Parasites and hosts, predators and prey, even males and females, are all coevolving, caught up in a complex series of interactions where genetic changes in one can lead to genetic changes in the other. But not all coevolution is hostile. Some interactions, known as symbioses, are mutually beneficial. For instance, many aphids harbour bacteria that provide crucial amino acids. Without the bacteria, the aphids would not be able to feed; without the aphids, the bacteria would not exist. Even humans benefit this way: colonies of bacteria in our guts help with digestion.
Coevolution can also occur on all sorts of levels. Sometimes genes coevolve, suppressing the action of the selfish genetic elements that arise from time to time and pervert the process of meiosis, propagating themselves at the expense of other genes.
