ABD
Hybridization3
4 Cell division error
and self-fertilization
T. tauschii(wild)(14 chromosomes)
T. turgidumEmmer wheat(28 chromosomes)
AA BB DD
T. aestivumBread wheat(42 chromosomes)
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14.7 Reproductive barriers may evolve as populations diverge
How do reproductive barriers arise?
Initial sampleof fruit flies Maltose mediumStarch medium
Mating frequenciesin experimental groups
Starch
Mating frequenciesin starch control groups
Maltose
Mating experiments
ResultsFemale
Female
Sta
rch
Ma
lto
se
22 9
8 20Ma
le
Population Population
Po
p#1
Po
p#2
18 15
12 15M
ale
#1 #2
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14.8 Hybrid zones provide opportunities to study reproductive isolation
What happens when isolated populations renew contact?
In hybrid zones, members of different species meet and mate to produce hybrid offspring
Ancestralspecies
21
Gene flow
Population(five individualsare shown)
Barrier togene flow
Newspecies
Hybridzone
Hybrid
Gene flow
3
4
Malecollaredflycatcher
Malepiedflycatcher
Allopatricpopulations
Sympatricpopulations
Pied flycatcher fromallopatric population
Pied flycatcher fromsympatric population
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What may happen in a hybrid zone?
Reinforcement: If hybrids are less fit than parent species, natural selection strengthens reproductive barriers
Fusion: Weak reproductive barriers between the two species, with considerable gene flow, reverses speciation and two species become one again
14.8 Hybrid zones provide opportunities to study reproductive isolation
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Stability: Many hybrid zones are stable, continuing to produce hybrids; this allows some gene flow between populations, but each species maintains its own integrity
– Which of these three outcomes—reinforcement, fusion, or stability—is happening to the Pundamilia species of cichlids in Lake Victoria?
14.8 Hybrid zones provide opportunities to study reproductive isolation
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Peter and Rosemary Grant have worked on medium ground finches on tiny, isolated, uninhabited Daphne Major in the Galapágos Islands for 35 years
Medium ground finches and cactus finches occasionally interbreed
– Hybrid offspring have intermediate bill sizes and survive well during wet years, when there are plenty of soft, small seeds around
– During dry years, hybrids are outcompeted by both parental types
14.9 TALKING ABOUT SCIENCE: Peter and Rosemary Grant study the evolution of Darwin’s finches
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The occasional hybridization between finch species introduces new genes into both populations
During drought years, hybrids die out
– This keeps medium ground finches and cactus finches on separate evolutionary paths
14.9 TALKING ABOUT SCIENCE: Peter and
Rosemary Grant study the evolution of
Darwin’s finches
Cactus-seed-eater(cactus finch)
Seed-eater(medium ground finch)
Tool-using insect-eater(woodpecker finch)
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14.10 Adaptive radiation may occur when new opportunities arise
In adaptive radiation, many diverse species evolve from a common ancestor
Adaptive radiations occur
– When a few organisms colonize new unexploited areas
– After a mass extinction
Adaptive radiations are linked to new opportunities: lack of competitors, varying habitats and food sources, evolution of new structures
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14.11 Speciation may occur rapidly or slowly
What is the total length of time between speciation events (between formation of a species and subsequent divergence of that species)?
– In a survey of 84 groups of plants and animals, the time ranged from 4,000 to 40 million years
– Overall, the time between speciation events averaged 6.5 million years and rarely took less than 50,000 years
Animation: Macroevolution
Prezygotic barriers
• Temporal isolation
• Habitat isolation
• Behavioral isolation
• Mechanical isolation
• Gametic isolation
Zygote
Gametes Postzygotic barriers
•
•
•
Viable,fertile
offspringReduced hybrid
viability
Reduced hybrid
fertility
Hybrid breakdown
Species
may interbreedin
outcome may be
a.
a fewhybrids
continue tobe produced
f.e.
b. c. d.
speciesseparate
speciationreversed
reproductivebarriers
when
are
keeps
whenwhen
are
and
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You should now be able to
1. Explain how the diverse assemblage of cichlid species evolved in Lake Victoria; explain why many of these species no longer exist
2. Compare the definitions, advantages, and disadvantages of the different species concepts
3. Describe five types of prezygotic barriers and three types of postzygotic barriers that prevent populations belonging to closely related species from interbreeding
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4. Explain how geographical processes can fragment populations and lead to speciation
5. Explain how sympatric speciation can occur, noting examples in plants and animals
6. Explain why polyploidy is important to modern agriculture; explain how modern wheat evolved
7. Explain how reproductive barriers might evolve in isolated populations of organisms
You should now be able to