  A natural phenomenon occurring in a living system is an outcome of the
dynamics of the specific biological network underlying the phenomenon. The
collective dynamics have both deterministic and stochastic components. The
stochastic nature of the key processes like gene expression and cell
differentiation give rise to fluctuations (noise) in the levels of the
biomolecules and this combined with nonlinear interactions give rise to a
number of emergent phenomena. In this review, we describe and discuss some of
these phenomena which have the character of phase transitions in physical
systems. We specifically focus on noise-induced transitions in a stochastic
model of gene expression and in a population genetics model which have no
analogs when the dynamics are solely deterministic in nature. Some of these
transitions exhibit critical-point phenomena belonging to the mean-field Ising
universality class of equilibrium phase transitions. A number of other
examples, ranging from biofilms to homeostasis in adult tissues, are also
discussed which exhibit behavior similar to critical phenomena in equilibrium
and nonequilbrium phase transitions. The examples illustrate how the subject of
statistical mechanics provides a bridge between theoretical models and
experimental observations.
