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How do species occur?
Concept 24.2: Speciation can take
place with or without geographic
separation
• Speciation can occur in two ways:
–Allopatric speciation
–Sympatric speciation
Both work through a block of gene
flow between two populations.
Example
• Pupfish populations in Death
Valley.
• Generally happens when a
specie’s range shrinks for some
reason.
Fig. 24-5
(a) Allopatric speciation
(b) Sympatric speciation
Allopatric Speciation
• Allopatric = other homeland
• Ancestral population split by a
geographical feature.
• Comment – the size of the
geographical feature may be
very large or small.
The Process of Allopatric
Speciation
• In allopatric speciation, gene flow is
interrupted or reduced when a
population is divided into geographically
isolated subpopulations
• The definition of barrier depends on
the ability of a population to disperse
• Separate populations may evolve
independently through mutation, natural
selection, and genetic drift
Fig. 24-6
A. harrisi
A. leucurus
Evidence of Allopatric Speciation
• Regions with many geographic
barriers typically have more
species than do regions with
fewer barriers
Fig. 24-7
Mantellinae
(Madagascar only):
100 species
Rhacophorinae
(India/Southeast
Asia): 310 species
Other Indian/
Southeast Asian
frogs
100
60
80
1
2
40
20
0
3
Millions of years ago (mya)
1
3
2
India
Madagascar
88 mya
65 mya
56 mya
• Reproductive isolation between
populations generally increases as
the distance between them
increases
Degree of reproductive isolation
Fig. 24-8
2.0
1.5
1.0
0.5
0
0
50
200
100
150
Geographic distance (km)
250
300
• Barriers to reproduction are
intrinsic; separation itself is not
a biological barrier
Fig. 24-9a
EXPERIMENT
Initial population
Some flies
raised on
starch medium
Mating experiments
after 40 generations
Some flies
raised on
maltose medium
Fig. 24-9b
RESULTS
Female
8
20
Mating frequencies
in experimental group
Starch
population 1
9
Starch
population 2
22
Starch
population 1
Male
Male
Maltose Starch
Female
Starch
Maltose
Starch
population 2
18
15
12
15
Mating frequencies
in control group
Conditions Favoring Allopatric
Speciation
1. Founder's Effect - with the
peripheral isolate.
2. Genetic Drift – gives the
isolate population variation as
compared to the original
population.
Conditions Favoring Allopatric
Speciation
3. Selection pressure on the
isolate differs from the parent
population. (environment is
different on the edges)
Result
• Gene pool of isolate changes
from the parent population and
new species can form.
Sympatric Speciation
• Sympatric = same homeland
• New species arise within the range
of parent populations.
• Can occur in a single generation.
• In sympatric speciation,
speciation takes place in
geographically overlapping
populations
Plants
• Polyploids may cause new
species because the change in
chromosome number creates
postzygotic barriers.
Polyploidy
• Polyploidy is the presence of
extra sets of chromosomes due
to accidents during cell division
• An autopolyploid is an individual
with more than two chromosome
sets, derived from one species
Polyploid Types
1. Autopolyploid - when a
species doubles its
chromosome number from 2N
to 4N.
2. Allopolyploid - formed as a
polyploid hybrid between two
species.
–Ex: wheat
Fig. 24-10-1
Autopolyploid
2n = 6
4n = 12
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
Fig. 24-10-2
Autopolyploid
2n = 6
4n = 12
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
2n
Gametes
produced
are diploid..
Fig. 24-10-3
Autopolyploid
2n = 6
4n = 12
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
2n
Gametes
produced
are diploid..
4n
Offspring with
tetraploid
karyotypes may
be viable and
fertile.
Autopolyploid
• An allopolyploid is a species with
multiple sets of chromosomes
derived from different species
Fig. 24-11-1
allopolyploid
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Meiotic
error
Species A
2n = 6
Normal
gamete
n=3
Fig. 24-11-2
allopolyploid
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Meiotic
error
Species A
2n = 6
Normal
gamete
n=3
Hybrid
with 7
chromosomes
Fig. 24-11-3
allopolyploid
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Meiotic
error
Species A
2n = 6
Normal
gamete
n=3
Hybrid
with 7
chromosomes
Unreduced
gamete
with 7
chromosomes
Normal
gamete
n=3
Fig. 24-11-4
allopolyploid
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Meiotic
error
Species A
2n = 6
Normal
gamete
n=3
Hybrid
with 7
chromosomes
Unreduced
gamete
with 7
chromosomes
Normal
gamete
n=3
Viable fertile
hybrid
(allopolyploid)
2n = 10
Allopolyploid
• Polyploidy is much more common
in plants than in animals
• Many important crops (oats,
cotton, potatoes, tobacco, and
wheat) are polyploids
Habitat Differentiation
• Sympatric speciation can also
result from the appearance of
new ecological niches
• For example, the North American
maggot fly can live on native
hawthorn trees as well as more
recently introduced apple trees
Sexual Selection
• Sexual selection can drive
sympatric speciation
• Sexual selection for mates of
different colors has likely
contributed to the speciation in
cichlid fish in Lake Victoria
Fig. 24-12
EXPERIMENT
Normal light
P.
pundamilia
P. nyererei
Monochromatic
orange light
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
• 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
Adaptive Radiation
• Rapid emergence of several
species from a common ancestor
( often Allopatric speciation)
• Common in island and mountain
top populations or other
“empty” environments.
Mechanism
• Resources are temporarily
infinite.
• Most offspring survive.
• Result - little Natural Selection
and the gene pool can become
very diverse.
When the Environment
Saturates
• Natural Selection resumes.
• New species form rapidly if
isolation mechanisms work.
• Examples
–Galapagos – Finches
–Usambaras Mountains –
African violets
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