Download Biology 4974/5974 Evolution

Allopatric Speciation
Evolution
Biology 4974/597
D.F. Tomback
Biology 4974/5974
Evolution
Allopatric Speciation
And Hybridization
Grant and Grant 2002
R.J. Abbott (2003)Science 302: 1189-1190
Learning goals
Learning Goals--Know and understand:
• The reproductive isolating mechanisms and their
functions.
• General steps common to allopatric speciation models.
• Allopatric speciation models: geographic, parapatric,
peripatric.
• The speciation mechanisms involving polyploidy and
autopolyploidy, and diploid hybridization.
• The role of chromosomal rearrangements in speciation.
• Punctualism and gradualism as different speciation
time frames and models.
• The Shifting Balance Theory as a model of speciation.
• How speciation in Darwin’s finches may be explained
by peak shifts.
Reproductive isolating mechanisms
Isolating mechanisms prevent hybridization (nonadaptive gene exchange). (Mayr 1963)
Premating (prezygotic) isolating mechanisms:
• Temporal or habitat isolation
• Behavioral or sexual isolation
• Mechanical isolation
Postmating (postzygotic) isolating mechanisms
(these are a by-product of genetic differences):
• Gametic mortality
• Zygotic mortality
• Hybrid inviability
• Hybrid sterility
Benefits to avoiding hybridization
• Preserve adaptation to specific environments.
• Prevent disruption of adaptive gene complexes or
arrangements.
• Prevent investment of energy and gametes if hybrids are
inferior or inviable--reduced reproductive success.
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Allopatric Speciation
Evolution
Biology 4974/597
D.F. Tomback
General steps in allopatric speciation
Fig. 22.5
• Starts with geographic isolation
between populations. What
does this mean?
• Next, genetic differentiation
between populations occurs
over time.
• Eventually both populations
may come into contact again
through overlap (sympatry).
• Hybridization can result in
“reinforcement” of initial phases
of reproductive isolation.
• When gene flow no longer
occurs, speciation is achieved.
• Or, speciation does not occur
because hybrids are viable, and
genetic introgression (mixing)
occurs, and one population
emerges again.
Other allopatric speciation models
Peripatric speciation
Speciation in small population
outside of a main population. Also
called Founder Effect speciation
(Mayr 1954).
• Small, founding population “buds
off” from main species range.
• No gene flow occurs between the
small population and main range.
• Small population then undergoes
rapid genetic change through
genetic drift and genetic
reorganization.
• See “peak shift” process later in
lecture.
Fig. 23.1 b
Other allopatric speciation models
Parapatric speciation
Speciation in contiguous (neighboring) but nonoverlapping populations.
• Neighboring populations exist in different environments.
• Populations experience reduced gene flow and
diverge genetically.
aa
• Populations become reproductively isolated.
AA
• “Hybrids” are at a disadvantage, reinforcing
reproductive isolating mechanisms.
Aa
Selection for genetic differences between the two
populations must be greater than the rate of gene flow.
e.g., The grass, Anthoxanthum odoratum, has diverging populations with
heavy metal tolerance. Populations in mine tailings and
uncontaminated soil exist side-by-side. Populations with heavy metal
tolerance flower at different times and self-pollinate more frequently
than other populations.
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Allopatric Speciation
Evolution
Biology 4974/597
D.F. Tomback
Allopatric speciation models
Speciation by polyploidy
40-70% of plants are polyploids (p. 466), and
could have arisen from either mechanism
below. This mechanisms applies primarily to
plants.
Autopolyploidy—sympatric speciation
Allopolyploidy
• Change in ploidy level within a species is a single
genetic event.
• Speciation is instantaneous.
• Results from abnormal reduction division during
meiosis, which produces a 2n gamete followed by
fertilization.
• E.g., triploid (2n + n) or tetraploid (2n + 2n).
Allopolyploids—allopatric speciation
• Sterile hybrid initially forms.
• Chromosomal doubling produces a fertile hybrid..
Fig. 16.2
Allopatric speciation: Diploid hybridization
Hybridization is widespread in plants but there are cases of
hybridization in animals as well. Hybridization leads to important
outcomes:
1.Generating novel genotypes.
2.Founding new evolutionary lineages.
Fertile hybrids mediate gene flow from one species to another.
•e.g., (Grant and Grant 2008). They note that hybridization between
ground finches occurs in about 1% of matings. Most offspring die.
•In 1983, after heavy rains, many small seeds were available. Hybrids
(two ground finches and ground finch/cactus finch) survived using these
seeds. (Inprinted on wrong species’ song.)
