  We investigate the error threshold for the emergence of quasispecies in the
Eigen model. By mapping to to an effective Hamiltonian ruled by the
"imaginary-time" Schr\"odinger equation, a variational ansatz is proposed and
applied to calculate various quantities associated with the quasispecies. The
variational ansatz gives correct predictions for the survival population of the
wild-type sequence and also reveals an unexpected universal scaling behaviors
near the error threshold. We check the validity of the variational ansatz by
numerical methods and find excellent agreement. Though the emergence of the
scaling behaviors is not yet fully understood, it is remarkable that the
universal scaling function reigns even for relatively short genome length such
as L=16. Further investigations may reveal the mechanism of the universal
scaling and extract the essential ingredients for the emergence of the
quasispecies in molecular evolution.
