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Speciation Level 1 Biological Diversity Jim Provan Campbell: Chapter 24.

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Presentation on theme: "Speciation Level 1 Biological Diversity Jim Provan Campbell: Chapter 24."— Presentation transcript:

1 Speciation Level 1 Biological Diversity Jim Provan Campbell: Chapter 24

2 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

3 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

4 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

5 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

6 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

7 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

8 Polyploidy in plants

9 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

10 Evolution of wheat

11 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

12 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

13 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

14 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

15 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

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