the theory of evolution, but it was in his family. Darwin’s grandfather, Erasmus Darwin (1731–1802) was an eminent physician and keen amateur naturalist who proposed all organisms had descended from a ‘primal filament’. Yet Charles Darwin did not look likely, initially, to follow in his grandfather’s illustrious footsteps. His father, Robert Darwin, often harangued the young man, declaring, ‘You care for nothing but dogs and rat catching, and you will be a disgrace to yourself and your family’ This assessment looked increasingly probable as Charles dropped out of first, Medicine at Edinburgh and then Divinity at Cambridge. When, in 1831, the opportunity arose to serve as unpaid naturalist aboard the HMS Beagle, his father was at first opposed to the idea. It was only after the intervention of Darwin’s uncle, Josiah Wedgwood, that Robert Darwin was persuaded to allow his son to board ship.
Perhaps the elder Darwin did have a point about his rat-catching son because although Charles Darwin published The Voyage of the Beagle and several geological works in the years after his return from his voyage, he sat on his theory of evolution for twenty years. But in 1858 a letter arrived from Alfred Russell Wallace with an essay entitled, ‘On the tendency of varieties to depart indefinitely from the original type’. Horrified to find that Wallace had independently come to the same conclusions, Darwin’s first reaction was to abandon his lifework, but after much soul-searching and advice, he was persuaded to make a joint presentation with Wallace at the Linnaean Society. Darwin then set about organizing the mass of material he had collected over the years and published an abstract of his work, The Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life.
What was original to the theory of Darwin and Wallace was that they proposed a mechanism to drive evolution – natural selection. The idea owes much to Malthus’ Essay on Population which argued that mankind will always produce more offspring than the available resources can support. The inevitable consequences, according to Malthus, are war, famine, poverty and disease, as increasing populations fight for limited resources. To Darwin, the same pressures amongst living organisms lead to natural selection or survival of the fittest. Only the fittest individuals will capture the resources needed to generate offspring.
The survival of the fittest is not enough in itself to bring about evolution because there is as yet no mechanism to generate change. The vital extra ingredient for natural selection is therefore inherited chance variation within a species. Darwin observed how all individuals within a species were very slightly different. Breeders of domesticated animals, such as dogs, have exploited these small differences to select animals with the desired traits, such as the shape of a face or the length of a tail. After centuries of breeding, quite distinct breeds, such as the poodle and the Great Dane, have been generated from the same ancestral wild dog. In the same way, nature could act as the selective force. These naturally occurring, randomly varying individuals within a species would compete for resources. With progeny always outstripping resources, only the fittest variants would survive to reproduce. Natural selection would tend to breed from those fittest individuals who possessed the most desirable traits. Over millennia, natural selection would bring about gradual change in plant and animal characteristics towards increasing fitness – descent with modification – evolution.
Publication of The Origin of Species aroused a great deal of criticism. But although the ensuing theological debate generated the greatest controversy (and still does), a very real scientific problem led Darwin to at least a partial abandonment of his original hypothesis. The difficulty arose with the commonly held belief at the time that sexual reproduction always led to the blending of characteristics. This blending would tend to wipe out variation – so that if poodles and Great Danes were allowed to mate freely, then their offspring would tend towards nondescript mongrels. The physicist H. C. Fleming Jenkin and many others held that no amount of selection for either the characteristics of poodles or Great Dane could result in their offspring being anything other than a blend of the two.
Without variation, natural selection had nothing to select. Darwin needed some mechanism to replenish the variation that was being washed out of populations by blending. His answer was to embrace the discredited Lamarckian theory of inheritance of acquired characteristics. In the 1868 edition of The Origin of Species, Darwin proposed that parts of the body threw off minute particles congregating in reproductive cells to be inherited by the offspring. These bits, or gemmules, were held to transmit characteristics acquired during an individual’s lifetime and thereby provided a new source of variation to offset the loss of variation by blending. The gemmules theory had more than a whiff of desperation about it and did much to discredit Darwinism in the latter half of the nineteenth century.
GENES
Two years before the 1868 edition of The Origin of Species, an obscure monk working in an Augustinian monastery in Brünn had discovered and published the right answer to Darwin’s problem. Born into a peasant family in Silesia, Gregor Mendel joined the monastic life to escape poverty. In the monastery garden, Mendel crossed pea plants and carefully recorded the inheritance of parental traits – such as round or wrinkled peas – in those hybrids he generated. He demonstrated that, contrary to expectations, discrete characteristics such as the shape of the pea seed did not blend in crosses. Instead they bred true – were unchanged when they appeared in subsequent generations – though sometimes skipping a generation. To account for his peas, Mendel proposed that organisms contain within them discrete factors, passed unaltered from one generation to the next. We now call Mendel’s discrete factors, genes. This was the exactly the answer Darwin needed – genetic variation would not be lost during sexual reproduction but emerge, unscathed by its passage, in each generation.
Darwin died in 1882, completely unaware that the work that would rescue his seriously flagging theory was languishing in the Linnaean Society library, the very place his own theory had been triumphantly unveiled. Even more tragically, Mendel died in obscurity in 1884, his revolutionary work unknown or forgotten. It was not until 1900 that three botanists, Hugo de Vries, Carl Correns and Erik von Tschermak, independently rediscovered his experiments. Working on the inheritance of variation in plants, they were each finding evidence for discrete patterns of inheritance. Searching the literature for any related similar work, independently each came across brief references to Mendel’s publications and immediately realised their significance. Mendel was posthumously recognized as the father of modern genetics. Mendelian genetics went on to revolutionize twentieth-century biology and medicine.
The early twentieth century saw the fusion of Darwinian evolutionary theory (his original rather than the Revised Version) and Mendelian genetics in what has come to be known as the neo-Darwinian synthesis. In 1901 De Vries published the first volume of his Mutation Theory, in which he proposed that evolution occurs by discrete steps or mutations that were rare chance modifications of Mendelian genes. These mutations were the source of the variation for Darwinian natural selection and evolution. Evolutionary theory was, at last, complete.
But what were genes? What were they made of? How were they inherited? How were they modified? In 1901 nobody had any idea. The first real clue did not come until 1945 when the bacteriologist Oswald Aver/s experiments at New York’s Rockefeller Institute, demonstrated that genetic characteristics could be transferred from one bacterial cell to another, simply by transferring a chemical called deoxyribonucleic acid, or DNA. Avery purified DNA from some bacterial cells which produced a capsule (a slimy protective layer made of strings of sugars that surrounds the bacterial cells). He found that if he added it to cells that didn’t produce a capsule then some of them would be transformed into capsule-producing bacteria. It appeared that the genetic information, the gene for the capsule, was made of DNA and could be transferred from one bacterial cell to another simply by transferring the DNA chemical.
However, not everyone was convinced of the significance of Avery’s demonstration that DNA encoded bacterial slime. DNA was considered an unlikely vehicle for heritable information. Different species were assumed to have
