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Speciation
Professor Andrea Garrison
Biology 3A
Illustrations ©2010 Pearson Education,
Inc. , unless otherwise noted
Speciation
• Natural selection drives evolution by favoring
adaptive traits
• Works on level of gene pool
– Gene pool = all alleles for all genes in population
– Affects entire population
• Agents of change
– Mutation (= chance changes in DNA)
• Rare; 1 in100,000 to 1 in 1,000,000 events
• Over evolutionary time scales, they add up
– Sexual reproduction (mixes alleles; speeds up
opportunities for selection of alleles)
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Speciation
• To understand how speciation can happen,
and to see how to test it’s effects in
experiments, it helps to understand what
prevents speciation
• This is a conceptual model that is assumed to
work as a control
– It provides a theoretical reference point against
which observations can be evaluated
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Speciation
• Hardy-Weinberg Principle
– Null model
• Predicts what would happen if evolution has no effect
• Specifies condition under which population of diploid organisms achieves
genetic equilibrium
– Genetic equilibrium = genotype and allele frequencies remain stable in succeeding
generations
– Population’s gene pool often has 2 or more alleles for each gene
– Genotype frequency = % of individuals possessing each genotype
• AA, Aa, aa
• AA + Aa + aa = 1 (=100% of genotypes)
– Allele frequency = relative abundance of each allele
• For gene with 2 alleles, p = frequency of one allele, q = frequency of second
allele
• p + q = 1 (=100% of alleles)
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Speciation
Speciation; Table ©2014 Cengage Learning
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Speciation
• Hardy-Weinberg Principle
– Null model
• Genetic equilibrium possible only if all of the following are true
–
–
–
–
–
No mutations occurring
No immigration from other populations
Very large population
All genotypes survive to reproduce equally well
Mating is random
• Under the above conditions, no microevolution occurs
• Allele frequencies will never change, and genotype frequencies
stop changing after one generation
• If population’s genotype frequencies do not match model’s
predictions, or if allele frequencies change over time,
microevolution may be occurring
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Speciation
• Hardy-Weinberg equation
p2 + 2pq + q2 = 1 (100% of population)
If equation is true, population is at genetic equilibrium
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Speciation
For snapdragon population allele frequencies are
p2 + 2pq + q2 = 1
p = 0.7
q = 0.3
0.72 + 2(0.7 x 0.3) + 0.32 = 0.49 + 0.42 + 0.09 = 1
Out of 1000 plants, Hardy-Weinberg predicts:
490 CRCR (red), 420 CRCW (pink), 90 CWCW (white)
Observations showed 450 red, 500 pink, 50 white; so population is not in equilibrium
Speciation; Table ©2014 Cengage Learning
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Speciation
• Allele frequencies will change over time if
Hardy-Weinberg conditions are not met
• Processes that foster microevolutionary
change :
– Mutation
– Gene flow
– Genetic drift
– Natural selection
– Nonrandom mating
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Speciation
• Mutation
– Spontaneous change in DNA (causes microevoution only
when hereditary, meaning in DNA of germ cells)
– Have little or no immediate effect on allele frequency, but
are available to be selected for by natural selection
• Deleterious mutations: harmful, not usually passed on
• Lethal mutations: kill all carriers (if dominant) or only
homozygous carriers (if recessive), not usually passed
on unless there is some advantage to heterozygote
• Neutral mutations: are neither harmful nor helpful
• Advantageous mutations: confer a benefit, natural
selection may increase frequency
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Speciation
• Gene flow
– Movement of
organisms or their
gametes (pollen) to a
different population
– Animals often
migrate to other
populations
– Dispersal
mechanisms provide
gene flow
• Wind pollination
Speciation; photos ©2014 Cengage Learning
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Speciation
• Genetic Drift
– Chance loss of alleles
• Found in small populations
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Genetic Drift
• Founder Effect
– Establishment of small population whose gene pool differs from
parent population
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Genetic Drift
• Bottleneck Effect
– Natural disasters decrease population to extent some alleles
underrepresented by chance
– Difficult to maintain healthy population
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Speciation
• Natural selection
– Environment may select for specific phenotypes
• Predators choose:
– Smallest of prey population
» Selects for larger individuals
– Slowest of prey population
» Selects for faster individuals
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Speciation
• Nonrandom mating
– Many species mate nonrandomly
– Selecting mate with specific phenotypes increases
the alleles associated with that phenotype
– Inbreeding (mating with closely related
individuals) occurs within small populations
• increases frequency of homozygous genotypes and
decreases frequency of heterozygous genotypes
• Recessive phenotypes often expressed
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Patterns of Speciation
• How does one species evolve from another?
