Transcript Ch. 24 The Origin of Species notes

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)
(a) Anagenesis
(b) Cladogenesis
Animation: Macroevolution
Right-click slide / select “Play”
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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 Barriers
Habitat 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.
Video: Albatross Courtship Ritual
© 2011 Pearson Education, Inc.
Video: Giraffe Courtship Ritual
© 2011 Pearson Education, Inc.
Video: Blue-footed Boobies Courtship Ritual
© 2011 Pearson Education, Inc.
Mechanical Isolation
• Morphological differences
Gametic 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 Separation
Allopatric (“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
2n = 6
4n = 12
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.
Unreduced gamete
with 4 chromosomes
Species A
2n = 4
Hybrid with
7 chromosomes
Unreduced gamete
with 7 chromosomes
Viable fertile hybrid
(allopolyploid)
Meiotic error;
chromosome
number not
reduced from
2n to n
2n = 10
Normal gamete
n=3
Species B
2n = 6
Figure 24.9
Normal gamete
n=3
• 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)
Monochromatic
Normal light
P. pundamilia
P. nyererei
orange light
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
Fire-bellied
toad range
Hybrid zone
Yellow-bellied
toad, Bombina
variegata
Frequency of
B. variegata-specific allele
Yellow-bellied
toad range
Fire-bellied toad, Bombina bombina
0.99
Hybrid
zone
0.9
Yellow-bellied
toad range
0.5
Fire-bellied
toad range
0.1
0.01
40
10
0
20
10
20
30
Distance from hybrid zone center (km)
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.