Transcript Sympatric speciation

Mechanisms of
Evolution
Macroevolution
Speciation
Sympatric (“Same Country”) Speciation
• In sympatric speciation
– Speciation takes place in geographically
overlapping populations
• Sympatric speciation: A new species
originates in within the range of the
parent population.
• Reproductive isolation can evolve
without geographical isolation.
– This can occur quickly (in one generation) if a
genetic change results in a reproductive barrier
between the mutants and the parent
population.
Polyploidy
• Polyploidy
– Is the presence of extra sets of chromosomes
in cells due to accidents during cell division
– Has caused the evolution of some plant
species
• Speciation by the formation of polyploids
• Many plant species have originated from
improper cell division that results in
extra sets of chromosomes. This is a
mutant condition called polyploidy.
• Depending on the origin of the extra set
of chromosomes, polyploids are either:
autopolyploids or allopolyploids.
• Autopolyploid: An organism that has
more than two sets of chromosome, all
derived from a single parent species.
See Figure 24.8.
• 1. Nondisjunction in the germ cell line (in either mitosis or meiosis)
results in diploid gametes.
• 2. Self-fertilization would double the chromosome number to
tetraploid.
• 3. Tetraploids can self-pollinate or mate with other tetraploids.
• 4. The mutants cannot interbreed with diploids of the parent
population because triploid hybrids would be sterile due to unpaired
chromosomes interfering with meiosis.
Failure of cell division
in a cell of a growing
diploid plant after
chromosome duplication
gives rise to a tetraploid
branch or other tissue.
Gametes produced
by flowers on this
branch will be diploid.
Offspring with tetraploid
karyotypes may be viable
and fertile—a new
biological species.
2n
2n = 6
4n = 12
Figure 24.8
4n
• An instantaneous special genetic event
could thus produce a postzygotic barrier
which isolates the gene pool of the
polyploid in just one generation.
• Allopolyploid: A polyploid hybrid with
parents of two different species. See
Figure 24.9.
•
•
•
•
1. More common than autopolyploidy.
2. Potential evolution of an allopolyploid begins when two different
species interbreed and a hybrid is produced.
3. Such interspecific hybrids are usually sterile: The haploid sets
of chromosomes from each species cannot pair with the
chromosomes from the other species.
4. These sterile hybrids may propagate asexually.
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Species A
2n = 4
Unreduced gamete
with 7 chromosomes
Viable fertile hybrid
(allopolyploid)
Meiotic error;
chromosome
number not
reduced from
2n to n
2n = 10
Normal gamete
n=3
Species B
2n = 6
Figure 24.9
Normal gamete
n=3
• Two mechanisms can transform sterile
allopolyploids into fertile polyploids.
• 1. Mitotic nondisjunction in the
reproductive tissue of the hybrid may
double chromosome number.
– The hybrid clone will then be able to produce
gametes by meiosis because each
chromosome will have a homologue.
– This would produce a new species of
interbreeding individuals, reproductively isolated
from both parent species.
• 2. Meiotic nondisjunction in one species produces an
unreduced (diploid) gamete (24.9).
– This abnormal gamete fuses with a normal haploid gamete of a
second species and produces a triploid hybrid.
– In the sterile triploid clone, meiotic nondisjunction again produces
an unreduced gamete (triploid).
– Combination of this triploid gamete with a normal haploid gamete
from the second parent species would result in a fertile hybrid with
homologous pairs of chromosomes.
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Species A
2n = 4
Unreduced gamete
with 7 chromosomes
Viable fertile hybrid
(allopolyploid)
Meiotic error;
chromosome
number not
reduced from
2n to n
2n = 10
Normal gamete
n=3
Species B
2n = 6
Figure 24.9
Normal gamete
n=3
• Speciation by polyploidy (especially
allopolyploidy) has been important in plants.
(relatively rare in animals)
• Some allopolyploids are very vigorous because
they contain the best qualities of both parent
species.
• The accidents required to produce these new plant
species (interspecific hybridization coupled with
nondisjunction) have occurred often enough that
between 25% and 50% of all plant species are
polyploids.
