If you naively raise a chimpanzee infant as a human child, you'll eventually come to the conclusion it must be autistic (severely autistic). But that same chimpanzee infant reared among other chimpanzees would be assessed as quite normal.
To some degree, all individual organisms differ genetically from one another, but organisms tend to clump together into tight genetic categories we recognize as species. Within a tolerance of difference, each organism becomes a member of, and develops alongside, the other organisms who also exist within that same tolerance of difference.
What happens when the tolerance is exceeded? The chimpanzee infant raised as a human child would be an extreme example, but each case need not be so extreme.