text: Currently, there is no consensus among biologists on the main selective forces maintaining sex.  The competing models to explain the adaptive function of sex have been reviewed by Birdsell and Wills.
  Evolution of aging
  The Red Queen hypothesis has been invoked by some authors to explain evolution of aging. The main idea is that aging is favored by natural selection since it allows faster adaptation to changing conditions, especially in order to keep pace with the evolution of pathogens, predators and prey.
  Interspecies race
  A number of predator/prey species couple compete via running speed.
  "The rabbit runs faster than the fox, because the rabbit is running for his life while the fox is only running for his dinner." Aesop
  The predator-prey relationship can also be established in the microbial world, producing the same evolutionary phenomenon that occurs in the case of foxes and rabbits. A recently observed example has as protagonists M.xanthus (predator) and E.coli (prey) in which a parallel evolution of both species can be observed through genomic and phenotypic modifications, producing in future generations a better adaptation of one of the species that is counteracted by the evolution of the other, thus generating an arms race that can only be stopped by the extinction of one of the species.