  We propose an extension of the evolutionary Prisoner's Dilemma cellular
automata, introduced by Nowak and May \cite{nm92}, in which the pressure of the
environment is taken into account. This is implemented by requiring that
individuals need to collect a minimum score $U_{min}$, representing
indispensable resources (nutrients, energy, money, etc.) to prosper in this
environment. So the agents, instead of evolving just by adopting the behaviour
of the most successful neighbour (who got $U^{msn}$), also take into account if
$U^{msn}$ is above or below the threshold $U_{min}$. If $U^{msn}<U_{min}$ an
individual has a probability of adopting the opposite behaviour from the one
used by its most successful neighbour. This modification allows the evolution
of cooperation for payoffs for which defection was the rule (as it happens, for
example, when the sucker's payoff is much worse than the punishment for mutual
defection). We also analyse a more sophisticated version of this model in which
the selective rule is supplemented with a "win-stay, lose-shift" criterion. The
cluster structure is analyzed and, for this more complex version we found
power-law scaling for a restricted region in the parameter space.
