Download Sympatric Speciation

Sympatric speciation
Evolution
Biol. 4974/5974
D.F. Tomback
Sympatric Speciation
Biol. 4974/5974
Evolution
Figures from Hall and Hallgrimsson, 2014, Strickberger’s Evolution, Jones and Bartlett
Learning goals
Know and understand:
• How populations with phenotypic plasticity are potential
candidates for sympatric speciation.
• How the process of sympatric speciation differs from that of
allopatric speciation.
• How sympatric speciation works by the process of disruptive
selection, and how assortative mating can lead to
reproductive isolation.
• Examples illustrating sympatric speciation, including the North
American fly (apple maggot), cichlid fish diversification, and
redwood salamander forming a “ring species”.
Reaction norms and phenotypic plasticity
For some species, phenotype varies with the environment,
which means that the genotype has more than one reaction
norm. This leads to the possibility of sympatric speciation.
• Different phenotypes or morphs are produced in response to
different abiotic or biotic conditions.
• This response is referred to as phenotypic plasticity.
Examples discussed in the text:
• Rotifers and water fleas develop protective spines in the
presence of predators.
• When food is restricted, tadpoles of spadefoot toads develop
a cannabilistic morph with bigger heads and jaws.
• In three-spined stickleback fish, the pelvic girdle and spines
may be reduced in the absence of predators.
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Sympatric speciation
Evolution
Biol. 4974/5974
D.F. Tomback
Example: North American emerald moth
Caterpillars feed on oak trees. They vary in morphology,
depending on time of hatching:
• Spring hatching: Caterpillars feed on catkins, which they
resemble, the catkin morphs.
• Summer hatching: Caterpillars feed on oak leaves and mimic
twigs, the twig morphs.
The difference in morphology is triggered by the tannins in oak
leaves.
Fig. 22.4
Sympatric speciation
Defined as speciation
without geographical
isolation.
Phenotypic plasticity
within a population can
lead to speciation.
Reproductive isolation
evolves between
different phenotypes.
Sympatric speciation by disruptive selection
Most sympatric speciation models rely on disruptive selection:
Individuals sort by phenotype (morph) into different
microhabitats or on different resources.
• Intermediate phenotypes are not well-adapted and selected
against.
• Natural selection increases an individual’s ability to
discriminate between phenotypes (“us” vs. “them”).
• Leads to assortative mating: like mating with like.
• This reduces gene flow between morphs.
Assortative mating may result from imprinting, pheromone
differences, host plant differences--where mating occurs, or
fertility differences.
Example: Pea aphids suck sap from plants in the pea family.
• Aphids raised on red clover prefer red clover, and aphids raised
on alfalfa prefer alfalfa.
• Hybrids do poorly.
• Populations are now diverging (Via 1991).
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Sympatric speciation
Evolution
Biol. 4974/5974
D.F. Tomback
Sympatric speciation in the North American fly
The North American fly or apple maggot is undergoing
sympatric speciation.
• Natural diet of the maggot (larva) is the fruit of the hawthorn
bush, which is a distant relative of apples.
• Cultivated apples were introduced to North America around
1800.
• Within 50 years some maggots fed on apples.
• The hawthorn-feeding morph and apple-feeding morph would
not switch between foods.
• The adults of these different morphs emerge at different
times, coinciding with fruit ripening.
• The different morphs are genetically differentiated.
• Speciation is not entirely complete, but changes have
occurred in the genome.
Sympatric speciation in cichlid fishes
Adaptive radiation occurs when a lineage rapidly diversifies,
producing many species adapted to a similar ecological lifestyle
or niche. Rapid diversification in cichlid fish in the African Rift
Valley and in Nicaragua and in the Amazon.
• Since the end of the ice age (14,000 years ago in Africa), 300
new cichlid species arose in Lake Victoria.
• Diversification also in other Rift Valley lakes.
• Adaptive radiation by sympatric speciation, as a consequence
of a new adaptation, pharyngeal jaws.
• With two two sets of jaws, the fish adapted to different,
specialized diet.
• The outer set of jaws used to capture prey; the pharyngeal set
used to grind up prey.
Cichlid diversification
Fig. 22 B2.1
Fig. 22.7
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Sympatric speciation
Evolution
Biol. 4974/5974
D.F. Tomback
Ring species
Fig. 22.10
Sympatric speciation in progress in
ring species. Known from several
groups: e.g., Greenish Warblers,
Black-headed gulls, and the
redwood salamander.
• Redwood salamander, Ensatina
escholtzii comprises 7
subspecies in California Central
valley.
• All overlap and interbreed,
except for several
southernmost populations of E.
s. klauberi and E. s. escholtzii,
which do not hybridize.
• Speciation appears complete in
these southern populations.
Study questions
• What is phenotypic plasticity? Why are populations with phenotypic
plasticity good candidates for sympatric speciation?
• Explain phenotypic plasticity in the North American Emerald moth, and
how it is triggered by a biotic factor. Why is phenotypic plasticity adaptive
for the moth?
• How does the process of sympatric speciation differ from that of allopatric
speciation? Explain how both disruptive selection and assortative mating
can lead tosympatric speciation.
• Explain the process of sympatric speciation, as it is now occurring in the
North American fly. What started this process?
• Cichlid fish in African lakes have experienced explosive sympatric
speciation. How rapidly has this occurred? Explain how a new adaptation
may have contributed to this tremendous adaptive radiation of fish.
• Explain how the redwood salamander “ring species” represents a gradient
in the degree of completion of sympatric speciation. The subspecies
appear to be in the process of speciating.
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