Natural selection can occur at the cellular level, where it is detrimental to health. Fortunately it is normally controlled by a well-known pattern of ongoing cell differentiation in the mature tissues of animals, according to a new study published December 14 in PLoS Computational Biology.

The failure of normal cell differentiation patterns may explain cancer and senescent decline with aging, say researchers at the University of Arizona, the Santa Fe Institute, the University of Pennsylvania, and the Wistar Institute.

Darwinian natural selection and evolution is usually studied in populations of organisms, but it also applies to cellular populations; this is called “somatic” evolution. Such somatic evolution tends to reduce cooperation among cells, thus threatening the integrity of the organism.

A Diagram of Serial Differentiation. The series includes stem cells (stage 0, in white), TACs (in gray), and finally, terminally differentiated cells (stage 3, in black). Stem cells divide asymmetrically with one daughter rejoining the stem cell compartment and one daughter differentiating (black arrows), unless the stem cell population is below homeostatic levels, in which case both daughter cells become stem cells (gray arrow). If there is an overabundance of stem cells, both daughter cells will differentiate (dotted arrow). TACs divide symmetrically so that both daughter cells advance to the next differentiation stage. Thus, every cell division outside the stem cell compartment entails differentiation into the next downstream stage, eventually ending in the terminally differentiated cells, which are purged from the tissue (e.g., sloughing of the outer layer of the skin, or the upper cells of an intestinal crypt into the lumen of the gut).

In this study the authors proposed that a well-known pattern of ongoing cell differentiation in the mature tissues of animals functions to suppress somatic evolution, which is essential to the origin and sustainability of multicellular organisms.

The team, lead by Dr. John Pepper, tested this hypothesis using a computer simulation of cell population dynamics and evolution. The results were consistent with the hypothesis, suggesting that familiar patterns of ongoing cell differentiation were crucial to the evolution of multicellular animals, and remain crucial as a bodily defense against cancer.

CITATION: Pepper JW, Sprouffske K, Maley CC (2007) Animal cell differentiation patterns suppress somatic evolution. PLoS Comput Biol 3(12): e250. doi:10.1371/journal.pcbi.0030250