Often occurs when some of the reproductive cells don't divide properly. Diploid, tetraploid, hexaploid, triploid etc. Many crop plants involve polyploidy in their formation. See table p. 47. Many types of polyploids are sterile.
Polyploids are usually larger than diploids etc.
This process often occurs by chance and humans have taken advantage of the rare plants in which it occurred. Geographic variation in plants Humans usually want uniform plants, but in nature just the opposite is favored. Variable offspring tend to have better survival. Humans provide a more or less uniform environment and enough care to make sure crops survive. In nature, gene frequencies vary in populations of plants over geographic distance.