Speciation

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. 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

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