Female fruit flies sometimes choose males who are aggresive, sometimes choose males who do not fight at all, and sometimes choose males for no reason science can explain, write a team of biologists, and the findings help explain the large variation in aggressiveness in most species, including humans.
So females are unpredictable, which means the fittest males don’t always get the girl. This explains why males don't evolve towards super-aggressiveness.
“If aggression makes you more likely to father children, all males should be selected to be very aggressive. Male fruit flies (like humans and other animals) show a lot of genetic variation in aggression, and we wanted to find out why,” explained study leader Brad Foley, a post-doctoral researcher at USC.