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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:

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speciation

Speciation

Level 1 Biological Diversity

Jim Provan

Campbell: Chapter 24

modes of speciation
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
  • 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
allopatric speciation in the pupfish
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
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:
      • 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 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
  • 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
polyploidy in plants9
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
sympatric speciation in animals
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
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
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
the cohesion concept of species
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
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