Download sympatric speciation

Speciation
Concept 22.2: Speciation can take place
with or without geographic separation
• Speciation can occur in two ways
• Allopatric speciation
• Sympatric speciation
Allopatric (“Other Country”) Speciation
• In allopatric speciation, a new species is created when a
population is divided into geographically isolated
subpopulations
• For example, the flightless cormorant of the Galápagos likely
originated from a flying species
on the mainland
The Process of Allopatric Speciation
• Different things can serve as geographic
barriers to different species
• For example, a canyon may create a barrier for
small rodents, but not birds, coyotes, or pollen
• The definition of a geographic barrier
depends on the ability of a population to
disperse
• A geographic barrier is anything that causes a
gene pool to separate into subgroups
• Once separated by a geographic barrier,
subpopulations may evolve independently
through mutation, natural selection, and
genetic drift
• Reproductive isolation may arise as a result
of genetic divergence
• Regions with many
geographic barriers
typically have more
species than do regions
with fewer barriers
• Reproductive isolation
between populations
generally increases as
the geographic distance
between them
increases
Sympatric (“Same Country”) Speciation
• In sympatric speciation,
speciation takes place in
populations that live in the
same geographic area
• Less common than
allopatric speciation
• Sympatric speciation occurs
when gene flow is reduced
between groups that
remain in contact through:
• Polyploidy
• Habitat differentiation
• Sexual selection
Polyploidy
• Polyploidy is the presence of
extra sets of chromosomes
due to accidents during cell
division
• Polyploidy is much more
common in plants than in
animals
• 80% of today’s plant species
were formed by polyploidy
speciation
• There are two forms of
polyploidy observed in
plants:
1. Autopolyploid
2. Allopolyploid
Polyploidy
• An autopolyploid is an individual with more than two
chromosome sets, derived from one species
• The offspring of matings between autopolyploids
and diploids have reduced fertility, which can create
reproductive isolation without any geographic separation
• Autopolyploids must self polinate or mate with other
autopolyploids in order to create viable offspring
Figure 22.UN01
Cell
division
error
2n  6
2n
Tetraploid cell
4n  12
New species
(4n)
Gametes produced
by tetraploids
• An allopolyploid is a species with multiple sets of
chromosomes derived from different species
• Allopolyploids cannot interbreed with either parent
species
• This creates reproductive isolation in populations that have
not been physically separated
Figure 22.9-4
Species A
2n  6
Species B
2n  4
Meiotic error; chromosome
number not reduced from 2n to n
Normal
gamete
n3
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Normal
gamete
n3
Unreduced gamete
with 7 chromosomes
New species:
viable fertile hybrid
(allopolyploid)
2n  10
• Many important crops (oats,
cotton, potatoes, tobacco, and
wheat) are polyploids
• Wheat is an allohexaploid (6 sets
of chromosomes, 2 each from
three different species)
• Wheat underwent spontaneous
polyploidy events as early as
8,000 years ago to become what
we use for bread
Habitat Differentiation
• Sympatric speciation can also result from the appearance
of new ecological niches
• New species can from when genetic changes enable a
subpopulation to exploit a habitat or resource not used by the
parent population
Sexual Selection
• Sexual selection can drive sympatric speciation
• Mate choice can serve as a main reproductive barrier
that keeps two gene pools separate
Allopatric and Sympatric Speciation: A
Review
• In allopatric speciation, geographic isolation restricts
gene flow between populations
• Reproductive isolation may then arise by natural
selection, genetic drift, or sexual selection in the isolated
populations
• Even if contact is restored between populations,
interbreeding is prevented by reproductive barriers
• In sympatric speciation, a reproductive barrier isolates a
subset of a population without geographic separation
from the parent species
• Sympatric speciation can result from polyploidy, natural
selection, or sexual selection