Sympatric speciation can be regarded as speciation where parent species gives rise to a daughter species without the individuals of a species being separated by space or territory. Both instantaneous and gradual models of sympatric speciation have been proposed. Barring one mode of instantaneous speciation by a mechanism known as polyploidy, other modes of sympatric speciation have remained quite controversial. Polyploidy is quite common among plants. (For a detailed discussion on polyploidy refer to Unit 10 of Block 2 of LSE-03 of Genetics course). A cross between two diploid plants could result in a tetraploid hybrid. The hybrid would remain largely reproductively isolated from its diploid parents. The reason for such isolation is that due to back-crossing if a triploid individual were to be formed, it will produce a high proportion of nonviable gametes. There is also a possibility that interbreeding between deploid and tetraploid forms or between different tetraploids may give rise to other polyploids.