Piero Scaruffi(Copyright © 2013 Piero Scaruffi | Legal restrictions )
These are excerpts and elaborations from my book "The Nature of Consciousness"
The Logic Of Replication
In 1865 the Austrian botanist Gregor Mendel, while studying pea plants, proposed a mechanism for inheritance that was to be rediscovered in 1901. Contrary to the common-sense belief of the time, he realized that traits are inherited as units, not as "blends". Mendel came to believe that each trait is represented by a "unit" of transmission, by a "gene" (a term coined in 1909 by Wilhelm Johanssen). Furthermore, traits are passed on to the offspring in a completely random manner: any offspring can have any combination of the traits of the parents.
Mendel proved that "blending inheritance" is false, that we do not "blend" the inheritances we receive from our parents. There is a unit of inheritance, the gene, and we either inherit a gene or we don't inherit it. Our eyes are either blue or brown, but not a blend of blue and brown. We are either male or female, but not a blend of male and female. (The color of the skin may be intermediate between the colors of the parents, but that is because the color is due to the sum of numerous genetic effects).
The British biologist William Bateson coined the term "genetics" in 1906. The Danish botanist Wilhelm Johannsen coined the term "gene" in 1909. In the 1920s the US biologist Thomas Hunt Morgan discovered that genes are arranged linearly along "chromosomes".
The model of genes provided for a practical basis to express some of Darwin's ideas. For example, Darwinian variation within a phenotype can be explained in terms of genetic "mutations" within a genotype: when copying genes, nature is prone to making typographical errors that yield variation in a population.
In the 1920s population genetics (as formulated by the US biologist Sewall Wright and the British biologist Ronald Fisher) turned Darwinism into a stochastic theory (i.e., it introduced probabilities). Evolution became a shift in gene frequencies within a population over time. Fisher, in particular, proved that natural selection requires Mendelian inheritance in order to work the way it works. Fisher unified Darwin and Mendel (initially Mendel had even been viewed as anti-Darwin): what changes in evolution is the relative frequency of discrete hereditary units, each of which may or may not appear (more or less randomly) in successive generations. In the 1940s the two theories were merged for good in the so called "modern synthesis". In practice, the synthetic theory of evolution merged a theory of inheritance (Mendel’s genetics) and a theory of species (Darwin’s evolutionary biology).
Since those days, the idea of natural selection has undergone three stages of development, parallel to developments in the physical sciences: the deterministic dynamics of Isaac Newton, the stochastic dynamics of Clerk Maxwell and Ludwig Boltzmann, and finally the dynamics of self-organizing systems. Originally, Darwin's theory was related to Newton's Physics in that it assumed an external force (natural selection) causing change in living organisms (just like Newton posited an external force, gravity, causing change in the motion of astronomical objects). However, with the formulation of population genetics by Ronald Fisher and others, Darwinism became stochastic (the thermodynamic model of genetic natural selection, in which fitness is maximized like entropy), just what Physics had become with Boltzmann's theory of gases. In the 1990s self-organizing systems provided a new model to think about the organization of life at different levels, from cells to societies.
Darwin had not explained what he set out to explain: the origin of species. The Russian geneticist Theodosius Dobzhansky came closer: what makes a species a species is sex. Different populations of the same species actually have different genomes, but only some genes need to be compatible for sexual reproduction to occur. If those genes are affected by mutations, then two populations may become incompatible and a new species is born.
In 1944 the Canadian physician Oswald Avery identified the vehicle of inheritance, the substance that genes are made of, the bearer of genetic information: the deoxyribonucleic acid (DNA for short).
In 1953 the British biologist Francis Crick and the US biologist James Watson figured out the double-helix structure of the DNA molecule. It appeared that genetic information is encoded in a rather mathematical form, which was christened “genetic code” because that’s what it is: a code. The “genome” is the repertory of genes of an organism.
In 1957 Crick, by using only logical reasoning, reached the conclusion that information must flow only from the nucleid acids to proteins, never the other way around.
In 1961 the South African biologist Sydney Brenner and the French biologist Francois Jacob discovered that cells of ribonucleic acid (messenger RNA), carry the genetic instructions from the DNA to the ribosomes, the sites within a cell that manufacture proteins.
Also in 1961 Jacob and the French biologist Jacques Monod discovered the mechanism of gene regulation: genes turn each other on and off, i.e. genes are organized in a network. Jacob and Monod also noticed that the interaction among genes might explain cell differentiation (the fact that cells containing the same genetic information end up doing completely different things).
By 1966 the US chemist Marshall Nirenberg had cracked the "genetic code", the code used by DNA to generate proteins. He and the Indian biologist Har Gobind Khorana discovered how the four-letter language of DNA is translated into the twenty-letter language of proteins (the DNA is made of four kinds of nucleotides, proteins are made of twenty types of aminoacids).
In the 1980s we started deciphering the genome of different animals, including our own.
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