•Backcrosses survived leading to genetic introgression. (Showed that
species not isolated genetically)
Hybrids may be better adapted to a unique habitat or environment
than either parental species.
•e.g., fleabane in Colorado (Apocynum androsaemifolium x A.
cannabinum = A. x-floribundum) (Johnson et al. 1998). These hybrids
are clonal and represent separate hybridization events.
•e.g., hybrids between sunflower species (next slide)
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Allopatric Speciation
Evolution
Biology 4974/597
D.F. Tomback
Example: the role of hybridization in sunflowers
Rieseberg et al. ( 2003): Recreated
hybridization events between the
sunflowers Helianthus annuus and H.
petiolaris.
• Believed to produce three hybrid
species H. anomalus, H. deserticola,
and H. paradoxus.
• These natural hybrids formed less
than 60 generations ago.
• The extreme phenotypes observed in
the wild were produced among
hybrids in the lab.
• These phenotypes were favored by
selection when planted in the natural
habitats occupied by hybrids in the
wild.
These hybridization events favored
ecological divergence. New species
adapted to novel environments.
Abbott 2003 Science 301: 1189-1190.
Chromosomal rearrangements in speciation
Closely related species, and even populations of the
same species, may differ in karyotype. Chromosomal
differences result from simple rearrangements, such
as inversions, translocations, and fusions between
chromosomes.
• Rearrangements may reduce or prevent gene flow.
• Rearrangements may spread within small, isolated
populations, the result of Founder Effects or genetic drift.
In small populations, they spread by drift, even if
deleterious.
• Chromosomal rearrangements can lead to reproductive
isolation and speciation.
Punctualism and gradualism
Punctualism and gradualism represent opposing
views of the time-frame of speciation, i.e., how fast
speciation occurs.
• Rates may be fast, such as the radiation of species after
a mass extinction or within a new adaptive zone, e.g.,
cichlid fish.
• Or slow, such as in living fossils such as the horseshoe
crab (Limulus) or the coelocanth (Latimeria), which have
not speciated in hundreds of millions of years.
• Gradualism reflects “Descent with modification,”
supported by geographic speciation—slow change over
time.
• Punctualism or punctuated equilibrium (advocated by
Mayr and later by N. Eldredge and S.J. Gould) suggests
that most speciation occurs sympatrically, often initiated
by chromosomal rearrangements, and proceeds quickly.
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Allopatric Speciation
Evolution
Biology 4974/597
D.F. Tomback
Punctualism and gradualism: graphic models
Avers 1989
Wright’s Adaptive Landscape Theory
Sewall Wright developed the concepts of Adaptive
Landscape and Shifting Balance to explain adaptation
within species.
The peaks represent genotypes of higher fitness than the
genotypes in the valleys, with height directly related to fitness.
End Box 19.1
Speciation by Peak Shift
A peak shift can lead to
speciation. It may involve
substantial genetic
reorganization and a new
phenotype.
• Mathematical models
suggest that once a
population slips off a peak,
natural selection rapidly
drives it up another adaptive
peak. Can culminate with
speciation.
• The mathematical timeframe
provides some support for
“punctuated equilibrium.”
Avers (1989) Patterns and processes in evolution.
Oxford University Press.
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Allopatric Speciation
Evolution
Biology 4974/597
D.F. Tomback
Example: speciation in Darwin’s finches and the
Adaptive Landscape Model
• Darwin’s finches speciated faster
than other similar groups (e.g.,
Hawaiian honeycreepers).
• Rather than a strict allopatric
model, the Grants favor a more
complex process involving
hybridization and events
represented by Wright’s Adaptive
Landscape Model.
• This involves a dynamic process
with changing environments and
food resources over time.
• As food resources change, the
peaks shift, so hybrids can
become more successful than
parent species.
Grant and Grant 2002
Study questions
• What are the general steps in allopatric speciation?
• What are the main features of each of the following allopatric
speciation models?
a. Geographic, b. Parapatric, c. Peripatric
• What are the key features of speciation by
polyploidy, allopolyploidy, and diploid hybridization?
• How did the work by Rieseberg et al. (2003) on hybridization in
sunflowers illustrate how diploid hybrids arise and can be successful?
• Define: punctuated equilibrium and gradualism.
• Interpret and understand how punctualism and gradualism differ
referring to the graphic models.
• How does the Shifting Balance Theory depict a speciation event?
• Explain how Grant and Grant (2002) use the Shifting Balance Theory
to explain speciation in Darwin’s finches.
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