In the anime Kabaneri of the Iron Fortress, Kabane are shown with physical ability ranging from a slow, zombie-like walk to a high-speed animal-like sprint. In addition, Kabane are shown with various degrees of intelligence, ranging from mindless zombies to leader-types to being able to operate simple machinery.
What's the mechanism behind this variety? Do Kabane grow in strength, agility, and/or intelligence over time? Do their abilities depend on the original individual infected? Is it more related to the circumstances of the infection? Is there an intelligent agent and/or advanced evolutionary process behind it all?
The existence of Wazatori and Fused Colonies seems to indicate some form of guided evolution, but I'm frankly at a loss as to what mechanism could be behind all this.