The researchers suggested that only after using naturally sharp stones for cutting did ancient humans faced selective pressure that led them to start knapping their own stone tools.

by applying selective pressure, induce escape mutations in the viral genome. By sequencing the virus from three patients seen before seroconversion, the authors could identify the single founder ...

As proto-Europeans and Asians moved northward out of Africa, they experienced less sunlight and colder temperature, new environmental forces that exerted selective pressure on the migrants.

The dark variety had always existed in small numbers, but only had a selective advantage when the selection pressure changed.