Put simply, nature has figured out a way to continue going about makin' bacon when the males are in short supply. There are a ton of different kinds of this "virgin birth" in the insect and crustacean world, but I'll just touch on the one most commonly used in "higher" species. It basically works like this, with some variation:
The 15 species of whiptail are all female and reproduce exclusively through parthenogenesis, and often require "pseudocopulation" to get the process going:
Some species don't soley rely on this but use it as a survival mechanism, like sharks in captivity or komodo dragons in a dating crisis. This concerns scientists as such behaviors don't aid in increasing genetic diversity in the species.
I'm trying to find a citation now, but apparently parthenogenesis has also been observed in domestic turkey populations at alarming rates, making for quite a mess over at Butterball and other farms.
Makes you wonder though, if genetic diversity is beneficial to survival then why would these behaviors be possible, unless I guess it's just evolution hedging its bets as to whether genetic diversity doesn't do squat unless you have someone to mate with.