Polyploidy and speciation

There are two types of polyploidy:

Polyploids are usually infertile with members of their parent species, because diploid x tetraploid crosses produce sterile triploid progeny. If the triploid is viable, it is infertile, due to some chromosomes being inherited twice, others once, leading to a lack of gene dosage balance in the gametes.

One of the reasons polyploidy is commonest in plants is that they are often hermaphrodite, and hermaphrodites can often self. When you are the first "hopeful monster", such as a new hybrid allopolyploid, or often just an autopolyploid part or branch of a normal diploid individual, your only way of avoiding the production of triploid progeny will be to mate with yourself!