Speciation

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Speciation

• Level 1 Biological Diversity
• Jim Provan

Campbell Chapter 24
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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

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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
• 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

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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

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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
  populations range
• Peripheral isolates are good candidates for
  speciation
• 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

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Adaptive radiation on island chains

• Adaptive radiation is the evolution of many
  diversely adapted species from a common ancestor
• Example is Darwins finches in the Galapagos
• Multiple events of colonisation, adaptation,
  speciation and recolonisation

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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
• 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
• More common that autopolyploidy
• May initially be sterile due to incompatible
  chromosome numbers but may subsequently become
  fertile

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Polyploidy in plants
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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

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Evolution of wheat
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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

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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

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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
• 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

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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

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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