Before revision: Based on data on gene expression profiles from microarrays for E. Coli and tumor tissues, I show that the evolution for thousands of generations leads to a set of strongly over- and under-expressed genes, as compared to the initial expression profile. This fact is manifested as heavy tails in the gene expression distribution functions .
After revision: Based on data on gene expression profiles from microarrays for E. Coli populations, or the progression to Alzheimer disease in aged brains, or the progression from a normal tissue to the cancer state. The first two cases seem to belong to a class of transitions, where the initial and final states are relatively close to each other, and the distribution function for the differential expressions is short ranged, with a tail of only a few dozens of strongly varying genes. In the latter case, cancer, the initial and final states are far apart and separated by a low-fitness barrier. The distribution function shows a very heavy tail, with thousands of silenced and over-expressed genes. We characterize the biological states by means of their principal component representations, and the expression distribution functions .