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Copyright © 2010 Pearson Education, Inc.
Origin of Species
Part 1 – A few reminders from lecture 2
• Modern Synthesis of Genetics and Evolution
• Hardy-Weinberg Principle
• Factors Changing Allele Frequencies
BIOL102
Source of cover picture: Reece et al. (2010) , Campbell Biology, 9th edition, Pearson Benjamin Cummings, San Francisco (CA), Figure 24.4c
Copyright © 2010 Pearson Education, Inc.
Part 2 – Species Concepts
• Species
• Biological Species Concept
• Morphological Species Concept
• Ecological Species Concept
• Phylogenetic Species Concept
BIOL102
Origin of Species
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Part 3 – Speciation
• Allopatric Speciation
• Sympatric Speciation
• Rates of Speciation
• Dynamics
BIOL102
Origin of Species
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• A population is the smallest biological unit that can
evolve and is defined as a group of individuals of the
same species that live, interbreed and produce fertile
offspring in a particular geographic area
• A gene pool consists of all alleles (forms of genes)
for all loci in a population and is the source of
genetic variation that produces the phenotypes
and their traits on which natural selection acts
• A population evolves when individuals with different
genotypes survive or reproduce at different rates
Modern Synthesis of Genetics and Evolution
Part 1 – A few reminders from lecture 2
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• states that frequencies of alleles and genotypes in a
population remain constant from generation to generation
if certain conditions are met (Hardy-Weinberg equilibrium)
no mutations
random mating
no natural selection
extremely large population size (no effect of
genetic drift)
no gene flow (migration into or out of a population)
Part 1 – A few reminders from lecture 2
Hardy-Weinberg Principle
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• Hardy-Weinberg equilibrium is a null hypothesis, which
assumes that allele frequencies are not changed
• However, there are at least four mechanisms of evolution,
which cause changes in allele frequencies of populations:
mutations
gene flow
genetic drift
natural selection
Factors Changing Allele Frequencies
Part 1 – A few reminders from lecture 2
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• is defined as an evolutionarily independent
population or group of populations
• Biologists commonly use the following four
approaches to identify species:
the biological species concept
the morphological species concept
the ecological species concept
the phylogenetic species concept
A. Species
Part 2 – Species Concepts
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• defines a species as a population or group of populations
whose members have the potential to interbreed and
produce fertile offspring
• considers populations to be evolutionarily independent
if they are reproductively isolated from each other and
no gene flow occurs between them
B. Biological Species Concept
Part 2 – Species Concepts
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• Biologists categorize the mechanisms that stop gene
flow between populations into prezygotic barriers
(before fertilization) and postzygotic barriers (after
fertilization)
prezygotic barriers: individuals of different
species are prevented from mating
postzygotic barriers: individuals from different
populations do mate, but the hybrid offspring
produced have low fitness and do not survive or
produce offspring
Prezygotic and Postzygotic Barriers
Biological Species Concept
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Prezygotic and Postzygotic Barriers
Individuals ofdifferent species
Mating attempt
Fertilization(zygote forms)
Viable, fertileoffspring
Prezygoticbarriers
Postzygoticbarriers
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• block fertilization from occurring by:
impeding different species from attempting to mate
preventing the successful completion of mating
hindering fertilization if mating is successful
Prezygotic Barriers
Biological Species Concept
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• Habitat isolation: two species encounter each other
rarely, or not at all, because they occupy different
habitats, even though not isolated by physical barriers
• 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
Prezygotic Barriers
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• Floral traits of plants can influence the behavior of
pollinators, and thus whether plants can hybridize
two species of columbines (Aquilegia) in California
can produce fertile hybrids, but flower structure
determines that one species is pollinated by
hummingbirds, the other by hawkmoths, so
hybridization is rare
Behavioral Isolation
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• Mechanical isolation: morphological differences (e. g.,
size and shape of reproductive organs) can prevent
successful mating
• Gametic isolation: sperm of one species may not be
able to fertilize eggs of another species
Prezygotic Barriers
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• In plants, mechanical isolation may involve pollinators
many orchid flowers look and smell like the females of
particular pollinator species
male insects attempt
to mate, thereby
transferring
pollen
Mechanical Isolation
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Prezygotic barriers
HabitatIsolation
TemporalIsolation
BehavioralIsolation
MechanicalIsolation
