Nature is not just a dangerous place. Cooperation between organisms of the same species, or even between organisms of distantly related species, is just as important in evolutionary interactions as predation or parasitism. And often, once a beneficial interaction has been established, the evolution of the organisms proceeds in parallel. For example, a species of bioluminescent squid can be found anywhere from Japan to Australia. In order to light up, however, it requires a particular kind of bacteria to live inside its special light-organ. Recent experiments have shown that squid from Australia need the bacteria from Australian waters; squid from Japan need the bacteria from Japanese waters. If given the wrong bacteria ­ even though the bacteria are from the same species--the squid cannot light up properly. And one day, these differences may become so great that speciation takes place both for the squid and for the bacteria.
