Home > bio-, Michel Morange, teadus > Michel Morange “Genetics, Life and Death”

Michel Morange “Genetics, Life and Death”

Morange, Michel 2007. Genetics, Life and Death: Genetics as providing a definition. – Anne Fagot-Largeault; Shahid Rahman; Juan Manuel Torres (eds). The Influence of Genetics on Contemporary Thinking. Dordrecht: Springer, 51-62.

Among the different contemporary definitions of life which are given by biologists, one of the most frequent is that what characterizes organisms is their capacity to „reproduce imperfectly”. This definition includes two components, both essential: the power to reproduce, characteristic of life; and the fact that this reproduction is sufficiently faithful to allow a correct functioning of the progeny, but not perfect, not preventing the appearance of slightly different forms of organisms, likely to be better adapted to their environment. Imperfect reproduction is the condition required for the action of natural selection, and the historical development of life. (54)

Because genetic mechanisms are the main mechanisms of heredity to have been progressively forged and retained by natural selection, they are fundamental in this Darwinian definition of life. As sophisticated as they are – with the very precise copy at each generation of the long protein sequences –, they perfectly provide the basis on which natural selection can efficiently play its role. (55)

Despite its apparent success, such a vision of genes controlling the duration of life – and death – was rapidly challenged both by cell biologists and population geneticists. The former showed the existence of a specific form of cell death – by apoptosis –, totally different from the form of cell death which results from the absence of nutrients and oxygen occurring when the organisms die. During apoptosis, cells play an active role in their own death – whence its name of programmed cell death. (56)

The first observations of apoptosis did not question the existence of a genetic program of death or its link with a developmental program, since they were made on the massive cell death process that accompanies metamorphosis, a specific phase of development for insects and amphibians (Lockshin and Williams, 1964, 1965). But as findings accumulated on this specific form of death, it appeared less and less linked with the program of development and more and more related to the capacities of organisms to adapt to changing environments. It is the absence of cell death, not its occurrence, which in most cases constitutes a threat to the organism. (56-57)

The present model proposed by population geneticists is that the duration of life (and the occurrence of death) is not actively controlled by genes, but that specific forms of genes have been progressively introduced by chance during evolution and can affect the duration of life, and increase the probability of death simply because they do not sufficiently affect fitness to be eliminated by the action of natural selection. Two slightly different possibilities remain: these mutations affect the organisms too late, at a time when, in natural conditions, most have already succumbed by accident; or their negative effect is balanced by the positive effect they have on reproduction (antagonistic pleiotropism). A later modification has been made to the evolutionary theory of aging and to the “antagonistic pleiotropism” model by appealing to the “disposable soma” theory (Kirkwood and Rose, 1991): the organism differentially allocates a fixed number of resources to reproduction and maintenance. (57)

Human genome sequencing – and the discovery of the low number of human genes – was clearly a blow to the idea that human higher activities are directly reflected in the complexity of the genome, whereas the isolation of master genes controlling behavior has frequently been disappointing, and has lost its attraction – possibly with the exception of thefruitlessgene ofDrosophila(Manoli et al., 2005). Gene variations can have a dramatic effect on behavior, but this does not mean that the genes that are involved are behavioral genes. However, the possibility that the human mind consists of cognitive modules that evolved through the pressure of natural selection still leaves a place for the genes in the construction of the mind (Buller, 2005). (58)

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