Transcript Chapter 14

◦ Until recently, over 500 species of cichlid
fishes lived in East Africa’s Lake Victoria
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Africa
Uganda
Kenya
Lake
Victoria
Lake
Tanganyika
Tanzania
Lake
Malawi
◦ Lake Victoria’s cichlids diversified 100,000
years ago (young species!)
◦ Why??
– Specialized mouthparts that allow them to
specialize on different food sources
– 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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
Species: a group of organisms whose members
can breed and produce fertile offspring, but who
do not produce fertile offspring with members of
other groups
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◦ Over the last 30 years, 200 species of cichlids
have disappeared from Lake Victoria
– Introduced predator (perch) eats them up
– Pollution makes it difficult for females to see
colors
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 Speciation is the emergence of new species
and increases diversity
 Millions of species all arose from ancestor that
lived 3.6 billion years ago
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◦ Taxonomy is the field of biology that names
and classifies species and groups them into
broader categories
◦ Binomial system gives every organism a genus
and species name
 Example: Homo sapien, Escherichia coli
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Order
Family
Genus
Species
Felis
catus
(domestic
cat)
Mephitis
mephitis
(striped skunk)
Lutra
lutra
(European
otter)
Canis
latrans
(coyote)
Canis
lupus
(wolf)
But how do we decide what is
actually a species??
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◦ The biological species concept
 Species is 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
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◦ Can the biological species concept always
distinguish species from each other?
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◦ The morphological species concept :
 classifies organisms based on observable
phenotypic traits
 It can be applied to asexual organisms, fossils,
and when we don’t know about inbreeding
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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
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◦ The phylogenetic species concept:

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
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◦ Reproductive barriers serve to isolate a species
gene pool and prevent interbreeding
1. Prezygotic (before sex)
2. Postzygotic (after sex)
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◦ Prezygotic Barriers: prevent mating or fertilization
between species
1. Temporal isolation- two species breed at
different times (seasons, times of day, years)
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◦ Prezygotic Barriers: prevent mating or fertilization
between species
2. Habitat isolation: two species live in the same
general area but not in the same kind of place
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Reproductive barriers keep species separate
◦ Prezygotic Barriers: prevent mating or fertilization
between species
3. Behavioral isolation: there is little or no sexual
attraction between species, due to specific
behaviors
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Reproductive barriers keep species separate
◦ Prezygotic Barriers: prevent mating or fertilization
between species
4. Mechanical isolation: female and male sex
organs are not compatible
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Reproductive barriers keep species separate
◦ Prezygotic Barriers: prevent mating or fertilization
between species
5. Gametic isolation: female and male gametes are
not compatible
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Reproductive barriers keep species separate
◦ Postzygotic Barriers: operate
after hybrid zygotes are formed
1. Zyogotic mortality
2. Reduced hybrid viability most
hybrid offspring do not survive
3. Reduced hybrid fertility hybrid
offspring are vigorous but sterile
4. Hybrid breakdown- first
generation hybrid fertile, loss in
second generation
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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?
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MECHANISMS
OF SPECIATION
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◦ 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 __________?????
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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
A. harrisi
South
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A. leucurus
North
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:
1. Polyploidy: multiplication of the chromosome
number due to errors in cell division (plants)
2. Habitat differentiation (animals)
3. Sexual selection (animals)
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a.
b.
How do you know if two species are
reproductively isolated?
◦ Hybrid zones: regions in which members of
different species meet and mate producing
some hybrid offspring
New
species
Ancestral
species
1
3
2
4
Gene
flow
Gene flow
Population
(five individuals
are shown)
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Hybrid
zone
Hybrid
Barrier to
gene flow
Outcomes of hybrid zones
Reinforcement
Stability
Fusion
◦ 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
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Zygote
Gametes Prezygotic barriers
• Temporal isolation
• Habitat isolation
• Behavioral isolation
• Mechanical isolation
• Gametic isolation
Postzygotic barriers
• Reduced hybrid
viability
• Reduced hybrid
fertility
• Hybrid breakdown
Viable,
fertile
offspring