Sympatric speciation may also occur animals,
but the mechanisms are different.
• A group of animals may become isolated
within the range of a parent population if
genetic factors cause them to become fixed
on resources not used by the parent
population as a whole.
Resource Fixation
• Example:
• 1. A particular species of wasp pollinates a
particular species of figs. The wasps mate
and lay their eggs in the figs.
• 2. A genetic change causes some wasps to
select a different fig species. This would
segregate mating individuals of the new
phenotype from the parent population.
• 3. Divergence could then occur after such
an isolation.
Sympatric speciation may also occur animals,
but the mechanisms are different.
• Sympatric speciation could result from
balanced polymorphism combined with
assortative mating (sexual selection).
Polmorphism and assortative mating
• • Example: if birds in a population that is
dimorphic for beak size began to selectively mate
with birds of the same morph, speciation could
occur over time.
• Also blue/white snowgeese example
Sympatric speciation may also occur animals, but
the mechanisms are different.
• Sympatric speciation
– Can also result from the appearance of new
ecological niches
Habitat Differentiation and Sexual Selection
• In cichlid fish
– Sympatric speciation has resulted from nonrandom
mating due to sexual selection
EXPERIMENT
Researchers from the University of Leiden placed males and females of Pundamilia pundamilia and
P. nyererei together in two aquarium tanks, one with natural light and one with a monochromatic orange
lamp. Under normal light, the two species are noticeably different in coloration; under monochromatic orange
light, the two species appear identical in color. The researchers then observed the mating choices of the fish
in each tank.
Monochromatic
Normal light
orange light
P. pundamilia
P. nyererei
RESULTS
CONCLUSION
Figure 24.10
Under normal light, females of each species mated only with males of their own species. But
under orange light, females of each species mated indiscriminately with males of both species.
The resulting hybrids were viable and fertile.
The researchers concluded that mate choice by females based on coloration is the main
reproductive barrier that normally keeps the gene pools of these two species separate. Since
the species can still interbreed when this prezygotic behavioral barrier is breached in the
laboratory, the genetic divergence between the species is likely to be small. This suggests
that speciation in nature has occurred relatively recently.
Allopatric and Sympatric
Speciation: A Summary
• In allopatric speciation
– A new species forms while geographically
isolated from its parent population
• In sympatric speciation
– The emergence of a reproductive barrier
isolates a subset of a population without
geographic separation from the parent species
Allopatric vs. Sympatric speciation.
• Both allopatric speciation and sympatric
speciation have important roles in plant
evolution, but in animals, allopatric speciation
is much more common.
Adaptive Radiation
• Adaptive radiation
– Is the evolution of diversely adapted species from a
common ancestor. Generally occurs when new
environments open up to colonization.
Figure 24.11
Adaptive Radiation
• The Galapagos are volcanic islands close
enough to the coast of South America to allow
for occasional colonization, yet far enough to
prevent significant gene flow.
• They are relatively young islands, and they
provided new uninhabited environments
waiting to be exploited by colonizing species.
• As organisms adapted to the unique
environments on the islands, many endemic
species diverged.
Adaptive Radiation
•
Another example is the Hawaiian Archipelago, volcanic islands 3500 km
from the mainland.
•
1. Hawaii is the youngest (<one million years old), largest island and has
active volcanoes.
•
2. The islands grow progressively older in a northwesterly direction away from
Hawaii.
•
3. As each island was formed and cooled, flora and fauna carried by ocean
and wind currents from other islands and continents became established.
•
4. The physical diversity of each island provided many
environmental opportunities for evolutionary divergence.
•
5. Multiple invasions and allopatric speciation have caused so much adaptive
radiation that there are thousands of species endemic to the islands.
• The Hawaiian archipelago
– Is one of the world’s great showcases of
adaptive radiation
Dubautia laxa
1.3 million years
MOLOKA'I
KAUA'I
MAUI
5.1
million
years O'AHU LANAI
3.7
million
years
Argyroxiphium sandwicense
HAWAI'I
0.4
million
years
Dubautia waialealae
Figure 24.12
Dubautia scabra
Dubautia linearis