The discovery of heredity

by Matthew Cobb

In a previous post on the genetics of dog coat variation, I stated that ‘dog “breeds” are an incredibly recent invention – less than 200 years’. In the discussion that followed, David Burbidge commented that this wasn’t quite right. Citing  two 17th century examples (Shakespeare’s Macbeth and the King Charles spaniels) as proof, he wrote: “Major breeds like greyhounds, wolfhounds, mastiffs, and spaniels all go back much further than 200 years, though of course the modern pedigree breeds are not identical to their ancestors.”

There are two points here. First, he’s right, of course. There was indeed artificial selection for general dog shape/behaviour, presumably going back even before the 17th century. However, the point being made in the article (and in my post) is that contained in the final part of David’s comment. Pedigree breeds, with strict control of breeding, and the development of so many more kinds of dog breed, beyond the key hunting types, with a few pets thrown in (like the King Charles Spaniel), is indeed extremely recent. Most of the examples of coat variation described in the article have accumulated in less than 200 years.

The second, and more intriguing point, relates to what on earth all those dog breeders thought they were doing prior to the 19th century. For evolution by natural selection to occur, you need three conditions: characters need to be variable between individuals, some element of that variability needs to be inherited, and the variability must affect fitness (ultimately expressed in terms of the number of copies of the genes underlying the character that are present in the next generation).

With those three conditions, any set-up, be it organic life-forms or a computer model, will lead to evolution by natural selection.

The existence of the second of these three conditions for evolution – heredity – seems completely obvious to us today. But less than 200 years ago, the concept of “heredity” in biological terms simply did not exist. In the 17th century, the great physician William Harvey tried to work out why some characters seemed to reappear in different generations, others seemed to “skip” a generation, some (like skin colour) appeared to be a mixture of  the parents, while others again (like sex) were either one or the other but never (or very rarely) a mixture.

As he put it: “why should the offspring at one time bear a stronger resemblance to the father, at another to the mother, and, at a third, to progenitors both maternal and paternal, farther removed?” In the end, Harvey simply gave up – it was too complicated for him.

In a way, this isn’t surprising. Genetics is complicated – there is not, at first sight, a common explanation for skin colour (blending inheritance), eye colour (dominance in most cases) and sex (chromosomal determination – in humans at least). Harvey couldn’t see a pattern because, on the surface, there isn’t one.

You might think that although “scientists” like Harvey didn’t get it, there must have been some general folk knowledge that “like breeds like”. Yes and no, is the answer. For example, in the second half of the first century CE, Lucius Junius Moderatus Columella summarized Roman agricultural knowledge in De Re Rustica (‘On Rural Matters’). Problems associated with animal breeding take up only a few sentences, but Columella did note that, althought most coat colours were a mixture of the parental kinds, the grandsire’s coat colour could reappear in the second generation. What that meant, of course, was completely opaque. In other words, like does not always breed like.

Furthermore, until the experiments of Francesco Redi and Jan Swammerdam in the mid-1660s, it was thought that insects and all other “bloodless animals” appeared from decay, rather than from mating between male and female. It took Redi’s careful experimentation, and Swammerdam’s observations, for them to come to the radical conclusion that “all animals come from an egg laid by a female of the same species”.

The question of the role of the male and the female in producing the offspring remained hugely contentious right up until the 1840s. Aristotle had argued that the male produced a “seed” (= “semen”), which the female nourished. Swammerdam, Steno and others turned this argument on its head and argued that the egg was the origin of all life – including in humans. Within a few years, matters became even more complicated when Antoni Leeuwenhoek observed spermatozoa in human semen. Leeuwenhoek thought that these sperm were the origin of life, and that the egg was simply food. Most people, however, argued they were parasitic worms (hence the name we still use today – “spermatoZOA”, or animal found in semen).

While the mechanics of what eventually became known as reproduction were finally worked out in the 1840s, in the middle of the 18th century people had begun to suspect that something else was going on. Two parallel sources of information helped gel the idea of heredity. Both involved large data sets – which is when the patterns of heredity start to become really clear.

The transmission of polydactyly – having five or more fingers – in human families was separately studied by two French thinkers, Réaumur and Maupertuis, and they began to suspect that something was passing across the generations. Then, in the middle of the 18th century, Robert Bakwell, an entreprising English sheepfarmer, decided to make a better breed of sheep that would grow quicker and make him more money. By carefully selecting sheep in a very large flock, he soon produced his desired changes.

These ideas were soon applied to the centre of the European wool industry, in Moravia, where local thinkers started to predict what plant hybrids would look like. In 1837, one of the leading intellectuals of the area, Abbot Napp of the Brno monastery, asked “what is inherited and how?” Six years later, he welcomed a new recruit to the monastery – Gregor Mendel.

The final certainty that there was such a thing as “heredity” came with French thinkers looking at patterns of disease. People knew that some diseases reappeared across the generations. For example, in the seventeenth-century, the philosopher Sir Francis Bacon wrote that: “Long Life, is like some Diseases,a Thing Hereditarie”. By the end of the 18th century, French physicians had become convinced that many human characteristics were “hereditary”.

However, at this stage “heredity” did not exist, either as a concept or as a word. You won’t find the word in the first Edition of The Origin of Species, either, although the idea is fundamental for Darwin’s view of evolution. The English word was first used in print by Spencer in 1863, and Darwin used it in his notes at about the same time.

It is well known that although Darwin knew there was a “force” of heredity, he did not understand how it worked (any more than Harvey did). Many of his ideas were wrong, and he recognised this was a major weakness in his theory.

To the modern reader, what appears so surprising at first sight is that it took so long for something to be discovered that now appears so obvious. What this history in fact shows is that the facts of heredity are in fact remarkably complicated, and it required  a lot of work by some very smart people over a long period of time to finally work out what was going on.

To learn more about this, you can download this article I published in 2006 (PDF format), or, even better, read my book, entitled Generation in the US, or The Egg & Sperm Race in the UK. You can also visit the website associated with the book.