The primary disadvantage to asexual reproduction is the lack of genetic diversity among the resulting offspring. In contrast to sexual reproduction, which involves the recombination of two different sets of genes, an asexual organism possesses the same genes as its parent did. This can lead to problems if the parent has a genetic defect; without another parent to replace the gene, the offspring possesses the defect as well.
Asexual reproduction is more common among simple organisms such as bacteria than it is among complex ones such as mammals and birds. Nevertheless, highly evolved species occasionally exhibit asexual reproduction. A group of American lizards called whiptails, which occur in all-female populations, best illustrates this phenomenon. However, these lizards have more chromosomes than most other lizards, which helps offset the problems associated with asexual reproduction.
There are many different types of asexual reproduction. Some animals, such as sponges, produce structures called gemmules, which turn into daughter organisms. Other organisms such as planarians reproduce through the process of fragmentation. Fragmentation involves the splitting of the parental organism into two daughter organisms. Parthenogenesis is a special type of asexual reproduction that occurs when an unfertilized ova develops into an offspring. Parthenogenesis has been documented in many different reptiles and fish.