We do not even in the least know the final cause of sexuality; why new beings should be produced by the union of the two sexual elements, instead of by a process of parthenogenesis.Charles Darwin (1862)
The mystery which Darwin struggled with, the existence of sex in the plant and animal kingdoms, continues to fascinate biologists today. While many plant and animal species reproduce sexually, others continue to succeed with asexual reproduction.
Consider, for example, Prorodon utahensis, a small animal which flourishes in the hypersaline waters of the Great Salt Lake (Figure 0.1). There are few other forms of life that can tolerate these salinities, which have been measured at up to 27%. The quivering hair-like cilia of Prorodon provide its tiny body – scarcely the width of a human hair – with sufficient locomotion to zip about its otherwise lethal environment, consuming organic detritius, cyanobacteria and the salt-tolerant green alga Dunaliella. In the shallow waters of the Great Salt Lake, which are too salty for fish, these tiny Prorodon are the major hunters, the equivalent of sharks at the microscopic level. Reproduction in Prorodon is a simple matter – it simply splits in half. Without resorting to sexual recombination, Prorodon is able to lock in its genetic combination for survival and success in this most hostile of environments. Asexual reproduction also grants Prorodon utahensis a significant numerical advantage in progeny. A single individual splits, producing two, then four, then eight, then sixteen, then thirty-two genetically identical offspring.