  The presence of phenomena analogous to phase transition in Statistical
Mechanics, has been suggested in the evolution of a polygenic trait under
stabilizing selection, mutation and genetic drift.
  By using numerical simulations of a model system, we analyze the evolution of
a population of $N$ diploid hermaphrodites in random mating regime. The
population evolves under the effect of drift, selective pressure in form of
viability on an additive polygenic trait, and mutation. The analysis allows to
determine a phase diagram in the plane of mutation rate and strength of
selection. The involved pattern of phase transitions is characterized by a line
of critical points for weak selective pressure (smaller than a threshold),
whereas discontinuous phase transitions, characterized by metastable
hysteresis, are observed for strong selective pressure. A finite size scaling
analysis suggests the analogy between our system and the mean field Ising model
for selective pressure approaching the threshold from weaker values. In this
framework, the mutation rate, which allows the system to explore the accessible
microscopic states, is the parameter controlling the transition from large
heterozygosity (disordered phase) to small heterozygosity (ordered one).
