B737-700
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 175; Flight Crew Type 7000
Situational Awareness
ATC Issue All Types; Conflict NMAC
Flight Crew Took Evasive Action
Human Factors; Procedure
About ten-miles from ZZZ; we were cleared to descend to 1600' and cleared for the ILS XX. Weather was Day; CAVU (Clear Visibility Unlimited); and we were at about 3000'; flaps 5; landing gear down; and about 175 knots. The autopilot caught the localizer and glideslope; and I disconnected the autopilot and autothrottle at about 3000'. At about 2500' on glideslope; the Approach Controller told us to expedite our descent for traffic at 1 to 2 o'clock. As I was hand flying; I asked for flaps 15; and began descent below the glideslope; and we could see the traffic on the horizon (appeared to be a high-wing; single-engine GA aircraft); and got a TA on TCAS. The aircraft showed 300' above us on TCAS. Shortly thereafter; we received a 'DESCEND' RA. I increased my descent rate to comply with the guidance. I would guess I was doing about 1500 fpm descent at about 175 knots. The GA aircraft passed directly over the ILS XX final; and directly over us by less than 1000'; and probably closer to 500'. We received 'clear of conflict' at about 1300'; leveled; configured; and complied with stabilized approach criteria by 1000' AGL; so we continued the approach and informed TRACON; who said they thought ZZZ Tower was trying to get him to stop his descent. I don't believe Passengers were aware of the incident.Airspace that provides more protection to IFR aircraft on a published instrument approach going into ZZZ. We should not be getting DESCEND RAs below 2;000' while on (or actually below) glideslope on approach into a towered airport. (unless; of course; the GA aircraft was at fault and not compliant with regulations. Then the preventative measure would be corrective action against the pilot.)
B737 flight crew reported a near miss with a GA Aircraft on final.