— A world-wide radiation disaster might eventually give rise to two populations, research on bacteria indicates.… Starting with a culture of ordinary (wild-type) bacteria, the scientist added copper ions that produced a “disaster.” Most of the bacteria died…. But as time passed, a small number of survivors, called variants, began reproducing at a rapid rate. They were resistant to effects of poisonous copper and differed in many ways from ordinary bacteria. Eventually a second population of survivors began to appear, resembling the original bacteria in every respect except that they had just enough resistance to copper to grow slowly in the presence of that poison. Finally, the completely resistant variants disappeared.