Darwin and DNA: How Genetics Spurred the Evolution of a Theory
Peter Bowler
Our understanding of evolution today stems from the combination of two very different ideas. One came from a monk who studied pea plants in a Moravian monastery in the 1850s. The other came from a Victorian gentleman who spent five years as a naturalist on a voyage around the world, 20 years previously.
Although Gregor Mendel and Charles Darwin were alive at the same time, they never met and Darwin wasn't aware of Mendel's work. With hindsight, the union of the two men's work seems like a marriage made in heaven (or hell, if you're a creationist). In fact, for many years, it wasn't obvious that Mendel's studies of heredity had any relevance to Darwin's theory of evolution by natural selection. It would take nearly 60 years for this jigsaw to be pieced together and give rise to the "modern synthesis" of evolution, which framed Darwin's idea in terms of genetics.
How exactly did this new understanding arise? And why did it take so long?
The explanation starts with natural selection itself. According to this, only the fittest--the best adapted to the local environment - survive and breed, and in this way the population as a whole gradually transforms. The idea of evolution was already accepted by many biologists in the mid-19th century, but there was considerable opposition to the notion that it happened by means of natural selection. The plausibility of this mechanism rests on the assumption that beneficial characteristics are passed more or less intact from one generation to the next. But it was not clear how this might happen.
To explain heredity, Darwin proposed a hypothesis he called pangenesis. It posited that each organism produces particles called "gemmules", which transmit its characteristics to the next generation. Darwin suggested that the offspring develops from a mix of the parents' gemmules and thus exhibits a blend of their characters. But the idea had a major flaw, seized upon by his opponents: blending would result in the useful characteristics of one parent becoming diluted as it mated with individuals that do not have those traits. Over successive generations these characteristics would gradually disappear. It was a problem no one was able to solve during Darwin's lifetime.
Unbeknown to Darwin and his compatriots, the key to solving this puzzle had already been found. Sometime in the 1840s, Gregor Mendel joined the friary at Brno, in what is now the Czech Republic. In the years that followed, he made detailed studies of how the characteristics of pea plants were passed from one generation to the next. He found that parents' traits were not blended in their offspring; rather, they were transmitted unchanged in predictable ratios. This led him to devise laws of inheritance, published in 1866 (see "Mendel and the birth of genetics"). No one imagined, though, that the characteristics Mendel studied in peas, such as flower colour, were of any general significance, and the work was largely ignored for decades.
Then in 1900, Mendel's laws were rediscovered by the botanists Hugo De Vries and Carl Correns. Through studying inheritance, each independently came to the view that an organism's characteristics are fixed units that are transmitted unchanged. Only later did they discover that Mendel had carried out similar work.
Dawn of the gene
A new science of heredity emerged. First dubbed "Mendelism", it was soon christened "genetics" by the biologist William Bateson, who translated Mendel's paper into English and was a key promoter of his work. Bateson derived the name from the Ancient Greek word "genesis", meaning "origin". Mendel had expressed his laws in terms of characteristics transmitted from parent to offspring. The early geneticists were convinced that some material entity in the organism must encode that information.
Before long, the biologist Thomas Hunt Morgan had identified genes as units arranged along the chromosomes inside the cell's nucleus. Working on the fruit fly Drosophila in 1910, Morgan showed that the trait for eye colour could be traced to a specific place on the X chromosome. This led to a burst of discoveries about the links between different genes, and to the creation of genetic maps showing the positions of genes on chromosomes.
Morgan's research eventually won him a Nobel prize and confirmed that genes were the physical substance of inheritance. It would take another three decades, however, to discover that they were made of DNA and that each gene codes for a specific protein.
The concept of the gene seemed to be the missing piece in Darwin's jigsaw. It completed his picture of natural selection by showing that traits can't be blended away to insignificance, although this wasn't recognised immediately. Genetics also solved another problem of Darwin's theory: the source of the variation within a population. Darwin's starting point was that any population naturally contains a variety of individuals, providing the raw material for natural selection. A key source of this variation was now shown to be mutation--spontaneous changes in the structure of a gene, leading it to code for something new. Such changes had been observed by Morgan and others as they traced the position of the genes on chromosomes.
