  We study the proximity effect in a topological nanowire tunnel coupled to an
s-wave superconducting substrate. We use a general Green's function approach
that allows us to study the evolution of the Andreev bound states in the wire
into Majorana fermions. We show that the strength of the tunnel coupling
induces a topological transition in which the Majorana fermionic states can be
destroyed when the coupling is very strong. Moreover, we provide a
phenomenologial study of the effects of disorder in the superconductor on the
formation of Majorana fermions. We note a non-trivial effect of a quasiparticle
broadening term which can take the wire from a topological into a
non-topological phase in certain ranges of parameters. Our results have also
direct consequences for a nanowire coupled to an inhomogenous superconductor.
