It's no only about quanity. Let's look at small fact: the book takes place after 20 years after the war in bombed and irradiated Moscow. Alright, then. How much of new lifeforms could you expect to appear after 20 years? Something tells me not much. But Glukhovsky thinks otherwise. There are hybrids of scorpions and spiders, giant amoebas, shrimps, nosalises, GHOSTS and the final nail in the coffin - Dark Ones (they are Black Ones but whatever), Something Glukhoy took out of his nose, a 'next step in evolution.' All of it is purely because of radiation. In a mere 20 years. Right. Intense radiation exposure have higher chances to kill than trigger mutations, even in Fallout.