ABSTRACT
This process has been possible because separate, interbreeding populations share a poo l o f genes w hich is distinct from that o f any other group o f animals. T he genes are to some extent variable so that individual m em bers o f a population are no t identical bu t vary around a m ean w hich represents the type best fitted to the par ticular circumstances. E xtrem e forms (and the genes w hich deter m ine them ) are elim inated because they cannot com pete w ith fitter organisms. This is natural selection w hich, under constant condi tions, tends to keep a population stable. B ut, w hen circumstances change, slightly abnorm al individuals m ay becom e fitter than no r m al ones. So the gene poo l w ill slowly shift to produce a new norm . H ence this natural selection brings about evolution.
