If you keep a garden and have never been bothered by aphids, you are lucky.  But one day you may find your rose bushes or bean plants covered-almost overnight-with these green, greedy sap-suckers.  What allows aphids-and thousands of other species-to appear so fast is a trick these insects have developed for circumventing the old rule about the birds and the bees.

During the summer months when aphid-food is plentiful, aphids reproduce through a process called "parthenogenesis," from the Latin words "virgin birth."  What that means is that female aphids give birth to more female aphids without ever seeing a male.  As soon as those daughters mature, they produce their own offspring, and so on.

Without the search for mates or the ensuing courtship rituals, parthenogenesis means big savings in time and energy: in just one season, a single aphid can give rise to billions of descendants.

But in the fall, fresh food gets harder to find and there'd no longer be any reason for producing so many offspring.  So now a generation of aphids is born that includes males as well as females.  And what happens next between the males and females is just what you might expect.

Why aphids would turn from parthenogenesis to something as messy and inefficient as sex is another story, but in short, sexual reproduction is nature's way of shuffling the genes to create variety.  Each of the offspring inherits a random mix of the parents' genes.  And variety is what enables species to adapt and evolve as the environment changes.

But when summer returns and the living is easy, the aphids give up on variety and reproduce once again in the fastest way they know how.