Evolution Choices.

Demon evolution used to be random before the advent of the system. Evolution will occur once the sin mark upgrades and if there are available resources for it. The system has changed how things used to be done.

In the past, when the evolution process begins, a demon will then upgrade based on the number of resources they have with the priority being in the UPGRADES tab before moving on to the UPGRADABLE. 

Only after those two have been sorted out will the matter of the sin ability be determined. By that time, most of the resources if not all of them have been consumed so they mostly get the one at the common rank because it is free. The STATS section will is hardly ever touched.