If a patient had 12% of their body covered with a psoriasis rash what would be the severity

Severe

Several methods to assess the severity of psoriasis are available including Psoriasis Area and Severity Index (PASI), Physician Global Assessment (PGA) for psoriasis, and the National Psoriasis Foundation Psoriasis Score (NPF-PS). In terms of body surface area, psoriasis is generally classified as mild when it affects less than 5% of the body, moderate when it affects 5-10%, and severe for >10%. Based on PASI, which includes a rating or score for the surface area of the body affected by psoriasis, the "Area Score" would be 2.

PASI Area Score rating for Psoriasis: 
0: 0 (clear)
1: <10%
2: 10–<30%
3: 30–<50%
4: 50–<70%
5: 70–<90%
6: 90-<100%

It is evolution, because we all have some of the same traits like eye color or shape or ears.