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Chapter 24
The Origin of Species
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Biological Species Concept
• Speciation, “the origin of new species”, is the
focal point of evolutionary theory
– Species -group of populations whose
members have the potential to interbreed in
nature and produce viable, fertile offspring
• Microevolution consists of adaptations that
evolve within a population, confined to one
gene pool
• Macroevolution - evolutionary change above
the species level
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-2
(a) Similarity between different species
(b) Diversity within a species
Reproductive Isolation
• Reproductive isolation - existence of
biological factors (barriers) that impede two
species from producing viable, fertile offspring
• Hybrids - offspring of crosses between
different species
• Reproductive isolation can be classified by
whether factors act before (prezygotic) or after
fertilization (post zygotic)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Prezygotic barriers block fertilization from
occurring by:
– Impeding different species from attempting to
mate
– Preventing the successful completion of
mating
– Hindering fertilization if mating is successful
• Habitat isolation: Two species encounter each
other rarely, or not at all, because they occupy
different habitats, even though not isolated by
physical barriers
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Temporal isolation: Species that breed at
different times of the day, different seasons, or
different years cannot mix their gametes
• Behavioral isolation: Courtship rituals and other
behaviors unique to a species are effective
barriers
• Mechanical isolation: Morphological differences
can prevent successful mating
• Gametic isolation: Sperm of one species may
not be able to fertilize eggs of another species
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Postzygotic barriers prevent the hybrid zygote from
developing into a viable, fertile adult:
– Reduced hybrid viability
– Reduced hybrid fertility
– Hybrid breakdown
• Reduced hybrid viability: Genes of the different parent
species may interact and impair the hybrid’s development
• Reduced hybrid fertility: Even if hybrids are vigorous, they
may be sterile
• Hybrid breakdown: Some first-generation hybrids are fertile,
but when they mate with another species or with either
parent species, offspring of the next generation are feeble or
sterile
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Other Definitions of Species
• The biological species concept cannot be applied
to fossils or asexual organisms (including all
prokaryotes)
• Morphological species concept defines a
species by structural features
– It applies to sexual and asexual species but relies
on subjective criteria
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Ecological species concept views a species
in terms of its ecological niche
– It applies to sexual and asexual species and
emphasizes the role of disruptive selection
• Phylogenetic species concept: defines a
species as the smallest group of individuals on
a phylogenetic tree
– It applies to sexual and asexual species, but it
can be difficult to determine the degree of
difference required for separate species
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Allopatric (“Other Country”) Speciation
• Allopatric speciation, gene flow is interrupted
or reduced when a population is divided into
geographically isolated subpopulations
• Sympatric speciation, speciation takes place
in geographically overlapping populations
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-5
(a) Allopatric speciation
(b) Sympatric speciation
Polyploidy
• Polyploidy - presence of extra sets of
chromosomes due to accidents during cell
division
• Autopolyploid - individual with more than two
chromosome sets, derived from one species
• Allopolyploid - species with multiple sets of
chromosomes derived from different species
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 24.3: Hybrid zones provide opportunities
to study factors that cause reproductive isolation
• Hybrid zone - region in which members of
different species mate and produce hybrids
– can occur in a single band where adjacent
species meet
– often have reduced fitness compared with
parent species
• The distribution of hybrid zones can be more
complex if parent species are found in multiple
habitats within the same region
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-13
EUROPE
Fire-bellied
toad range
Hybrid zone
0.99
Allele frequency (log scale)
Yellow-bellied toad,
Bombina variegata
Yellow-bellied
toad range
Fire-bellied toad,
Bombina bombina
0.9
0.5
0.1
0.01
40
20
30
10
0
10
20
Distance from hybrid zone center (km)
•When closely related species meet in a hybrid zone, there are
three possible outcomes:
Isolated population
diverges
Possible
outcomes:
Hybrid
zone
Reinforcement
OR
Fusion
Gene flow
Hybrid
Population
(five individuals
are shown)
OR
Barrier to
gene flow
1. Strengthening of reproductive barriers
2. Weakening of reproductive barriers
3. Continued formation of hybrid individuals
Stability
Fig. 24-15
Sympatric male
pied flycatcher
28
Allopatric male
pied flycatcher
Pied flycatchers
24
Collared flycatchers
Number of females
•Reinforcement of
barriers occurs
when hybrids are
less fit than the
parent species
•Over time, the
rate of
hybridization
decreases
•Where
reinforcement
occurs,
reproductive
barriers should
be stronger for
sympatric than
allopatric
species
20
16
12
8
4
(none)
0
Females mating Own
Other
with males from: species species
Sympatric males
Own
Other
species species
Allopatric males
Fig. 24-16
•Fusion:
Weakening
Pundamilia pundamilia
ReproductivePundamilia nyererei
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
Pundamilia “turbid water,”
hybrid offspring from a location
can fuse into a
with turbid water
single species
Stability: Continued Formation of Hybrid
Individuals
• Extensive gene flow from outside the hybrid
zone can overwhelm selection for increased
reproductive isolation inside the hybrid zone
• In cases where hybrids have increased fitness,
local extinctions of parent species within the
hybrid zone can prevent the breakdown of
reproductive barriers
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Patterns in the Fossil Record
• The fossil record includes examples of species
that appear suddenly, persist essentially
unchanged for some time, and then apparently
disappear
• Niles Eldredge and Stephen Jay Gould coined
the term punctuated equilibrium to describe
periods of apparent stasis punctuated by
sudden change
– The punctuated equilibrium model contrasts
with a model of gradual change in a
species’ existence
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-17
(a) Punctuated pattern
Time
(b) Gradual pattern