I don't think they evolved based off of thier surroundings. That would require multiple generations breeding with traits that kept the better suited ones alive. Evolving sudden changes like that isn't Darwinian, but more Lamarckian. Even so, my guess is that neither are the case. The 
Little Ones appear to have been created or came from somewhere. Where? I have no clue? For all I know Pinstripes grew them in a test tube, mixed zombie blood with super baby blood, or something. But once the original batch of Little Ones came to exist, I believe they grew up at a slow rate. 
We witnessed them as being little all the way up until the assault on the Ice Rink by 
the Mallers (that's based off my memory, correct me if we've seen Little Ones in the story beyond that point). So presumably, the original batch of tattooed Little Ones have had months (from about August to November) to grow into this full sized Inklings. My guess is that the original batch of tattooed and numbered Inklings took those months to grow up. But if bitten by one, the change to an inkling is almost instant (instant being 24 hours maybe?). 
Also, I think the Little One's possessed some of thier adult counterpart traits, just in a weaker form. Sure 
Bricks severed ones arm, but I think he used the truck door as a sort of axe to repeatedly slam it on the Little One's arm until it severed. But prior to that, 
Lizzy narrates that 
Tardust shot at the Little One but only managed to piss it off. Does this mean he shot it but didn't pierce it's skin, or did he shoot 
at it and annoy it. Can't tell. Maybe the Little Ones had tough skin, but it wasn't until they were a full grown Inkling that they possessed this very noticeable thick hide.
-also, in regards to the spray and pray/one shot, one kill discussion: I shoot about 20 to 40 million per milliliter but it only takes one to get the job done...just saying.
 
			
		
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