It was not until the late 1930s that Darwin’s mechanism for evolution – natural selection – obtained widespread acceptance. At this time, leading scientific figures like Ronald A. Fisher, J.B.S. Haldane, and Sewall Wright became the founding fathers of neo-Darwinism, which fused natural selection with Mendelian genetics. The theory of heredity was essential for the connection between the theory of evolution and cell theory. In the 19th century, biologists Schleiden, Schwann, and Virchow explained that cells were the basic unit of all living things. In 1944, Canadian Bacteriologist Oswald Avery identified DNA in the cell nucleus as the material forming the basis of heredity. The discovery by Francis Crick and James Watson of the double helix of DNA further clarified the pathway of evolution. Darwin’s variations in offspring were due to changes in DNA, arising from random mutations and internal molecular rearrangements, on which natural selection would act.

Gregor Johann Mendel, an Austrian monk, and amateur botanist in the 1860s made a careful study of the inherited characteristics of plants, which discovered the phenomenon of genetic inheritance. Mendel, a shy and modest man, sent his findings to an eminent biologist, who, as one might have expected, dismissed the whole idea as nonsense. Deeply discouraged, Mendel hid his ideas from the world and returned to his plants. His revolutionary work was only rediscovered in 1900, when the science of genetics was really born. Improvements in microscopes made it possible to see inside the cell, leading to the discovering of genes and chromosomes.

Genetics allows us to understand the ever-continuing development of life. The evolution of life meant the appearance of a self-replicating molecule which could transmit the characteristics of the life-form to future generations. Such a mechanism is deoxyribonucleic acid (DNA). This self-reproducing DNA molecule is not concentrated in a particular part of the body, but is contained in every animal or plant cell. The highest evolved species, a product of over 3 billion years of evolution, is the human species. At adulthood, humans are made up of a trillion cells, but at conception there existed only a single-celled embryo. How does this happen? The secret is in the DNA. Within this single cell was contained the DNA molecule that held the genetic code for the construction of a human being. The genetic information carried by the genes is stored in a chemically coded form. One gene is a section of DNA that has the information to make a particular type of protein.

The genes contained in every cell are that part of the organism that contains all the necessary information for creating animals and plants. Most genes carry information that direct cells to make proteins. Some genes tell the cells in an embryo where they are and whether they should grow into an arm or a leg. The sequences of bases stored in the genes determine what the living creature will be. The heredity information is stored in the nucleus of each cell in the form of chains of genes called chromosomes. Like a living textbook, two sets of chromosomes carry all the genes allotted to an individual, defining the nature of the structure of the proteins that do most of the work in the body.

Only in the 1950s was the chemical composition of genes identified as DNA. In 1953 Francis Crick and James Watson made a revolutionary breakthrough in genetics with their discovery of the famous double-helix model of the nucleic-acid molecule, for which they shared the Nobel Prize in 1962. This makes clear how chromosomes are duplicated in cell division. DNA is present in the simplest life-forms: a virus possesses a single DNA molecule. All life as we know it depends on DNA in the last analysis. The discovery and development of genetics further unlocked the secrets of evolution. The laws of evolution discovered by Darwin were enriched by the understanding of genetics, through the work of Fisher, Haldane and Wright, the founders of neo-Darwinism.

The gene is the unit of heredity. The entire collection of genes possessed by an organism is called the genome. At present scientists are engaged in a project to identify all the genes of the human genome, which number around 100,000**. The genes themselves in each generation of cells reproduce themselves; proteins in the shape of special enzymes play an important role in the process. Through this self-reproduction, genes are formed once again for each new cell. So the genes indirectly produce the proteins that construct and maintain all cells. From bacterial cells, plant cells and animal cells; cells specialised to form leaf and stem, muscle and bone, liver and kidney, and many more, including the brain. Each cell contains the same complement of genes as was present in the original cell. Each human cell probably contains the genetic information needed to make any type of human cell, and therefore an entire human being, but in each cell only a selected portion of that information is used. It is analogous to a book of instructions, where only certain pages, and even only certain lines and words are selected to code the necessary proteins needed in the production of various cells.

The effect of sexual reproduction is to mix or shuffle the genes. The sex cells (egg or sperm) only contain 23 chromosomes each in humans, but when fused make up the normal 46 chromosomes. The new cell would, in the words of Dawkins, be “a mosaic of maternal genes and paternal genes.” As the two sets of chromosomes merge, if two gene signals differ, then one characteristic will prevail over the other. The gene for brown eyes, for instance, is dominant to that for blue. They are what is termed as recessive and dominant genes. Sometimes a hybrid compromise is produced.

It is through reproduction that variation is achieved. From an evolutionary view this is vital. The asexual reproduction of primitive organisms makes identical copies of the parent cell, where mutation is very infrequent. On the other hand, sexual reproduction, with the new combination of genes from two sources, increases the possibilities of genetic variation and has the effect of accelerating the rate at which evolution can proceed. Each life form carries the DNA code of genetic information. The evidence of our common ancestry is the similarity of cell structure of all living things. The mechanism of inheritance is the same, where DNA determines that mice look like mice, humans look like humans, and bacteria look like bacteria. Some organisms, such as bacteria, possess only one main DNA molecule, whereas our own cells, and those of higher organisms, contain a number of separate bundles of DNA (chromosomes).

  1. This was written before the results of the Human Genome Project were known which has shown that the actual number of human genes is far less than what scientists supposed. See the comments in the preface to the second edition.