Why does mutations and mutation balance among alleles occur

Why does mutations and mutation balance among alleles occur at the highlighted region? PLEASE HELP

HAPLOID Gametes O O Gene pool Adults DIPLOID zygotes DIPLOID O O O Juveniles DIPLOID Population

Solution

Simply because it is adult who faces environmental challenges mostly ... The environmental adaptive challenges causes a selective pressure in gene pool to select best fit mutations/alleles along with balancing mutations . It is like an adult girrafe tried to eat canopy on higher heights which evolutionarily gave him high neck by mutations thereby required off.

Adults eventually accuire mutations and genetic drift subsequently selects forward beneficial ones

Hope it helps .... Regards