Duplication within the species isn't the only means by which the number of cooperating genes has increased in evolution. An even rarer, but still possibly very important occurrence, is the occasional incorporation of a gene from another species, even an extremely remote species. There are, for example, haemoglobins in the roots of plants of the pea family. They don't occur in any other plant families, and it seems almost certain that they somehow got into the pea family by cross-infection from animals, viruses perhaps acting as intermediaries.
