Species  “A species is a group of similar individual organisms that can usually breed among themselves to produce fertile offspring.” Ernst Mayr  Geneticists and molecular biologists define a species as a group of organisms that share the same gene pool.

Speciation  The formation of a new species.  Two types: Allopatric: similar species living in a separate geographical area. Sympatric: species of same genus living together in same area.

Allopatric 1. Population moves into new area with many ecological niches unfilled. 2. Population explosion follows with increased variation. 3. Migration into fringe environments. 4. Geographical barrier arises so geographically isolated. 5. Isolated population have different mutations

6. Different populations are exposed to different environmental selection pressures. 7. Some subspecies develop genetic and chromosomal differences that mean they cannot breed with the parent population. 8. Now are genetically isolated so therefore a new species.

 A cline: when we move in a particular direction through a range of species and find a continuous increase or decrease in some characteristics.

Isolating mechanisims 1. Geographical barriers: mountains, rivers, continental plates moving, oceans, deserts. 2. Ecological barriers: populations may have developed genetic differences to cope with different ecological niches or habitats. 3. Reproductive barriers: under natural conditions, sympatric populations of different species can be prevented from mating by pre or post zygotic isolating mechanisims.

Prezygotic  Habitat isolation (nocturanl/diurnal)  Temporal isolation (differences in breeding times)  Biochemical incompatibility sperm and egg fail to unite - due to chemical differences, eg, hormone signals.

Prezygotic 2  Behavioural Differences in courtship behaviour are a major isolating factor.  Mechanical differences Appearance and colouration patterns can differ enough between closely related species to make them “unattractive” to each other. The fit of the mating apparatus of many insects is very specific.

Postzygotic These mechanisms stop gene flow between species even if mating is successful.  Hybrid Sterility The resulting offspring from two different species may be healthy, but is sterile, eg. horses and donkeys can breed to produce mules, which are sterile.  Hybrid inviability The zygote fails to develop.  Hybrid breakdown

When Isolating Mechanisms Fail  Sometimes isolating mechanisms are weak. When this happens, species hybrids can occur.  Eg: ○ the native grey duck and the introduced mallard duck hybridise freely. ○ The endangered black stilt is interbreeding with the pied stilt, producing a range of intermediate hybrids.

Sympatric  Remember non geographical isolation To illustrate the following example is imaginary: there is a bird population on an island. E.g Sympatry involves natural selection but allopatry does not

Other ways of speciation  Polyploidy – instant speciation: the abrupt formation of a new species with more than two sets of chromosomes. E.g. wheat.  Chromosomal rearrangement: duplications or delations may make individuals or small populations that are genetically incompatible with the rest of their species.  Isolation by time.