Two species of Crotoniidae, a type of mite, have regained the ability to mate after their ancestors had lost it (they reproduce asexually):
The Crotoniidae reproduce by having sex, which wouldn't be too strange except that they are very similar physically to Camisiidae, a family of some 80 mite species that all reproduce asexually via parthenogenesis, in which females give birth to young without having sex with males.
Evolutionary biologist Katja Domes at the Technical University of Darmstadt in Germany and her colleagues examined genetic sequences in two Crotoniidae species and a diverse range of 13 other mite species. Their calculations show the sexual Crotoniidae evolved from the asexual Camisiidae, the first known reversal to sexuality from asexuality within the animal kingdom. (The only other known such reversal is a plant, the mouseear hawkweed, or Hieracium pilosella.)