  We consider the prospects for future ultrahigh energy cosmic ray and neutrino
observations to constrain the evolution of sources producing a proton flux
above 10 EeV (1 EeV = 10^18 eV). We find that strong constraints on the source
evolution can be obtained by combining measurements of the cosmic ray proton
fraction above 30 EeV with measurement of the neutrino flux at 1 EeV, if
neutrinos are predominantly of cosmogenic origin. In the case that interactions
in the source environment produce a significant astrophysical neutrino flux,
constraints on the source evolution may require measurement of the observed
proton fraction, as well as, the neutrino flux at multiple energies, such as 1
EeV and 10 EeV. Finally, we show that fits to current UHECR data favor models
which result in a >30 EeV proton fraction and 1 EeV neutrino flux that could
realistically be discovered by the next generation of experiments.
