When I was younger, Special Forces Assessment & Selection required a prior Army enlistment (ie - you serve your 4-6 with a perfect record and then apply for Selection) or cross qualifications (Navy UDT or Marine Recon with Para and Scuba).

Special Forces Qualification Course (Q) had an enormous failure rate even among Tri-Bodies (Jump School, USMC Recon or Ranger School, and Scuba).

This information is coming from a pre-SOC knowledge. Now that Special Operation Command is basically a branch of military within all the branches of military I can't speak to the SPAS process.