Reading the Wiki with some early promises that still gots quoted
" When aliens are victorious in combat, they may mutate more in order to use captured weapons and other technology. In contrast, aliens that are consistently defeated will continue to mutate in a natural selection process which mimics evolution. For example, a mutation might generate aliens with a new melee attack ability or a new defensive counter to certain types of weapons used by the players’ soldiers. These mutations are somewhat random; however, the game’s AI works in the background to find mutations that can defeat players’ soldiers by discarding iterations that are unsuccessful. Aliens will continue to evolve until they develop a mutation that allows them to prevail in battle. Aliens with successful mutations then will be deployed in increasing numbers. Thus, the Pandoravirus responds and adapts to the tactics and technology used by players."
However playing latest Necromicon patch plus DLC1 Blood and Titanium,
I find mutations always in same direction.
No any adoptive selection, no new limbs, no mutated weapons.