Evolution within ourselves

We normal*ly think of Darwinian evolution, or evolution by natural selection*, as something that occurs in populations of organisms, over a relatively long time. A process whereby selective pressure by the environment on variants in a population generates organismal varieties and new species. We don't think of it as something going on inside us, every day, but it does. Our bodies are environments made of tissues and cells, and the rules of evolution apply to the cell populations that live and reproduce there as well. They may change through natural selection acting upon the variation present in their population, just like the organisms within an ecosystem, but on the timescales of the physiological events of our lives.

The B cells that produce antibodies against disease-causing organisms are a great example. B-cell populations are hugely variable: almost every cell you look at is able to recognize a particular structural motif on a protein or sugar chain. Upon invasion by a disease-causing organism, the cells that happen to recognize a structural feature on its surface are selectively stimulated to reproduce and make antibodies against the disease agent. The antibodies produced after the first round of selection are not very effective, but selection and reproduction repeats many times during the disease course, at every round improving the quality of the antibodies, and evolving B-cell populations better and better adapted to changes in the environment (invasion by the pathogen).

Evolution by natural selection also turns out to be the explanation for some very undesirable phenomena of the human body: neoplastic disease and progression to metastatic cancer. An article on the January 19th 2012 issue of Nature (“Clonal evolution in cancer”) explains that neoplasms evolve by accumulating mutations that are selected for if they confer a competitive advantage over other cells. At the precancerous stage, mutations are introduced by physical and chemical agents, and eventually loss of growth control allows some cells to multiply and become established as a tumor. At this stage cells can become “genetically unstable”, meaning they can accumulate mutations at a much faster rate than normal, even in the absence of outside chemical or physical mutagens. This is important, because mutations are the source of traits on which natural selection acts to produce evolution. The faster it mutates, the more likely a tumor is to become fast-growing and invasive, and therefore life-threatening. Progression to the next stage happens when a tumor cell acquires, by chance, traits that allow it to move and grow away from the primary tumor—the ability to cause metastases. Why is this selected for? Because that cell will have a big competitive advantage in the form of access to more resources for growth and reproduction.

Similarly, Darwinian evolution explains how cancers become more resistant to therapy after a round of treatment: a few cells with resistance traits normally present in the population will survive (or be selected by) the adverse environment, and eventually regrow into a nastier, harder-to-treat, more “evolved” cancer.