A → A1
non-branching evolution
A1
A
divergence (branching evolution)
A2
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Speciation
• Key to speciation is presence or absence of
reproduction
– Local subgroups may have chance mutations
– Not new species until so different they cannot
interbreed
• Population = all members of a species within
given geographic area (often taken to mean
individuals interbreeding with one another)
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Speciation
• Non-branching evolution A → A1
– changes over time, A1 cannot interbreed with A
• Divergence – A1 and A2 must be kept from
interbreeding until they are so different they
cannot interbreed
A1
A
A2
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Speciation
• Divergence occurs because local populations
breed only with their neighbors, and not those
members of the same species on “the other side
of the hill”
– If members of two subgroups interbreed, no
divergence
• They share any chance mutations
– If members of two subgroups do not interbreed,
divergence can occur
• No sharing of chance mutations
– Chance mutations are different
– Two different environments select differently for different
mutations
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Speciation
• Some organisms recognized as separate
species today are actually capable of
interbreeding, and would converge back to
one species if the opportunity existed
• REPRODUCTIVE ISOLATING MECHANISMS
– Prevent reproduction between two populations
– Maintain separate species
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Reproductive Isolating Mechanisms
• Prezygotic mechanisms
– Geographical isolation
• Geographical ranges do not coincide
• Species never encounter each other
– Example: English Oak in Europe and Valley Oak in
California
• Never close enough to interbreed, although they could
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Reproductive Isolating Mechanisms
• Prezygotic mechanisms
– Ecological (habitat) isolation
• Two species live in same area, but different habitats
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Reproductive Isolating Mechanisms
• Prezygotic mechanisms
– Temporal isolation
• Closely related species mate at different times of the year
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Reproductive Isolating Mechanisms
• Prezygotic mechanisms
– Behavioral isolation
• Closely related species with different mating rituals
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Reproductive Isolating Mechanisms
• Prezygotic mechanisms
– Mechanical isolation
• Structural differences between species (copulatory organs, pollen
grains, etc.
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Reproductive Isolating Mechanisms
• Prezygotic mechanisms
– Gametic isolation
• Chemical differences between gametes (e.g., proteins on surface of
gametes)
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Reproductive Isolating Mechanisms
• Postzygotic mechanisms
– Reduced hybrid viability
• Hybrid is weak or dies
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Reproductive Isolating Mechanisms
• Postzygotic mechanisms
– Reduced hybrid fertility
• Hybrid is sterile
Speciation; left illustration Cengage
Learning 2014
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Reproductive Isolating Mechanisms
• Postzygotic mechanisms
– Hybrid breakdown
• Hybrid reproduction
produces weak offspring
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Allopatric Speciation
• Speciation while subpopulations separated
– Geographical separation breaks population into
two subgroups preventing gene flow
– Mutations eventually cause reproductive isolation
(mechanical, temporal, gametic)
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Allopatric Speciation
1 At first, a
22 A geographical
population is
change, such as a
distributed over a change in the river’s
large geographical course, separates
area. A river flows the original
along one edge of population, creating
the population’s a barrier to gene
geographical range. flow.
33 In the absence
of gene flow, the
separated
populations evolve
independently and
diverge into
different species.
44 When the river
later changes
course again,
allowing individuals
of the two species
to come into
secondary contact,
they do not
interbreed.
Figure 22-9, p. 486
Sympatric Speciation
• Speciation while
subgroups are not
separated
• Generally insects that
live entire life (feed,
mate, lay eggs) on same
bush
– Which bush determined
by genetics or what
larvae ate
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