Darwin’s theory and the genome

Abstract

Even today, the unshakable basis of the theory of evolution remains natural selection. Modern genetics added another fundamental factor: the mechanism of changes of hereditary characteristics. For Darwin’s predecessor, Jean Baptiste de Lamarck, all changes taking place in the life of a parent are transmitted to children. Darwin tried to give a more satisfactory explanation of how adaptations produced during the life of individuals can be transmitted to children, an essential part of the theory of evolution by natural selection that can apply only to inherited characters. The explanation had to wait for modern genetics: working on fruit flies (Drosophilas) in T.H. Morgan’s laboratory, Herman Joseph Muller showed that hereditary changes, which were called mutations, are a spontaneous, rare and probably random phenomenon due to a chemical change, and obtained the Nobel prize 1946 for his discovery. When the nature of the hereditary material, DNA, was understood, the chemical nature of mutations could also be understood: it is a copy error in the DNA a parent makes of his own DNA and transmits to a child. The error is inevitably transmitted to the child’s descendants because the erroneous DNA is the model the child uses for making DNA for his own children. DNA is a long filament made of units called nucleotides, which can be of four types. The four nucleotides constituting DNA are called with the initials of their chemical name A, C, G and T; the DNA forming the genome that a parent passes to the progeny is made of three billion nucleotides and is organized in 23 long filaments each of which is called a chromosome. The simplest mutation is the replacement of a nucleotide in a giving position of a chromosome with another of the four. There are also mutations of longer DNA segments, even loss or duplication of entire chromosomes. The first mutations studied were spontaneous ones and they are more or less maladaptive for the normal environment of life, and are often responsible for hereditary diseases, or even lethal. But there are mutations, perhaps less frequent, that increase survival and/or fecundity and are therefore selected positively in evolution. But the great majority of mutations are