Saturday, July 22, 2006

Identity and Composition in Genetics

The idea of survival of the fittest in evolutionary theory would seem to imply that identical organisms would not compete for survival. However, in nature, no two things can be identical since they would differ in space and time. The philosophical question is to what extent that difference makes a difference.

In the theories of physics and chemistry, electrons or specific elemental atoms are more or less interchangeable when each type of thing is considered within a specfic context of conditions.

The case in biology seems different because background conditions are not so easily subject to ceteris paribus conditions. For example with biological entities, according to the idea of the survival of the fittest, identical organisms could not compete with each other since that would result in no advantage for survival, yet, in a recent issue of Nature Reviews: Genetics, competition between genetically identical cells are discussed in the context of improving fitness of the organism:

Competition between genetically identical organisms is considered insignificant in evolutionary theory because it is presumed to have little selective consequence. We argue that competition between genetically identical cells could improve the fitness of a multicellular organism by directing fitter cells to the germ line or by eliminating unfit cells, and that cell-competition mechanisms have been conserved in multicellular organisms. We propose that competition between genetically identical or highly similar units could have similar selective advantages at higher organizational levels, such as societies. (Khare, 577)

If this were true, then two problems need solution:

(1) If natural selection in biological evolutionary theory is that individual organisms with favorable traits are more likely to survive and reproduce, then it would seem to follow logically that identical organisms would have the same probability of survival and reproduction. However, if Khare and Shaulsky are correct that competition between genetically identical individuals have higher selective advantages, then the theory of natural selection would not be meaningful because it would not be falsifiable. Whichever individual survives, whether being alike or different from the others, would have, by definition, survivable (favorable traits). But if those traits were identical with those individuals who perish, then survivable traits would be identical to nonsurvivable traits. Even so, traditional evolutionary theory describes the orgins of organisms, not the origins and survival of differentiated cells within an organism . Fitness among genetically identical cells, it seems to me, should be discussed. at best, in an analogical sense with the fitness of organisms because of the inplicity different scales of phenomena described.

(2) There is no reason to presuppose that biological organization and change on various levels of phenomena (viz. cellular, organism, and society) would be subject to the same rules of natural selection. Khare and Shaulsky argue, "[cell competition] has a selective advantage in increasing the overall fitness of the individual by increasing tissue and organ fitness" (Khare, 578). But fitness of cells and fitness or organisms are considerably different orders of phenomena.

Just as an all-star football team is normally not a better team than the league leader, so likewise individually fit cells do not necessarily produce the most fit of organisms. To make such a claim without a supporting a mechanism and theory is to commit the fallacy of compostion. A cancer stem cell, on this view, would seem to confer a selective advantage to a cancer patient. The interrelationships among cells, organisms, and societies are not presently reducible to the same biological theory any more than cellular biology can explain both interpersonal and sociological problems of modern life.

In truth, of course, Khare and Shaulsky note "No two cells are identical" (Khare, 580) because "Although genetically identical cells have the same DNA sequence, they might differ in epigenetic modification, nutritional status, signaling proficiency, cell cycle position, and other factors that affect fitness." (Khare, 581).

If genetically identical cells differ in factors that affect fitness, then by definition these factors are selective advantages. Nonetheless, these factors could not be intrinsic to the genotype under the traditional interpretations of the terms "genotype" and "phenotype." Undoubtedly, these terms can no longer be used in a precise sense in current genetic discussions.

In any case, this realization reduces the claim that "competition between genetically identical cells could improve the fitness" of an organism to a tautological statement. Any such "competition" among cells would have to be based on phenotypic differences. We are left with the unenable position that fitness, in such cases, for the organism, could only be shown in a Lamarkian world.

In truth, Khare and Shaulsky are suggesting that competition among epigenetically different cells of the same organism might improve the fitness of an organism.
Khare, Anupama and Shaulsky, Gad. "First among equals: competition between genetically identical cells." Nature Reveiws: Genetics. (July 2006) 7 (7), 577-583.