Separated web server and application server?
+1 for Ignacio's answer.   
Also note that separating a WSGI app from a server will lose one of the main benefits of WSGI (the server calls the wsgi app directly).  Additionally, WSGI apps have a response callback that was intended to communicate directly with the server.
Instead of decoupling the server from the app, have you considered keeping them paired together and replicating them over multiple servers/app pairs using nginx and/or haproxy to split and load balance requests?  I believe this is the usual solution to loading issues.
I am developing web service. My system is like this:

Client request --> Web Server(Apache) --> Application Server(Python)

I used WSGI for communicating between Apache and Python.
My question how can I separated the web server and app server. At the moment, I have to run them on the same server.