Chapter 24The Origin of
SpeciesPart B
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/SoutheastAsia): 310 species
Other Indian/Southeast Asianfrogs
Millions of years ago (mya)1 2 3
1 2 3
100 80 60 40 20 0
88 mya 65 mya 56 mya
India
Madagascar
• Reproductive isolation between populations generally increases as the distance between them increases
Fig. 24-8
Geographic distance (km)
Degr
ee o
f rep
rodu
ctive
isol
ation
00
50 100 150 250200 300
0.5
1.0
1.5
2.0
• Barriers to reproduction are intrinsic; separation itself is not a biological barrier
Fig. 24-9a
EXPERIMENT
Initial population
Some fliesraised on
starch medium Mating experimentsafter 40 generations
Some fliesraised on
maltose medium
Fig. 24-9b
RESULTS
FemaleFemale
StarchStarch Starch
Maltose population 1 population 2
Mal
e Star
chM
alto
se Mal
eSt
arch
Star
chpo
pula
tion
1po
pula
tion
2
22
8 20
9 18
12
15
15
Mating frequenciesin experimental group
Mating frequenciesin 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 Types1. 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
2n = 6 4n = 12
Failure of celldivision afterchromosomeduplication givesrise to tetraploidtissue.
Autopolyploid
Fig. 24-10-2
2n = 6 4n = 12
Failure of celldivision afterchromosomeduplication givesrise to tetraploidtissue.
2n
Gametesproducedare diploid..
Autopolyploid
Fig. 24-10-3
2n = 6 4n = 12
Failure of celldivision afterchromosomeduplication givesrise to tetraploidtissue.
2n
Gametesproducedare diploid..
4n
Offspring withtetraploidkaryotypes maybe viable andfertile.
Autopolyploid
Autopolyploid
• An allopolyploid is a species with multiple sets of chromosomes derived from different species
Fig. 24-11-1
Species A2n = 6
Normalgameten = 3
Meioticerror
Species B2n = 4
Unreducedgametewith 4chromosomes
allopolyploid
Fig. 24-11-2
Species A2n = 6
Normalgameten = 3
Meioticerror
Species B2n = 4
Unreducedgametewith 4chromosomes
Hybridwith 7chromosomes
allopolyploid
Fig. 24-11-3
Species A2n = 6
Normalgameten = 3
Meioticerror
Species B2n = 4
Unreducedgametewith 4chromosomes
Hybridwith 7chromosomes
Unreducedgametewith 7chromosomes
Normalgameten = 3
allopolyploid
Fig. 24-11-4
Species A2n = 6
Normalgameten = 3
Meioticerror
Species B2n = 4
Unreducedgametewith 4chromosomes
Hybridwith 7chromosomes
Unreducedgametewith 7chromosomes
Normalgameten = 3
Viable fertilehybrid(allopolyploid)2n = 10
allopolyploid
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 lightMonochromatic
orange light
P.pundamilia
P. nyererei
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
More next time
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