Grizzly bears and polar bears sometimes produce pizzly bears and grolar bears.26 Lions and tigers produce ligers and tigons.27 When we shift our attention from mammals and other multicellular organisms to the world of single-cell bacteria and archaea, we discover that anarchy reigns. In a process known as horizontal gene transfer, single-cell organisms routinely exchange genetic material not only with organisms from related species but also with organisms from entirely different genera, kingdoms, orders and even domains. Bacteriologists have a very difficult job keeping tabs on these chimeras.
