Evolution of individuality - Wiley Online Library

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power of group selection promoting greater levels of cooperativity and synergism ... For this to happen, selection at the lower level, selection among cells,.
8 BirkhBuser Verlag, Basel, 1998

J. evol.

biol. 11 (1998) 1010-061X/98/020225-03

225-227 $ 1.50 + 0.20/O

1Journal

of Evolutionary

Biology

Commentary

Evolution of individuality R. E. Michod University of Arizona, Arizona 85721, USA

Key words:

Department

Cooperation;

individual;

of Ecology and Evolutionary

Biology,

Tucson,

conflict.

The origin of multicellular life is probably the premiere example of the integration of lower levels in the biological hierarchy into a new higher level evolutionary unit. According to Jablonka and Lamb, “the EISs that evolved in early unicellular organisms probably played an important role in the transition of multicellularity”. In my commentary, I focus on the transition from single cell to multicellular organisms. The emergence of organisms as a new evolutionary unit depends in part upon the power of group selection promoting greater levels of cooperativity and synergism among cells. For this to happen, selection at the lower level, selection among cells, must be controlled; if not, selfish mutants may arise which threaten the existence of the emerging higher level entity, the organism. The main basis for the author’s claim that EISs promote the transition to multicellular life is that EISs make it easier for group selection to over power selection at the cell level. Specifically, they argue that selfish mutants are less likely in the presence of established EISs. “For a mutant cell to destroy the coherence of a group, it must forget its epigenetic heritage and defy the inductive influences of other members of the group, as well as proliferate more rapidly than its neighbours. The validity of these ideas about the importance of EISs in the evolution of multicellularity can be tested by modelling conipetition between groups of mutation cells with and without epigenetic inheritance” (p. 20). I have studied mutational models of the transition between unicellular and multicellular life (Michod, 1997a; Michod, 1997b; Michod and Roze, 1997; Michod, 1996), but not ones with epigenetic inheritance. Jablonka and Lamb’s article has stimulated me to extend my models to include an EIS which I am in the process of doing. I feel Jablonka and Lamb’s conjecture may be correct - EISs may help retard selfish mutants. However, I wonder about the risks of mutations in marker genes that are responsible for turning structural genes on and off (for example, mutations in genes responsible for states of methylation). If I understand Jablonka 225

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Michod

and Lamb correctly (see also (Jablonka, 1994)), EISs were a preadaptation for multicellularity. In addition to restricting variation and conflict within a group of cells, the authors note that EISs may also serve to increase the level of cooperation and synergism among cells expressed as increased fitness as the cell-group or organism level. With a well defined system of inheritance of cellular function in place, it seems only logical that cells should be able to do more for the organism. What explains the individuality of organisms? For an organism to be an individual, or evolutionary unit, requires functions at the organism level that protect it from conflict within. Although EISs may restrict the opportunities for selfishness on the cell level, within organism change does occur (Buss, 1987; Buss, 1985; Klekowski, Jr. and Kazarinova-Fukshansky, 1984; Michod, 1997a; Michod, 1996; Whitham and Slobodchikoff, 1981). According to a theory developed elsewhere (Michod, 1997a; Michod, 1997b; Michod and Roze, 1997; Michod, 1996) an individual is more than a group of cooperating cells related by common descent. Natural selection at any level requires heritable variations in fitness at that level. To emerge as an evolutionary individual, cell groups must acquire the means to guarantee fitness heritability at the group, or organism, level. Modifiers of within group change, for example germ line modifiers or self-policing modifiers, serve to protect the organism and increase the heritability of fitness at the organism level. By sequestering a group of cells in the germ line early in development, the opportunity for within organism change is limited (Buss, 1987). In addition, by protecting germ cells from the DNA-damaging effects of metabolism in the soma, the mutation rate in the germ line may be lower, again reducing the opportunity for conflict (Michod, 1997a; Michod, 1996). Self-policing modifiers serve to restrict the selfish tendencies of cells (Frank, 1995; Michod, 1997a; Michod, 1996), perhaps through immune system responses or programmed cell death (for a brief introduction to this large and rapidly developing area see (Carson and Ribeiro, 1993; Ameisen, 1996)). In the models I study, germ line and self-policing modifiers are the first uniquely organismal functions that evolve during the transition from unicellular to multicellular life. As these modifiers sweep through the population, the levels of cooperation among cells increases, the heritability of fitness at the organism level increases, and the amount of conflict and within organism change decreases (Michod, 1997b). Ironically, it is out of the need to cope with conflict that more harmony and well being is created for the organism.

References Ameisen, J. C. 1996. The origin of programmed cell death. Science (Washington, D.C.) 272: 12781279. Buss, L. W. 1985. The uniqueness of the individual revisited. In J. B. C. Jackson, L. W. Buss and R. E. Cook (Eds.), Population Biology and Evolution of ClonaJ Organisms. Yale University Press, New Haven. Buss, L. W. 1987. The evolution of individuality. Princeton University, Princeton, NJ. Carson, D. A. and J. M. Ribeiro. 1993. Apoptosis and disease. Lancet 341: 1251-1254. Frank, S. A. 1995. Mutual policing and repression of competition in the evolution of cooperative groups. Nature (London) 377: 520-522.

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Jablonka, E. 1994. Inheritance systems and the evolution of new levels of individuality. Journal of Theoretical Biology 170: 301-309. Klekowski, E. J., Jr. and N. Kazarinova-Fukshansky. 1984. Shoot apical meristems and mutation: selective loss of disadvantageous cell genotypes. American Journal Botany 71: 28-34. Michod, R. E. 1996. Cooperation and conflict in the evolution of individuality. II. Conflict mediation. Proceedings of the Royal Society of London B, Biological Sciences 263: 813-822. Michod, R. E. 1997a. Cooperation and conflict in the evolution of individuality. I. Multi-level selection of the organism. American Naturalist 149: 607-645. Michod, R. E. 1997b. Evolution of the individual. American Naturalist 150: S5-S21. Michod, R. E. and D. Raze. 1997. Transitions in individuality. Proceedings of the Royal Society of London B, Biological Sciences 264: 853-857. Whitham, T. G. and C. N. Slobodchikoff. 1981. Evolution by individuals, plant-herbivore interactions, and mosaics of genetic variability: the adaptive significance of somatic mutations in plants. Oecologia (Berlin) 49: 287-292.

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