Speciation
Level 1 Biological Diversity
Jim Provan
Campbell: Chapter 24
Modes of speciation
Reproductive barriers form boundaries around
species and the evolution of these barriers is the key
biological event in the origin of new species:
An essential episode in the origin of a species occurs when
the gene pool of a population is separated from other
populations of the parent species
This genetically isolated splinter group can follow its own
evolutionary course: selection, drift and mutation are not
balanced by gene flow
There are two general modes of speciation:
Allopatric speciation
Sympatric speciation
Allopatric speciation
Allopatric speciation occurs when the initial block to
gene flow is a physical barrier that isolates the
population:
Geological processes can fragment a population:
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Emergence of mountain ranges, movement of glaciers,
formation of land bridges, subsidence of large lakes
Small populations may become isolated after migration
Extent of barrier needed to isolate populations depends on
the ability of the organism to disperse (mobility in animals,
pollen, seed and spore movement in plants):
—
Both rims of the Grand Canyon are populated by the same
species of birds, but different, unique species of rodents
Allopatric speciation in the pupfish
In Death Valley, isolated
springs are remnants of a
historical river network
Each inhabited spring
contains its own species
of pupfish (Cyprinodon
spp.) which is found
nowhere else in the world
Probably derived from a
single ancestral species
whose range was
fragmented when the
region became arid
Conditions favouring allopatric
speciation
When populations become allopatric, speciation can
occur as isolated gene pools diverge genetically:
A small, isolated population is more likely to change substantially
enough to become a new species than a large one
The geographic isolation of a small population usually occurs at
the fringe of the parent population’s range
Peripheral isolates are good candidates for speciation:
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The gene pool of the peripheral isolate probably differs from that of
the parent population since fringe inhabiters usually represent the
most extreme genotypes
Genetic drift will continue to change the gene pool until a large
population is formed
Evolution caused by selection is likely to take a different direction in
the peripheral isolate than in the parent population
Most peripheral isolates do not survive long enough to speciate
Adaptive radiation on island chains
Adaptive radiation is the
evolution of many
diversely adapted
species from a common
ancestor
Example is Darwin’s
finches in the
Galapagos
Multiple events of
colonisation,
adaptation, speciation
and recolonisation
Sympatric speciation
Sympatric speciation is the formation of a new species
within the range of the parent population
Reproductive isolation without geographical isolation
Can occur if a mutation isolates a group from parent population
Many plant species have evolved through polyploidy:
Autopolyploids have chromosomes derived from a single species:
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Nondisjunction in the germ line cell results in diploid gametes
Selfing would lead to tetraploids which cannot breed with diploids
Allopolyploids arise from two different species:
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More common that autopolyploidy
May initially be sterile due to incompatible chromosome numbers
but may subsequently become fertile
Polyploidy in plants
Polyploidy in plants
Some allopolyploids are vigorous because they
contain the best qualities of both parents
25-50% of plant species are polyploid
Many are recent and/or important to humans:
The grass Spartina angelica (2n = 122) evolved in the 1870s
from S. maritima (2n = 60) and S. alternaflora (2n = 62)
Bread wheat (Triticum aestivum) is a 42 chromosome
hexaploid which originated from a 28 chromosome cultivated
wheat and a 14 chromosome wild grass
Other important polyploid species include oats,
cotton, potatoes and tobacco
Evolution of wheat
Sympatric speciation in animals
A group of animals may become isolated in the range
of a parent population due to resource utilisation:
Wasp which pollinate figs mate and lay their eggs in the figs
A genetic change which causes certain wasps to select
different fig species will segregate mating individuals
Divergence can occur after such an isolation
Cichlid fishes in Lake Victoria have probably evolved
numerous species due to exploitation of different food
sources and other resources
Sympatric speciation can also occur from a balanced
polymorphism combined with assortive mating e.g.
finches that are dimorphic for beak size
Genetic change and speciation
Classification as allopatric or sympatric speciation
emphasizes biogeographical factors
Taking genetic mechanisms into account, speciation
can be classed by adaptive divergence or by shifts in
adaptive peaks:
In adaptive divergence, adaptation to different environments
can lead to differentiation of gene pools followed by
reproductive isolation
Reproductive barriers can arise without being directly
favoured by natural selection i.e. may occur as a secondary
development after adaptation to separate environments
Hybrid zones
Allopatric populations may come back into contact:
If speciation has not occurred, they may interbreed freely,
re-establishing a common gene pool
If they are reproductively isolated, they will not interbreed
and speciation has occurred
They may form a hybrid zone
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Red shafted flicker and yellow shafted flicker in North America
are two phenotypically distinct woodpeckers that interbreed
Two populations came into renewed contact after separation
due to the ice ages
Integration of alleles between populations has not extended far
beyond hybrid zone
Genotypic and phenotypic differences that distinguish the two
populations form steep clines into the hybrid zone
The cohesion concept of species
Some researches suggest that the hybrids should be
classified as distinct species
This contradicts the biological species concept since
forces other than reproductive isolation must be
maintaining species
Cohesion species concept holds that cohesion may
involve a distinctive, integrated set of adaptations
that has been refined during the evolutionary history
of a population
How much genetic change is
required for speciation?
No generalisations can be
made!
Two species of Drosophila
(D. silvestris and D.
heteroneura) differ at only
one locus
Phenotypic effect of different
alleles at this locus is
multiplied by epistasis
involving at least ten other
loci
Only one mutation was
necessary to differentiate
the two species