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Speciation | OAT
Species
• Species - a group of individuals
who can interbreed and yield
fertile offspring
• Through a process called
speciation, individuals who were
once able to interbreed become
sexually isolated
o They become unable to
yield fertile offspring
• After speciation occurs, the
individuals are classified as
different species
Plant Cell with 3 Chromosomes
•
Allopatric (Geographic) Speciation
• Allopatric speciation occurs
when a physical barrier separates
members of a species
• Once separated, the individuals
are exposed to different
environments
o And therefore, different
selective forces
• Through a process termed
adaptive radiation, the nowseparated populations will evolve
in response to their unique
environments
• After many generations of
evolution, the populations may
become so evolutionarily
distanced that they can no longer
interbreed
o And thus, speciation has
occurred
If this cell undergoes meiosis I,
we’d expect each tetrad to be
pulled to opposite ends of the cell
o Such that 2 new cells are
yielded, each with 3
chromosomes
Plant Cell Undergoing Meiosis I
The tetrads are separated and pulled to opposite
ends of the cell.
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But suppose that nondisjunction
occurs, such that one of the
centrosomes fails to reel in its 3
chromosomes
Nondisjunction
In an event known as nondisjunction,
chromosomes are not evenly distributed during
meiosis I. One cell will end up with 6
chromosomes.
Sympatric Speciation
• Sympatric speciation is
speciation that occurs in the
absence of a physical barrier
• Suppose we have a plant cell that
contains 3 chromosomes
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Mutant 6-chromosomed cells
would be created
Meiosis II would then form
mutant 6-chromosomed gametes
The 6-chromosomed gametes
may be capable of growing into
full-fledged plants
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Speciation | OAT
•
o However, they will need
to be fertilized by other 6chromosomed gametes
6-Chromosomed Gamete
The original plant would ordinarily yield 3chromosomed gametes. But nondisjunction
induced the generation of mutant 6chromosomed gametes. These gametes must be
fertilized by other 6-chromosomed gametes.
•
•
The 6-chromosomed mutant will
no longer be able to breed with
the 3-chromosomed plant
o By virtue of this
polyploidy, speciation has
occurred
o Plants of the two
genotypes can breed, but
cannot interbreed
Thus, they are
different species
•
Reproductive Isolation
• Both allopatric and sympatric
speciation yield the same resultreproductive isolation
o Two species are left
unable to breed with one
another
• There are a two categories of
reproductive isolation
o Pre-zygotic isolation
o Post-zygotic isolation
• Pre-zygotic isolation occurs
when the speciation is such that
the members of the 2 species
don’t have the opportunity to
reproduce
o A zygote never even
forms
Examples of pre-zyogtic
isolation:
o Geographic isolation –
the species are separated
by a physical barrier
o Temporal isolation – the
species mate during
different times of the year
o Mechanical isolation –
the sizes/shapes of the
species’ reproductive
organs are not compatible
o Behavioral isolation –
mating rituals differ
between the populations
Post-zygotic isolation occurs
when speciation is such that a
zygote forms, but that zygote is
sterile or not viable for survival
Examples of post-zygotic
isolation
o A horse and a donkey can
breed to yield a mule
But the mule is
sterile
o Because the horse and
donkey could not yield
fertile offspring, they are
reproductively isolated
o But since the isolation
occurred after the zygote
(the mule) formed, this is
post-zygotic isolation
Post-Zygotic Isolation
A horse and donkey can mate, and successfully
yield a child (a mule). However, that mule is
infertile- it cannot reproduce- and so the horse
and donkey are still considered to be
reproductively isolated.
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Speciation | OAT
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Other post-zygotic barriers to
reproduction include:
o Zygote abnormality – the
zygote forms, but dies
during development
o Low viability – the
zygote develops, but dies
prior to reaching
adulthood
Convergent Evolution
The wings of birds and bats both serve to
facilitate flight. But birds and bats do not derive
their wings from a common ancestor.
Trends in Evolution
• Divergent evolution – multiple
species retain a similar structure
that they acquired by virtue of
their common ancestor
o i.e. humans and
chimpanzees both have
arms/legs by virtue of
their common ancestor:
4-legged tetrapods
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There’s a tendency to think that
evolution is a diversifying force,
always making species more
diverse
o This is not true
Consider a population of
genetically-diverse horses
A Population of Genetically Diverse Horses
•
Divergent Evolution
Humans and chimpanzees both have arms/legs
by virtue of their common ancestor: 4-legged
tetrapods.
•
Convergent evolution – multiple
species evolve similar structures,
but did not acquire those
structures by virtue of a common
ancestor
o i.e. birds and bats both
have wings to facilitate
flight, but did not acquire
their wings from a
common ancestor
Suppose that the environment
that these horses occupy becomes
exceedingly hot
o So hot that only a few
members are able to
withstand the heat
Rising Temperature of the Enivironment
With the increase in temperature, only a few
members of the once-diverse population are able
to survive.
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Speciation | OAT
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Much of the population’s
diversity has thus been lost
This phenomenon is called
evolutionary bottleneck and it
causes a population to converge
on a less-diverse population
Polymorphisms
• Certain physical differences exist
among members of all species
o i.e. subtle differences in
height
• These subtle differences are not
evolutionary differences
• Rather, they are slight genetic
differences called
polymorphisms
Polymorphisms
Certain genetic differences (i.e. subtle
differences in height) are not evolutionary
differences. They are the products of slight
genetic differences called polymorphisms.
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