GameticIsolation
Individuals of
differentspecies
MATINGATTEMPT FERTILIZATION
(a) (c) (e) (f)
(b)
(g)
(d)
Prezygotic Barriers
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• prevent the hybrid zygote from developing into a
viable, fertile adult due to:
reduced hybrid viability
reduced hybrid fertility
hybrid breakdown
• Hybrids are the offspring of crosses between
different species
Postzygotic Barriers
Biological Species Concept
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• 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
Postzygotic Barriers
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Reduced HybridViability
Reduced HybridFertility
HybridBreakdown
FERTILIZATIONVIABLE,FERTILE
OFFSPRING
Postzygotic barriers
(k)
(h) (i)
(j)
(l)
Postzygotic
Barriers
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• If two formerly isolated populations are reunited
before complete reproductive isolation has developed,
interbreeding can occur with three possible outcomes:
if hybrid offspring are as fit as those resulting from
matings within each population, hybrids will mate
with individuals of both parental species. The gene
pools will gradually become completely mixed
(no speciation)
Hybrid Zones
Biological Species Concept
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if hybrid offspring are less fit, reinforcement may result
in more prezygotic barriers and complete reproductive
isolation may evolve (speciation)
a hybrid zone may develop in the absence of
reinforcement, or before reinforcement is complete, and
may contain recombinant individuals resulting from
many generations of hybridization
Hybrid Zones
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• Example: two species of European toads have a long
narrow hybrid zone
the toad hybrids have many defects, some of which
are lethal
on average, a hybrid toad is significantly less fit as
a purebred individual
the hybrid zone is narrow, because there is strong
selection against hybrids. But it persists because
individuals of both species continue to move into it
and mate
Hybrid Zones
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Limitations
Biological Species Concept
• The criterion of reproductive isolation cannot be evaluated
in fossils or in species that reproduce asexually
for example, prokaryotic and viral species must be
defined differently
• this concept can only be applied to populations that
overlap geographically
• it also emphasizes absence of gene flow, which can occur
between distinct species
for example, grizzly bears and polar bears can mate
to produce “grolar bears”
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Grizzly bear (U. arctos)
Polar bear (U. maritimus)
Hybrid “grolar bear”
Limitations of
the Biological
Species
Concept
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• defines a species by differences in morphological or
structural features
is based on the idea that distinguishing features are
most likely to arise if populations are independent
and isolated from gene flow
applies to sexual and asexual species but relies on
subjective criteria
also cannot identify cryptic species that differ in
non-morphological traits
C. Morphological Species Concept
Part 2 – Species Concepts
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• views a species in terms of its ecological niche
applies to sexual and asexual species and
emphasizes the role of disruptive selection
is widely used for viral species (in addition
to genetic homologies)
D. Ecological Species Concept
Part 2 – Species Concepts
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• defines a species as the smallest group of individuals
on a phylogenetic tree (monophyletic group)
applies to sexual and asexual species, but it can be
difficult to determine the degree of difference
required for separate species
on phylogenetic trees, an ancestral population plus
all of its descendants is called a monophyletic group
or clade, which is identified by synapomorphies,
homologous traits inherited from a common ancestor
that are unique to certain populations or lineages
E. Phylogenetic Species Concept
Part 2 – Species Concepts
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• This concept can be applied to any population, but
there are disadvantages:
phylogenies are currently available for only a
tiny (though growing) subset of populations on
the tree of life
would probably lead to recognition of many
more species than either of the other species
concepts
Phylogenetic Species Concept
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• A key event in the potential origin of a species occurs
when a population is somehow severed from other
populations of the parent species. With its gene pool
isolated, the splinter population can follow its own
evolutionary course and become reproductively
incompatible
• Two modes leading to reproductive barriers can be
distinguished
allopatric speciation
sympatric speciation
Part 3 – Speciation
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• Allopatric speciation
occurs when geographic
isolation creates a
reproductive barrier
(extrinsic mechanisms)
• Sympatric speciation
occurs when a reproductive
barrier is created by
something other than
geographic isolation
(intrinsic mechanisms)
Part 3 – Speciation
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• Genetic isolation happens routinely when populations
become physically separated. Physical isolation, in turn,
occurs in one of two ways: dispersal or vicariance.
