Allopatric speciation occurs when a geographical barrier physically isolates populations of an ancestral species

An actual physical barrier that gets between members of a population can be something as small as a fallen tree for small organisms or as large as being split by oceans.

• The two populations begin to evolve separately as a result of cumulative mutation, genetic drift and natural selection

• Eventually the two populations reach a degree of genetic divergence whereby they can no longer interbreed (speciation)

• Usually populations of organisms get geographically separated (eg by river) and Gene flow stops --> Genetic isolation occurs.

• Differences in natural selection can cause differences in allele frequencies between the populations over time.

• If the populations was to come back together they now no may longer interbreed and are now defined as separate species.

• New Zealand's isolation and island formation have led to many examples of allopatric speciation.

• Snares and Rockhopper Penguin is an example of allopatric speciation, as they live on different islands. Because they now have different crests, the Rockhopper and Snares penguins don’t recognize each other as a mate, which is good, as if they did, the offspring may be infertile, so they have wasted time and energy. 

Sympatric speciation is divergence of species within the same geographical location (i.e. without a physical barrier). The individuals in the population are not separated at all and all live in the "same place."

• Sympatric speciation may result from the reproductive isolation of two populations as a result of genetic abnormalities

• Typically, a chromosomal error may arise which prevents successful reproduction with any organism lacking the same error

Sympatric speciation is the process through which new species are formed from one common ancestor while in the same geographic region. Sympatric speciation does not require large geographical distance to reduce gene flow in the population. An example of sympatric speciation is the three spined stickleback, a freshwater fish found in Canada. The three spined stickleback with larger mouths feed on prey in the depth at the waters edge while the smaller mouth spined stickleback feed on plankton near the surface of the water. This variation in phenotype is due to non random mating (ie large fish only mated with other large fish) and due to selection pressures in competition for food. 

The most common cause of sympatric speciation is reproductive isolation. Reproductive isolation may be due to individuals coming into their mating seasons at different times or preference of where to find a mate. In many species, choice of mates may be based on their upbringing. Many species return to where they were born to mate. Therefore, they would only be able to mate with others who were born in the same place, no matter where they move and live as adults.

Other reasons could be that different populations become dependent on different needs in the environment, such as food sources or shelter.

Sympatric speciation is where species don’t interbreed due to niche differences.

For instance, they may breed at different times, which means that there is limited gene flow. Eventually, genetic changes will mean they cannot interbreed.