1. Species and Speciation

2.  Within each population there are breeding subunits called demes  Individuals within a deme tend to breed with each other more often than with members of other demes.  They remain part of the same gene pool, but …gene flow between separate demes slows, and… … may even stop.  Each deme may evolve along separate lines, so… …when reunited they may nolonger be able to interbreed

3. Allopatric Speciation Demes are spearated by … … a physical barrier ‘Allopatric’ means …… ‘different fatherland’

4. Sympatric Speciation  Speciation which occurs when some factor prevents two populations living in the same area from interbreeding  ‘Sympatric’ means ‘same fatherland’  The mechanism which prevents interbreeding is called the isolating mechanism NB: the isolating mechanism is something other than a physical barrier.

5. Sympatric speciation: Example 1 Willow warbler (Phylloscopus trochilus) Chiffchaff (Phylloscopus collybita) Two small insect-eating birds which spend summer in Britain They are difficult to tell apart by their appearance.

6. For both species, their songs are an important part of courtship Willow warblerChiffchaff Their songs are very different Successful mating will only result from the correct behaviour pattern, so… … willow warblers and chiffchaffs do not interbreed

7. Example 2: Failure to transfer gametes Corn poppy (Papaver rhoeas) Corn poppies are pollinated by… … bees

8. Bees can distinguish between corn poppies and other species of poppy. Papaver alpinumPapaver orientale Papaver somniferum Bees visiting one species will not visit another species, so… …pollen is not transferred between species and interbreeding does not occur.

9. Example 3: Production of Infertile Hybrids Carrion crow (Corvus corone) Hooded crow (Corvus cornix) Found in England, Wales and Southern Scotland Found in Northern Scotland and Ireland

10. Territories of the two species do overlap slightly, so… Carrion crow Hooded crow … carrion crows and hooded crows do interbreed to produce hybrids, but… … hybrids are not as fertile as the parents, so… … selection favours the parents.