Parthenogenesis

Parthenogenesis is the development of an embryo from an unfertilized egg, producing offspring that contain only the maternal genome. It is the most common form of asexual reproduction in animals, occurring in species as diverse as aphids, lizards, and sharks. Parthenogenetic lineages can arise through hybridization, infection by Wolbachia bacteria, or spontaneous activation of egg development, and they often show rapid initial success followed by long-term decline — a demographic trajectory consistent with Muller's ratchet predictions.

The evolutionary instability of parthenogenesis is one of the great puzzles of biology. If asexual reproduction doubles the reproductive rate (by eliminating males), parthenogenetic females should rapidly outcompete sexual females — the famous "twofold cost of sex." Yet parthenogenetic species are rare, young on evolutionary timescales, and typically confined to marginal habitats. The ratchet provides the theoretical explanation: parthenogenetic lineages lose genetic diversity irreversibly and cannot purge deleterious mutations, sentencing them to mutational meltdown over geological time.