This is a unique type of speciation in which new species are created out of a preexisting species.
There are no physical barriers which seperate members of an existing species like in allopatric speciation(another way of new species are created).
Sympatric speciation is rare and is more common in plants than in animals.
It happens in plants when, due to some error during reproduction, offspring is produced that contains twice the number of normal chromosomes.
- We say that a normal cell is di ploid since it has two copies of each chromosome(one from each parent).
- But this certain offspring has four copies of each chromosome-hence it is called tetra ploid.
This process of getting more chromosomes than normal is given a fancy name Polyploidy.
This is an example of polyploidy in a cell with diploid number 6.
- As a general rule, when two things can't mate, it means they are not of the same species.
- A tetraploid plant cannot mate with a diploid plant.
- Hence, both are different species. And a new species, the tetraploid is made.
A tetraploid plant can reproduce asexually though, and that's how it's species continues.
Following is a real world example of sympatric speciation in animals :
It involves the apple maggot fly shown here:
The apple maggot fly lays it's eggs inside an apple causing it to rot.
The maggot originally laid it's eggs in a fruit called a hawthorn.
When apples trees were planted in North America in the 19th century, a maggot species evolved that laids its eggs only in apples, not in hawthorns.
The maggot flies that lay their eggs in apples, mate with the flies that also lay their eggs in apples; the same is true for flies that lay eggs in hawthorns.
So a new species of maggots has been created that lays it's eggs in apples, instead of hawthorns.
There are four more types of speciation (ways in which new species are formed).
If you want to learn more about these types of speciation (I know you do), go to http://science.howstuffworks.com/life/evolution/evolve-in-isolation1.htm