... the phoenix of the species ...

In a comprehensive review of what was known about the evolution of sex at that time Patrick Geddes and J. Arthur Thomson (1889. The Evolution of Sex. London: Walter Scott) concluded a chapter with this poetic statement:

"Sexual union in those infusorians, dangerous perhaps for the individual life,—a loss of time so far as immediate multiplication is concerned,—is in a new sense necessary for the species. The life runs in cycles of asexual division, which are strictly limited. Conjugation with unrelated forms must occur, else the whole life ebbs. Without it, the Protozoa, which some have called "immortal," die a natural death. Conjugation is the necessary condition of their eternal youth and immortality. Even at this low level, only through the fire of love can the phoenix of the species renew its youth." Geddes, and Thomson (1889, p. 166). 

This is a rejuvenation hypothesis framed in terms of the benefit of the species. The hypothesis is itself being rejuvenated in terms of individual advantages, for example, in Turke (2013. "Making Young from Old: How is Sex Designed to Help?" Evolutionary Biology, in press).