In sexual reproduction—the way most life-forms procreate, or produce offspring—each parent provides half an offspring's chromosomes. Over generations, this procreating shuffles the DNA cards, giving sexual reproducers genetic diversity. It is believed that this range of variation in genes can help organisms adapt more successfully to changing environments.
By contrast, asexual reproducers—which includes 70 vertebrate species and many less complex organisms—"use all the chromosomes they have" to solitarily produce offspring that are genetic clones, says molecular biologist Peter Baumann. Because the organisms are genetically identical, they're more vulnerable: A disease or an environmental shift that kills one could kill all.
Twice the Chromosomes
But there's a twist in the case of the genus Aspidoscelis, the asexually reproducing whiptail lizard. Baumann and his colleagues have been studying this species at the United States' Stowers Institute for Medical Research in Kansas City, Missouri. The lizards are all female and parthenogenetic, meaning their eggs develop into embryos without fertilization. But before the eggs form, Baumann's team discovered, the females' cells gain twice the usual number of chromosomes during meiosis. This means that the eggs get a full chromosome count as well as genetic variety and breadth (known as heterozygosity) rivaling that of those from sexually reproducing lizards.
This results in a standard pair of chromosones derived from two sets of pairs. This gives their asexually produced offspring genetic variability similar to that found in sexually produced offspring.
This occurs, according to Baumann, because at some time in the past lizards of the genus Aspidoscelis had "a hybridization event." Females of one species broke form and mated with males of another species. This gave whiptails robust heterozygosity, which has been preserved by the identical replication—essentially, cloning—that occurs in asexual reproduction. It's a genetic-diversity advantage that today's Aspidoscelis females still enjoy and replicate.