Are we evolving?

Bacteria evolve right before our eyes. Hospital acquired diseases are a very good example. Overexpose bacteria to some antibiotic, toxins, metals etc...they will change in a matter of days, adapting to their new environment. Unicellular organisms have absolute freedom to change because their own evolution, as individuals, can only benefit the population. If they fail to adapt, some individuals will be eliminated BUT those who are successful will become the ancestors of a resistant colony.

Why is it that this principle can not be applied to our own cells? Simply put, because the fate of our own cells is interconnected, a small change in the genome could mean disaster for the other ones. The reason why our own cells do not evolve in our lifetime (under normal circumstances) is discussed in an article by Pepper et al., soon to be published in PLoS computational biology. An article on the Nature website (september 21, 2007)explain why evolution within our own cells is unlikely and not desirable. Our tissues simply DO NOT evolve!

It is known that epithelial tissues have a rather high turnover...old cells are replaced on a constant basis. The speed at which they grow could suggest that these cells are more prone to mutations. This is not the case. As Philip Ball explains in its news feature (Nature): "Why a person doesn't evolve in one lifetime" epithelial cells take a long walk on the way to differentiation. Epithelial stem cells divide just a little before they commit to their final state. Mutations can occur within these cells but since they do not compete against each other because these mutations make differentiation of these stem cells more difficult.

On the other hand, the immune system is made to evolve, it has to adapt to new pathogens every single day but there is a price we pay for this: a higher incidence for cancer!

Reference: http://www.nature.com/news/2007/070917/full/070917-11.html
Image source: http://www.digitalapoptosis.com/archives/science/cells2.jpg