Thursday, January 7, 2010

Tasmanian Devils And One Selfish Gene

Genes don't have real motives. Selfish in the term "selfish gene"  really reflects the metaphorical motives of genes and not the real motives of individuals carrying those genes.

Ordinarily, different genes in the multicellular bodies of organisms co-operate. Such a system has evolved because in sexually reproducing multicellular organisms the reproductive fate of any gene depends on the successful reproduction of the individual. Genes in the somatic cells of the body, cells which form the body mass, can extend their lives only if a copy of themselves in the sex cells gets passed on. Co-operating to build successful bodies is the only way out.

But what if a gene is able bypass this system? What if a gene is able to unlink its reproduction from the reproduction of the individual?

Just such a situation has occurred in the spread of cancerous facial tumor in populations of the Tasmanian Devil - a carnivorous marsupial. Scientists working on these animals have identified a nerve cell type known as Schwann cells as the progenitor of this cancer which must have originated through mutations in one Schwann cell of one individual Tasmanian Devil.

Devils are aggressive creatures and they often bite each other especially during mating. The cancer cells graft themselves on facial tissue of the other individual and grow and spread. As cells divide, the bodies defense may kill some of these cells, but mutants among them which are better able to resist  thus become more common. This is natural selection acting on this cell lineage. Within the lifetime of an individual devil, cancer cells divide hundreds of times. Cells in the nth generation would be different somewhat from their ancestral state. This is evolution, though not in the form we are familiar with or think of commonly.  Ordinarily, this cancer cell lineage would die with the death of the individual. In this case, their evolution continues when they get grafted on another individual. The tumor gene thus spreads through the population by bypassing the normal channels of reproduction. The cancer cells have been able to successfully invade another body in this way because there is very little genetic variation in the immune system of the Tasmanian Devils. That may be because there are a small inbred population. Immune cells don't recognize grafted cancer cells as foreign and don't reject them.

The mutant gene (s) responsible for this cancer can be thought of as a special type of a selfish gene. In a broader sense all genes are selfish since they "try" to maximize copies of themselves by competing against variations of themselves.  Generally, the reproductive fate of the gene is tied to the reproductive fate of the individual and so genes cooperate to make bodies successful. However, in the more specific usage of the term, selfish genes may enhance their own reproduction by subverting the reproductive machinery of the cell and in the process causing harm to the population in the long run.  During sexual reproduction any gene on average has a fifty percent chance of being passed on to the next generation. A selfish gene subverts these odds and ensures a more than even chance for copies of itself to be passed on.

In the case of the Tasmanian Devils, the "selfish gene" is not in the sex cells but is a somatic cell mutant.

We don't think of somatic cells as a separate life form. The cells that make up our bodies are us. But occasionally as in the case of the facial cancer in the Tasmanian Devils the us can morph into the other. Most cancerous cell types which rebel against the body don't have an extended evolutionary future. The cell lineage dies with the death of the individual.  This particular cancer cell however, which was once part of the Tasmanian Devil is evolving into a parasite. It has developed a life of its own.

This situation illustrates several broader principles of evolution.

Firstly that evolution has no foresight. There is no long term benefit of the Tasmanian Devils that evolution is striving towards. Natural selection doesn't work for the benefit of the species. Evolution through natural selection is all about immediate advantage. Mutant cells have found a way to propagate independent of the body and because their life cycles operate quicker than that of an individual, selection will favor the spread of these cells regardless of whether that is harming Devil individuals and populations.

Second, it demonstrates that natural selection doesn't only act at the level of the individual.  It can act on any entities which shows certain properties. If entities vary in certain traits, if these traits are heritable and if these traits affect "fitness" i.e. they enable one variant to reproduce more than the other then natural selection is off and running. In the natural world these conditions are most familiarly met by whole organisms but in principal they can be met by cells or genes within cells.

In this case, selection is operating at two levels. At a lower level cells which contain the mutant gene are fitter than cells that don't contain this gene. But at a higher level individuals that don't contain this mutant gene are fitter than individuals that do. In most cases the "interests" of the gene and the individual coincide. Here, because their reproductive fates have been decoupled the relationship has turned antagonistic.

Finally, the study shows that the reconstruction of a complete evolutionary narrative requires thinking across the entire (or as much as possible) hierarchy of life.  The so called division between reductionists and practitioners of "holistic" biology is and always has been a false dichotomy.  Genes, cells, individuals, social behavior and population history all go to make the unfortunate story of the Tasmanian Devils more complete.

** Wishing readers coming to this post via Desipundit a very Happy New Year. If you missed my earlier announcement do note that my blog Reporting on a Revolution has been renamed Rapid Uplift. ...more in tune with the broad earth sciences / geology theme I write about.  Thanks for your support.


  1. Very nice! Thanks for the insight. A photo of a Tassie with the face cancer would improve the readability of the blog post!