dispersal occurs when a population moves to a new
habitat, colonizes it, and forms a new population
vicariance occurs when a physical barrier splits a
widespread population into subgroups that are
physically isolated from each other
• Speciation that begins with physical isolation via either
dispersal or vicariance is known as allopatric speciation
A. Allopatric Speciation
Part 3 – Speciation
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• Geographic separation prevents species from mating
• Speciation occurs only with the evolution of reproductive
barriers between the
isolated population
and its parent
population
Allopatric Speciation
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• The definition of barrier depends on the ability of a
population to disperse
for example, a canyon may create a barrier for
small rodents, but not birds, coyotes, or pollen
• Separate populations may evolve independently
through mutation, natural selection, and genetic drift
for example, speciation of snapping shrimp (Alpheus)
populations due to separation by the Isthmus of
Panama
Allopatric Speciation
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A. harrisii A. leucurus
Physical Isolation and Reproductive Barriers
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• Regions with many geographic barriers typically have
more species than do regions with fewer barriers
• Reproductive isolation between populations generally
increases as the distance between them increases
however, barriers to reproduction are intrinsic;
separation itself is not a biological barrier
Allopatric Speciation
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EXPERIMENT
Initial populationof fruit flies(Drosophila
pseudoobscura)
Some flies raisedon starch medium
Mating experimentsafter 40 generations
Some flies raised onmaltose medium
Allopatric Populations and Reproductive Isolation
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RESULTS
Female
Starch Maltose
Ma
leM
alt
ose
Sta
rch
Number of matingsin experimental group
22 9
8 20
FemaleStarch
population 1
Ma
leS
tarc
hp
op
ula
tio
n 2
Number of matingsin control group
18 15
12 15
Starchpopulation 2
Sta
rch
po
pu
lati
on
1
Allopatric Populations and Reproductive Isolation
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B. Sympatric Speciation
Part 3 – Speciation
• In sympatric speciation, speciation takes place in
geographically overlapping populations
• can occur if a genetic change produces a reproductive
barrier between mutants and the parent population
• may be the result of:
polyploidy
extreme habitat differentiation
sexual selection
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Polyploidy
Sympatric Speciation
• is 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
• is much more common in plants than in animals
many important crops (oats, cotton, potatoes,
tobacco, and wheat) are polyploids
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Extreme Habitat Differentiation
Sympatric Speciation
• Sympatric speciation can result from the appearance
of new ecological niches
for example, populations of the North American
maggot fly prefer to live either on native hawthorn
trees or on more recently introduced apple trees
although they are not yet separate species on the
basis of any species concept, apple flies and
hawthorn flies are diverging
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Sexual Selection
Sympatric Speciation
• Sexual selection can drive sympatric speciation
such selection for mates of different colors
has likely
contributed
to speciation
in cichlid fish
in Lake
Victoria
Normal lightMonochromatic
orange light
P. pundamilia
P. nyererei
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Comparison with Allopatric Speciation
Sympatric Speciation
• In allopatric speciation, geographic isolation restricts gene
flow between populations
reproductive isolation may then arise by e. g., natural
selection or genetic drift, in the isolated populations
• 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 selection, or sexual selection
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• differ among organisms and can occur in time scales
a slow rate of speciation evidenced by a living horseshoe
crab (13 species) and a 300 million year-old fossil
a rapid rate of speciation evidenced by Galapagos finches
which have diversified into 13 species within the last
100,000 years
Part 3 – Speciation
C. Rates of Speciation
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• Gradual model
traditional evolutionary
trees diagram the
descent of species as
gradual divergence
Part 3 – Speciation
D. Dynamics
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• Punctuated equilibrium
is a contrasting model
of evolution
states that species most
often diverge in spurts
of relatively sudden change
accounts for the relative
rarity of transitional fossils
and hence appears to be a
more accurate view of
speciation dynamics
Dynamics
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Learning Objectives and Check of Understanding
BIOL101 Introduction to Biology B
• Compare and contrast the different species concepts?
• Distinguish prezygotic and postzygotic barriers.
• Differentiate allopatric and sympatric speciation.
• Compare and contrast the models describing the
dynamics of speciation.
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Reading Assignments
• Campbell: Chapter 24
• Sadava: Chapter 23
BIOL101 Introduction to Biology B
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