The causes of human variability: a Darwinian view

We enjoy reading Edwards’ commentary1 on R A Fisher’s paper ‘The causes of human variability’.2 This subject is of great interest and importance in human genetics and evolution. It is also reminiscent of another paper, published in the same journal, on a great book in which Fisher and Stock wrote: So melancholy a neglect of Darwin’s work suggests reflections upon the use of those rare and precious possessions of man—great books. It was, we believe, the custom of the late Professor Freeman to warn his students that mastery of one great book was worth any amount of knowledge of many lesser ones. The tendency of modern scientific teaching is to neglect the great books, to lay far too much stress upon relatively unimportant modern work, and to present masses of detail of doubtful truth and questionable weight in such a way as to obscure principles. … How many biological students of today have read The Origin? The majority know it only from extracts, a singularly ineffective means, for a work of a genius does not easily lend itself to the scissors; its unity is too marked. Nothing can really take the place of a firsthand study of the work itself.3 The year 2018 marks not only the 100th anniversary of Fisher’s publication of his ‘The causes of human variability’,2 but more importantly, the 150th anniversary of Darwin’s publication of The Variation of Animals and Plants under Domestication, one of his ‘five great books’. Ironically, in his paper ‘The causes of human variability’,2 Fisher failed to cite this great book. It is in this book that Darwin came nearest to the subject of genetics, and devoted five chapters (Chapters 22 to 26) to variation, which was the objective of the entire book. Darwin’s primary concern was with the causes of variability. He considered the main cause of variability to be changed conditions of life. As he wrote: The greater dissimilarity of the successive children of the same family in comparison with twins, which often resemble each other in external appearance, mental disposition, and constitution, in so extraordinary a manner, apparently proves that the state of the parents at the exact period of conception, or the nature of the subsequent embryonic development, has a direct and powerful influence on the character of the offspring. … Thus I Geoffroy Saint-Hilaire asserts that poor women who work hard during their pregnancy, and the mothers of illegitimate children troubled in their minds and forced to conceal their state, are far more liable to give birth to monsters than women in easy circumstances.4 Of all the causes that induce variability, Darwin held that excess of food was the most powerful. He supposed that changed food could indirectly disturb the due and proper action of the reproductive system, thus influencing the characters of the offspring. He believed that ‘abundant and rich food supplied during many generations would give an inherited tendency to increased size of body’.4 He also noticed that ‘hemp-seed caused bullfinches and certain other birds to become black’, and that ‘caterpillars fed on different food sometimes either themselves acquire a different colour or produce moths differing in colour’.4 This led him to conclude that ‘the nature of the food sometimes definitely induces certain peculiarities, or stands in some close relation with them’.4 Now there is increasing evidence that during pregnancy, alterations in diet indeed affect the offspring across several generations.5 Darwin believed that the tendency of new character or modification to reappear in the offspring at the same age at which it first appeared in the parents, or in one of the parents, was of great importance in the study of inheritance. He gave many instances under the term of ‘inheritance at corresponding age’.4 Now it has been well documented that parental age may influence the characters of the offspring. Many studies have shown that children of older parents tend to be more intelligent than do children of younger parents. Either advanced or teenage parents are associated with risk of birth and health outcomes in offspring. Parental age can also influence dominant characters, longevity, sex ratio, personality and development process of the offspring.6 To fully analyse the effect of parental age on the offspring is of great interest and importance in deciding the optimal age for parenthood.7 In Darwin’s opinion, the effects of use and disuse, reversion or atavism, and hybridization were also responsible for variability. Many people today insist that Darwin did not know the phenomenon of dominance or Mendelian inheritance. This is not the case. In his book The Variation of Animals and Plants under Domestication, Darwin wrote: ‘The law of prepotency comes into action when species are crossed, as with races and individuals’.4 He noticed that: In some cases prepotency apparently depends on the same character being present and visible in one of the two breeds which are crossed, and latent or invisible in the other breed; and in this case it is natural that the character which is potentially present in both breeds should be prepotent. He concluded that ‘a character which is present in one form and latent in the other is generally transmitted with prepotent force when the two forms are crossed’. Obviously, Darwin’s ‘prepotency’ and ‘latency’ are essentially the same as Mendel’s ‘dominance’ and ‘recessivity’.8 Darwin himself conducted experiments with snapdragon, in which he found a ratio of 2.4:1 between two flower forms (peloric and radial) in the second generation of the hybrids.4 In his paper ‘The causes of human variability’,2 Fisher only mentioned Mendel’s dominance but ignored Darwin’s prepotency. In evolutionary biology, the Modern Synthesis or neo-Darwinism which brought Darwinian selection together with Mendelian genetics is well known. Little is known about Darwin’s own synthesis which combined his theory of evolution by natural selection with his theory of heredity and variation—Pangenesis. Darwin assumed that besides the ordinary multiplication, cells could also ‘throw off’ numerous particles or molecules called gemmules, which were able freely to be diffused from cell to cell or circulated through the body, modified by changes in environment or the effects of use and disuse, aggregated into germ cells and transmitted from parent to offspring. If the cells of the body were modified by changes in the environment or by the effects of use and disuse, they would release modified gemmules, which were transmitted with their acquired characters to the offspring. This provides an explanation for the inheritance of acquired characters. Now the discoveries of circulating cell-free DNA, mobile RNAs, prions and other extracellular vesicles indeed support his assumption that cells could shed particles or molecules into the circulation.9 If Darwin’s term ‘modified gemmules’ is replaced by ‘methylated DNA’ or ‘small RNAs’, it seems to be consistent with our current epigenetic explanation.10 In fact, Darwin’s Pangenesis explains a far wider range of phenomena than Mendel had considered. It appears to us that neo-Darwinists were not necessary to graft Darwinian evolution onto Mendelian genetics. The original Darwinism was far more comprehensive than neo-Darwinism, and there is a need to go back to Darwinian Synthesis.

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