I've always thought that polygamy is the most evolutionarily advantageous strategy an organism can pursue. Isn't the meaning of life to produce as many offspring as possible? However, I was reading 'Monkeyluv' by Robert Sapolsky, and he mentioned an experiment done with fruitflies: It showed that flies that were forced to be monogamous outbred the polygamous flies. This is because polygamous flies had to evolve strategies like toxic semen (males) and toxin neutralizers (females) in order to pass on their genes. [Male flies had toxic semen to kill off other sperm, females had to neutralize the semen so they don't die from the toxins] Hence monogamous flies, who didn't have to waste time playing these stupid games, could spend more time making babies.
Reading this made me slightly confused. Afterall, isn't evolution all about Genetic diversity; Competition between genes; Survival of the fittest etc? If you're monogamous, how do you know that your chosen mate is the best fit for your genes? I guess living longer and producing more offspring compensates somewhat for this. Better quality of life for you and your kids, maybe? Screw the species and the diversity of the gene pool.
Sapolsky also suggests a reason for the evolution of all this toxic strategies in polygamous species: It just takes one dude to evolve toxic sperm to give him an advantage over the others, and soon everyone else is also making toxins, and the females are like, 'Crap, now we have to neutralise the damn things.' I guess in this case, making love is analogous to making war.