CHAPTER 24
THE ORIGIN OF SPECIES
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OVERVIEW
A. Speciation
•Is defined as the origin of new species (process by
which one species splits into two or more species)
•Is at the focal point of evolutionary theory which must
explain how new species originate and how populations
evolve
B. Microevolution
•Consists of adaptations that evolve within a population,
confined to one gene pool
•Mechanisms—mutation, natural selection, genetic drift,
and gene flow
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C. Macroevolution
•Refers to evolutionary change above the species level
•Deals with the appearance of evolutionary novelties that
can be used to define high taxa
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II. Concept 24.1: Biological Species
Concept
• Emphasizes reproductive isolation
• Species is a Latin word meaning “kind” or “appearance”
• Today in addition to appearance, we use differences in
physiology, biochemistry, behavior, and DNA sequences to
differentiate species
A. Biological Species Concept
1. Proposed by Ernst Mayr in 1942
2. A species is defined as a group of populations whose
members have the potential to interbreed in nature and
produce viable, fertile offspring but who cannot produce
viable, fertile offspring with other such populations
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B. Reproductive Isolation
1. Defined as the existence of biological factors (barriers)
that impede two species from producing viable, fertile
offspring
2. Hybrids are the offspring of crosses between different
species
3. Development of reproductive isolation can possibly
lead to the formation of new species
4. Reproductive isolation can be classified by whether
factors act before (prezygotic) or after fertilization
(postzygotic)
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Prezygotic Barriers
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C. Prezygotic Barriers
• Five ways prezygotic barriers can block fertilization
between species
1. Habitat Isolation
Two species encounter each other rarely, or not
at all, because they occupy different habitats,
even though not isolated by physical barriers
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HABITAT ISOLATION
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2. Behavioral Isolation
Species use unique and elaborate courtship
behavior to attract mates
Courtship ritual of blue-footed boobies
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3. Temporal Isolation
Two species that breed during different times of day,
different seasons, or different years cannot mix
gametes
Eastern spotted skunk
(Spilogale putorius)
Western spotted skunk
(Spilogale gracilis)
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4. Mechanical Isolation
Closely related species may attempt to mate but fail
because they are anatomically incompatible and
transfer of sperm is not possible
Bradybaena with shells spiraling in opposite directions
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5. Gametic Isolation
Gametes of one species may not be able to fertilize
eggs of another species
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Postzygotic Barriers
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D. Postzygotic Barriers
• Three postzygotic barriers that prevent a hybrid zygote
from developing into a viable, fertile adult
1. Reduced Hybrid Viability
Genetic incompatibility between the two species
may stop the development of the hybrid at some
embryonic stage or produce frail offspring
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Ensatina hybrid
2. Reduced Hybrid Fertility
Even though hybrid offspring survives, it may be
sterile
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3. Hybrid Breakdown
Some first-generation hybrids are viable and fertile,
but when they mate with another species or with
either parent species, the offspring of the next
generation are sterile or feeble
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E. Limitations of the Biological Species Concept
The biological species concept cannot be applied to
fossils or asexual organisms (including all prokaryotes)
F. Other Definitions of Species
Other species concepts emphasize the unity within a
species rather than the separateness of different
species
1. The morphological species concept defines a species
by structural features
• It applies to sexual and asexual species but relies on
subjective criteria
• Oldest and still most practical
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2. The ecological species concept views a species in terms
of its ecological niche, the set of environmental
resources that a species uses, and its role in a
biological community
•Applies to sexual and asexual species
3. The phylogenetic species concept defines a species as
the smallest group of individuals that shares a common
ancestor and forms one branch on the tree of life
•It applies to sexual and asexual species, but it can be
difficult to determine the degree of difference required
for separate species
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II. Concept 24.2: Speciation
A. Two modes of speciation are distinguished by the way
gene flow among populations is initially interrupted:
1. Allopatric Speciation
• Occurs when geographic separation of populations
restricts gene flow
• Geological process can fragment a population into two
or more isolated populations.
• Definition of barrier depends on the ability of a
population to disperse
• Separate populations may evolve independently
through mutation, natural selection, and genetic drift
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• Evidence of allopatric speciation can be seen in regions
with many geographic barriers typically having more
species than do regions with few barriers
• Reproductive isolation between populations generally
increases as the distance between them increases
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2. Sympatric Speciation
• New species arise within the range of the parent
population
• Occurs when reproductive barriers evolve between
sympatric populations
• May occur rapidly
3. Types of Sympatric Speciation
a. Polyploidy
•Defined as the presence of extra sets of
chromosomes due to accidents during cell division
•An autopolyploid is an individual with more than two
chromosome sets, derived from one species
•An allopolyploid is a species with multiple sets of
chromosomes derived from different species (more
common)
•Polyploidy is more common in plants than animals 23
AUTOPOLYPLOID
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AUTOPOLYPLOID
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AUTOPOLYPLOID
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ALLOPOLYPLOID
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ALLOPOLYPLOID
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ALLOPOLYPLOID
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ALLOPOLYPLOID
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b. Habitat Differentiation
•Sympatric speciation in animals may result from
the appearance of new ecological niches
c. Sexual Selection
B. Adaptive Radiation
•Occurs when many new species arise from a single
common ancestor
•Typically occurs when a few organisms make their way
to new, distant areas or when environmental changes
cause extinctions thus opening up niches for survivors
•Ex: Darwin’s finches
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Adaptive Radiation
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Allopatric and Sympatric
Speciation: Review
•In allopatric speciation, geographic isolation restricts gene
flow between populations
•Reproductive isolation may then arise by natural selection,
genetic drift, or sexual selection in the isolated populations
•Even if contact is restored between populations,
interbreeding is prevented
•In sympatric speciation, a reproductive barrier isolates a
subset of a population without geographic separation from
the parent species
•Sympatric speciation can result from polyploidy, natural
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selection, or sexual selection
IV. Concept 24.4: Tempo of Speciation
A. Gradual—proposes that species descended from a
common ancestor and gradually diverge more and more
in morphology as they acquire unique adapations
-difficult to prove with fossils
B. Punctuated—spurts of rapid change rather than
gradual divergence
-based on fossils that suddenly appear and disappear
-proposed by Eldredge and Gould
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You should now be able to:
1. Define and discuss the limitations of the four species
concepts
2. Describe and provide examples of prezygotic and
postzygotic reproductive barriers
3. Distinguish between and provide examples of
allopatric and sympatric speciation
4. Explain how polyploidy can cause reproductive
isolation
5. Define the term hybrid zone and describe three
outcomes for hybrid zones over time
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