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Ch. 24 The Origin of Species
Objective:Show the different ways in which
speciation can occur.
Overview• Speciation is the process of the
creation of a new species.– Explains differences and
similarities among species.• Anagenesis: 1 species changes into
another• Cladogenesis: 1 species changes
into 2
– Connects micro- to macroevolution (evolution above species level)
(b) Cladogenesis(a) Anagenesis
© 2011 Pearson Education, Inc.
Animation: Macroevolution Right-click slide / select “Play”
24.1 Reproductive Isolation
Biological Species Concept• A species is individuals that can mate and
have viable offspring– Reproductively isolated (no gene flow)
• Barriers:– Prezygotic: prevents fertilization– Postzygotic: zygote won’t be fertile
Prezygotic BarriersHabitat Isolation• 2 species that occupy different habitats within
the same area may only encounter each other rarely if ever.
Temporal Isolation• Species that breed during different times of
the day, seasons, or year.
Leopard Frogs
Behavioral Isolation• Courtship rituals that attract mates and other
behaviors unique to a species are effective reproductive barrier.
© 2011 Pearson Education, Inc.
Video: Albatross Courtship Ritual
© 2011 Pearson Education, Inc.
Video: Giraffe Courtship Ritual
© 2011 Pearson Education, Inc.
Video: Blue-footed Boobies Courtship Ritual
Mechanical Isolation• Morphological differencesGametic Isolation• Gametes can’t fertilize each other.
Postzygotic Isolations
Reduced Hybrid Viability• The genes of different parent species may
interact and impair the hybrid’s development.
Reduced Hybrid Fertility• Hybrids created but, they may be sterile. If chromosomes
of the parents differ in number or structure, meiosis in the hybrids won’t produce normal gametes.
Hybrid Breakdown• Some first-generation hybrids are viable and fertile, but
when they mate with each other the next offspring are feeble or sterile.
24.2 Speciation Can Take Place With or Without Geographic SeparationAllopatric (“Other Country”) Speciation• Physical separation of 1 population into >1.• Founder’s Effect: Galapagos finches• Separation leads to reproductive isolation by:
– Different mutations– Sexual selection differs– Genetic drift
Sympatric (“Same Country”) Speciation• Speciation in overlapping populations due to
reduced gene flow.• Polyploidy
– Autopolyploidy: individual with more than two sets of chromosomes.
– Plants would be able to self pollinate to make correct gametes creating isolation from other plants nearby.
2n = 64n = 12
2n
4n
• Polyploidy– Allopolyploid: different species make a hybrid
offspring which should be sterile except for asexually reproducing organisms.
• Ex: type of weeds brought from Europe, oats, cotton, potatoes, tobacco, and wheat.
Figure 24.9
Meiotic error;chromosomenumber notreduced from2n to n
Unreduced gametewith 4 chromosomes
Hybrid with7 chromosomes
Unreduced gametewith 7 chromosomes Viable fertile hybrid
(allopolyploid)
Normal gameten = 3
Normal gameten = 3
Species A 2n = 4
Species B 2n = 6
2n = 10
• Habitat Differentiation and Sexual Selection– Apple flies: original fed on hawthorn trees but
started using apples, which mature more quickly. (temporal)
– Nonrandom mating: cichlids coloration in Africa differs. (behavioral)
P. nyererei
Normal lightMonochromatic
orange light
P. pundamilia
24.3 Hybrid Zones Reveal Factors That Causes Reproductive Isolation
• Hybrid zones are regions in which members of nearly different species meet and mate creating a hybrid.
EUROPE
Yellow-belliedtoad, Bombinavariegata
Fire-belliedtoad range
Hybrid zone
Yellow-belliedtoad range
Fire-bellied toad, Bombina bombina
Fre
qu
en
cy
of
B. v
ari
ega
ta-s
pe
cif
ic a
lle
le
Yellow-belliedtoad range
Hybridzone
Fire-belliedtoad range
Distance from hybrid zone center (km)40
0.99
0.9
0.5
0.1
0.0130 20 10 0 10 20
24.4 Speciation Can Occur Rapidly or Slowly and Can Result From Changes in Few or Many Genes
The Tempo of Speciation•Punctuated Equilibrium
– Periods of no change followed by sudden change according to fossil record.
•Gradualism– Organisms are constantly changing
(but not necessarily in bone structure or a way that shows up in fossils).
•Once initiated, speciation completes quickly
Time
Studying the Genetics of Speciation• Speciation can be traced at the genetic level.
– Ex: flower plants differ in their pollinators by having different genes for flower color and type of nectar.
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