Morgan himself came to believe that harmful mutations would quickly be eliminated from a population, thus recognising the negative side of natural selection. However, more work was needed to demonstrate how selection acted on genes to create positive evolutionary change."
With hindsight, the union of Darwinism and Mendelism seems a marriage made in heaven"
According to Darwin, evolution is a slow process of gradual adaptation to the environment, in which most characteristics have, or once had, an adaptive function. So giraffes with slightly longer necks were able to reach leaves higher up, and thus gradually evolved longer necks through the process of natural selection. In contrast, many of the original geneticists saw evolution as something that happened in large jumps, or saltations, whereby new characteristics appear abruptly as the result of some internal rearrangement of an organism's hereditary constitution. For example, a plant could suddenly start producing flowers of a colour not seen in its parents. The change would not necessarily have any adaptive benefit.
The early geneticists were attracted to Mendel's laws precisely because they seemed to support these ideas. Morgan thought that "Nature makes new species outright" through a "sudden change of the germ". Bateson saw no value in the Darwinians' studies of continuous variation and resisted the claim that natural characteristics appear as a result of adaptive pressures on the species. By the same token, the saltation mechanism of change seemed to have no relevance to the process of natural selection.
These entrenched positions made it hard for anyone to suggest a way to reconcile the two approaches. But that changed in the 1920s, thanks to the new field of population genetics--the study of how certain genes within populations change over time. Strange but true: Life may have emerged not once, but many times on Earth
The biologists Ronald Fisher, J. B. S. Haldane and Sewall Wright used sophisticated mathematical models to show that natural selection is able to enhance the frequency of any gene coding for a beneficial character, and eliminate those that are maladaptive. This concept was developed in Fisher's 1930 book The Genetical Theory of Natural Selection and in Haldane's more popular The Causes of Evolution in 1932. That same year, Wright introduced the idea of an adaptive landscape, a map depicting all possible gene combinations and the resultant fitness of the organism.
Collectively, their work demonstrated that genes accounted for both the abrupt changes in characteristics sometimes seen in an organism's offspring, and the continuous variation that Darwin had documented for large populations. These biologists showed that genetic selection is a genuinely creative force driving the adaptation of a species to its local environment, with continual mutation ensuring that the fund of variability is maintained. However, their theoretical models involved complex statistics and were hard to understand.
The gene-centred perspective of evolution only reached the wider scientific community in 1937, when Theodosius Dobzhansky published his Genetics and the Origin of Species, translating the mathematical formulations into terms that were more accessible. Dobzhansky's work also expanded our understanding of how genetics enabled evolution, showing, for example, how new species could emerge when isolated populations changed to adapt to their local environment.
In 1942, the biologist Julian Huxley's broad survey Evolution: The Modern Synthesis gave the new perspective a name. By the 1950s, this formulation had become dominant, although one key aspect would continue to be debated for decades to come.
A higher purpose?
More or less since its conception, Darwinian evolution was seen as an idea hostile to the Christian vision of nature as the product of some higher purpose. In the US especially, creationist opposition to Darwinism took hold in the 1920s and has continued ever since.The founders of the modern synthesis wanted to present Darwinism as able to accommodate the belief that evolution has a built-in tendency to produce higher levels of organisation. Dobzhansky, for example, came from a Russian Orthodox background and wrote Mankind Evolving in 1962 to promote the idea that evolution had an ultimate purpose. Huxley also wrote prolifically to promote the idea of evolutionary progress. These authors presented ideas about the modern synthesis in a way that did not challenge traditional hopes and values too openly. This, however, did not stop some of them being proponents of eugenics (see "From genetics to eugenics").
The quasi-religious portrayal would change in later decades, most notably with the publication of Richard Dawkins' The Selfish Gene in 1976 and his emergence as a leading proponent of the argument that nature has no ultimate moral purpose. Subsequent debates over the evolution of social behaviour and the emergence of altruism have taken place against the backdrop of an increasing tension between Darwinian evolution and religion--exactly what the founders of the synthesis hoped to avoid. Despite these difficulties, the modern synthesis remains at the heart of our understanding of evolution today. It is itself evolving as advances in genetics, developmental biology and ecology broaden our understanding of the relationship between genes, organisms and the environment.
