One case in which artificial selection has created substantial changes in a species is in the case of wild mustard. Humans have used artificial selection to create many plants such as kale, brussel sprouts, cabbage, broccoli, and kohlrabi. The wild mustard was bred to produce species carrying desired traits such as leaves, axillary buds, apical buds, flowers, and stems. Another case is the domestication of wolves over time into dogs. Humans have also used artificial selection to breed crops, livestock animals, and other pets with desired traits. The four sources of genetic variation are the formation of new alleles, altering gene number or position, rapid reproduction, and sexual reproduction. The formation of new alleles is caused by mutations, such as point mutations, that make slight changes in the genetic code. Gene number or position can be changed during meiosis, in which genes can be placed in a different place in the chromosome, or other genes can be accidentally duplicated. Even with a slow mutation rate, a rapid reproduction rate can still cause mutations to happen quickly, such as mutations in HIV viruses, which do not repair their genetic material. Sexual reproduction (crossing over and recombination) shuffles existing genes into new genotypes.