Bow-tie or hourglass structure is a common architectural feature found in many biological systems .

This offers a mechanism to understand a common architectural principle of biological systems , and a way to quantitate the effective rank of the goals under which they evolved.

Many biological systems show bow-tie (also called hourglass) architecture.

This offers a mechanism to understand a common architectural principle of biological systems , and a way to quan-titate the rank of the goals under Which they evolved.

Since complete absence of noise is a nonrealistic scenario in biological systems , we conclude that sum-mutations cannot account for bow-tie evolution.

Our evolutionary model could be further expanded to give a more detailed representation of specific biological systems .

Such assumptions seem justifiable on the basis of the biological systems that we are interested in.

Several biological systems are well suited to empirical exploration of this idea.

The general selection mechanism that we implement is representative of the biological systems on which our models are based; i.e.

In our opinion, this meticulous structure of the energy landscape for our simplified model is of general interest to other cell cycle dynamics, and the proposed methods can be applied to study similar biological systems .

This is a reasonable assumption for most biological systems .

However, it is still difficult to quantify the robustness and adaptivity of cellular networks, even for a small cellular network perturbed by intrinsic random fluctuations, due to the massive cross regulations and nonlinear nature of such biological systems .