'
Alien Resurrection' makes it clear Queens are parthenogenic. They're born without any need to mate.
In nature, both methods of reproduction have their pros and cons...
Creatures which need to mate, reproduce slower and, in the short term, can be out-competed for resources by those which reproduce as soon as they come of age. However, the chances of genetic mutation are vastly reduced. It happens, but they din't really have the benefit of genetic diversity, in the same way as one creature fertilising another would do. So, when something like a disease occurs, parthenogenesis means less likelihood of one amongst their number having a natural immunity to it. Evolution will probably take a much longer time for similar reasons, too.
With the fictional Alien, it effectively has the best of both worlds, since the normal adults aren't typically the ones which would prodice next Queens (and, thus, the next generation), but they do seem to somehow analyse and assimilate some genetic information from their hosts. So, one host might have some kind of useful environmental attribute, while another might not. The resulting Aliens might then have one of them which is better suited for their immediate survival. All of this in a system where the Queen doesn't require a mate and can just begin laying eggs and allow them to serve as her own protection.
I knew those environmental studies lessons in college would come in useful, some future day...