The gene-centred view of evolution that emerged from the ideas of Darwin and Mendel is being transformed by the growing recognition that the environment in which the organism develops does play a role in shaping its characteristics, and may even affect the way traits are passed on to future generations. Discoveries in the field of epigenetics are showing that chemical tags that attach to genes to switch them on and off might be as important for development as the hard-wired genetic code itself. The modern synthesis was an idea for the 20th century. In the 21st, the story of evolution is set to acquire a sophistication that Darwin could only have dreamed of.
Route to a new view evolution
5000 BC Humans begin to understand inheritance when they start to selectively breed more useful varieties of livestock and crops such as maize, wheat and rice
400 BC Ancient Greek philosophers contemplate mechanisms of human inheritance. Hippocrates believed that the material of heredity was tiny particles in the body which accumulate in a seminal fluid in the parents. These particles blend to create the traits of the offspring
1859 Charles Darwin publishes On the Origin of Species--his explanation of evolution by natural selection. It contains a wealth of evidence for how variable traits become more common in a population, but suggests no mechanism for their transmission
1866 Augustinian monk Gregor Mendel publishes meticulous studies of inheritance in pea plants, marking the birth of modern genetics. The findings go unnoticed for over three decades
1868 Darwin publishes The Variation of Animals and Plants under Domestication, outlining his hypothesis of pangenesis: that particles called gemmules transmit an organism's characteristics to its offspring
1900 Mendel's laws of inheritance are rediscovered by Dutch and German botanists
1905-6 The term "genetics" is coined by the biologist William Bateson, a key proponent of Mendel's work. Soon the concept of a gene is developed
1920s The new field of population biology begins to unite the ideas of Darwin and Mendel, establishing how evolution can work at the level of genes
1937 Theodosius Dobzhansky develops the modern synthesis, defining evolution in genetic terms as the "change in the frequency of an allele [gene type] within a gene pool"
1942 Ernst Mayr outlines how new species can evolve, for example when a geographical barrier results in a population becoming genetically incompatible with its original species
1944 DNA is proven to be the material of heredity, not protein as had been suspected
1951 Images of DNA are captured for the first time by Rosalind Franklin. Two years later, James Watson and Francis Crick determine the double-helix structure of DNA
1990 The Human Genome Project begins, completing its work 13 years later when the full sequence is revealed. Genomes of many other organisms follow
From genetics to eugenics
Some of the early pioneers of evolutionary theory were enthusiastic proponents of eugenics: the idea of enhancing the human population by eliminating "unfit" genes. Ronald Fisher, for example, devoted part of his 1930 book The Genetical Theory of Natural Selection to his hopes for improving the human race by this means. He even fathered eight children to further the cause. The dark reality of eugenics became clear in the early 20th century when several US states legislated to sterilise the "feeble-minded", and the Nazis took the idea to its ultimate, horrific extreme.
Mendel and the birth of genetics
Gregor Mendel had an unlikely background for someone known as the founder of modern genetics, not least because he carried out his work 50 years before genes were actually discovered. Born in 1822 on a farm in what is now the Czech Republic, he joined the Augustinian friary at Brno and here carried out his work on heredity. In the friary garden he bred thousands of pea plants, noting the presence of characteristics such as flower colour and wrinkliness of seeds. He found, for example, that when he crossed white-flowered plants with purple ones, the resulting plants weren't a light mauve, as might be expected if parental traits blended together, but appeared as either white or purple in fixed ratios.
These observations led him to devise the now famous laws of inheritance, published in 1866, which introduced the idea of dominant and recessive traits. This work went largely unnoticed until the turn of the century, when his ideas were incorporated into the new science of genetics.
The significance of Mendel's contribution is debated, however. His laws certainly helped to clarify how characteristics are transmitted from parent to offspring. But even Ronald Fisher, who used Mendel's ideas to create a genetic theory of evolution in the 1930s, suggested that Mendel's results were a bit too good to be true, perhaps "tidied up" by an overzealous assistant. And it is by no means clear that Mendel would have been a proponent of the theory that was based on his work. He expressed his ideas solely in terms of the transmission of characters from one generation to the next - there was no discussion of any mechanism.
Mendel eventually abandoned his studies when he became abbot at the age of 46. Little else is known about him as his correspondence and other personal papers were burned after his death.
https://www.newscientist.com/article/mg23130880-400-the-odd-couple-how-evolution-and-genetics-finally-got-together/