The Complete Guide to Darwin, Mendel & the Story of Inheritance

1. The Problem Nobody Could Solve

Why the variety of life was a genuine scientific crisis.

Here is a puzzle that tortured the best naturalists of the eighteenth century. Why does a mole, which lives in total darkness, have eyes? Why do whales — which are unmistakably fish-shaped — have finger bones inside their flippers, and hip bones attached to nothing? If every creature was designed for its life, these look less like design and more like something left over. But left over from what?

The standard answer was that species were fixed, created separately and unchanging. It was not a lazy view — it fitted what anyone could see, since nobody had ever watched a species turn into another one. And it had a serious observational problem, which naturalists kept politely stepping around: fossils. The rocks contained creatures that no longer existed anywhere on Earth.

Geology made it worse. In the 1830s Charles Lyell argued that the Earth's features were produced by ordinary slow processes — erosion, deposition, uplift — acting over immense time. Not a few thousand years. Millions. And once you have millions of years, very slow change stops being unthinkable and starts being unavoidable.

Vestigial structure: A body part that has lost most or all of its original function — the mole's eyes, the whale's pelvis, the human appendix and tailbone. Under separate creation these are baffling. Under descent with modification they are exactly what you would predict: leftovers from an ancestor that used them.

The nerve that controls part of your larynx does not run the short distance from your brain to your throat. It goes down into your chest, loops under an artery, and comes back up — a detour of a few centimetres in you, and about four metres in a giraffe. It follows the route it took in a fish-shaped ancestor, where the path was direct. Evolution cannot redesign; it can only modify what is already there.

What just happened

• Species were assumed to be fixed and separately created
• Fossils showed creatures that no longer exist, and vestigial structures made no design sense
• Lyell's geology established that the Earth was vastly older than anyone had assumed

The theory of evolution did not arrive into an empty field. It arrived into a growing pile of observations that the existing explanation was quietly failing to account for.

In 1831 a twenty-two-year-old with no qualifications and a passion for beetles was offered an unpaid place on a naval survey ship. His father tried to stop him going.

2. Five Years Around the World

What Darwin actually noticed — and what he noticed years later.

Charles Darwin (1809–1882)

Failed medical student, reluctant clergyman-in-training, and the most careful observer of the century.

Darwin was invited aboard HMS Beagle essentially as an educated companion for the captain, who feared the loneliness of command. He was chronically seasick for five years. He came home with crates of specimens, a reputation as a geologist, and an illness that would trouble him for the rest of his life.

The Galápagos story is usually told wrong. Darwin did not stand on the islands and have a revelation about finches. He collected the birds carelessly, without labelling which island each came from, and did not think they were all finches at all. It was the ornithologist John Gould, back in London, who identified them as thirteen closely related species found nowhere else — at which point Darwin had to reconstruct the locations from other people's notes.

That was the moment the pattern appeared. Species on adjacent islands were similar but distinct, and each resembled the mainland species most closely. If each had been created separately for its island, why would they look like each other's relatives rather than like their environments? A single ancestral population, isolated and slowly diverging, explained it exactly.

The mechanism came from an unexpected source. In 1838 Darwin read Thomas Malthus on population: living things produce far more offspring than can possibly survive, so competition for resources is inevitable and constant. Darwin saw what followed. If individuals vary, and if some variations improve the odds of surviving to breed, then those variations will become more common in the next generation. Nobody is choosing. The environment does the sorting.

Natural selection: Four requirements, and the conclusion follows automatically: (1) individuals vary; (2) some of that variation is inherited; (3) more offspring are produced than can survive; (4) some variations improve the chance of surviving and reproducing. Then the frequency of those variations must rise. It is less a discovery about biology than a piece of arithmetic that biology cannot escape.

He then did nothing about it for twenty years. He wrote a summary in 1844, sealed it with instructions for his wife to publish it if he died, and went back to studying barnacles for eight years. He knew what the theory implied about humans, his wife was devout, and the last person to publish something similar had been savaged. He waited.

What just happened

• Darwin spent five years on the Beagle collecting specimens, mostly as a geologist
• Gould's identification of the Galápagos birds revealed the pattern of divergence
• Reading Malthus in 1838 supplied the mechanism; Darwin then delayed for two decades

Natural selection is not a claim about progress or purpose. It is what must happen when heritable variation meets limited resources.

In June 1858 a letter arrived from a naturalist in the Malay Archipelago, containing Darwin's own theory, worked out independently, and asking for his opinion of it.

3. The Letter That Forced His Hand

Wallace, the joint paper, and the most consequential book of the century.

Alfred Russel Wallace was collecting specimens in what is now Indonesia, feverish with malaria, when he too thought of Malthus and reached the same conclusion. He wrote it up and posted it to the most respected naturalist he knew of, asking whether it was worth pursuing. Darwin read it and wrote to a friend that he had never seen a more striking coincidence — all his originality would be smashed.

Darwin's friends arranged a joint presentation to the Linnean Society in July 1858: Wallace's essay alongside extracts of Darwin's earlier writing establishing priority. Wallace, who was thousands of miles away and had not been consulted, later said he was entirely satisfied with the arrangement and remained a generous supporter of Darwin for the rest of his life. The presentation attracted almost no attention. The society's president reported at the end of the year that nothing remarkable had occurred.

It jolted Darwin into finishing. On the Origin of Species appeared in November 1859 — written for the general reader, dense with evidence, and carefully argued. The entire first printing sold on the first day. He had spent twenty years assembling objections to his own theory and answering them, which is why the book was so hard to dismiss.

Commonly believed: 'Survival of the fittest' means the strongest survive.

Actually: The phrase was coined by Herbert Spencer, and Darwin only adopted it in later editions — with reluctance. Fitness in biology means nothing but reproductive success in a particular environment. Sometimes that means being small, or camouflaged, or cooperative, or good at surviving cold. A creature that lives long and leaves no offspring has a fitness of zero, however strong it is.

Four misconceptions worth killing now

• 'Evolution is just a theory' — in science a theory is a well-tested explanatory framework, not a guess. Gravity is a theory too
• 'Individuals evolve' — they do not. Populations evolve, across generations. No animal has ever adapted during its own lifetime
• 'Evolution has a goal' — there is no direction and no ladder. Humans are not the top of anything; every living species is equally the current end of its own line
• 'Organisms evolve because they need to' — need does nothing. Variation arises regardless, and the environment sorts it. A species that needs a mutation and does not get one goes extinct

The reaction was more complicated than the caricature. Some clergy accepted it immediately; some scientists rejected it for decades on genuinely scientific grounds. The famous 1860 Oxford exchange, in which Bishop Wilberforce asked Thomas Huxley whether he was descended from apes on his grandfather's or his grandmother's side, is remembered as a rout — but the accounts were written years later by people who wanted it to have been one.

What just happened

• Wallace independently derived natural selection and sent it to Darwin in 1858
• A joint presentation was made, and the Origin appeared in 1859 and sold out immediately
• Reaction was mixed on both scientific and religious grounds, and the famous debate is largely legend

The theory survived because Darwin had spent twenty years anticipating objections — which is why the book persuaded working naturalists rather than merely provoking them.

But there was one objection Darwin could not answer, and it nearly sank the whole theory. He had no idea how inheritance worked.

4. The Monk Who Counted Peas

Mendel solves Darwin's fatal problem, and nobody notices for thirty-five years.

The objection was called blending inheritance, and it was devastating. Everyone assumed offspring were a blend of their parents — mix red and white paint, get pink. But if that is true, any advantageous new variation would be halved in the next generation, quartered in the one after, and diluted into nothing long before selection could act on it. A critic pointed this out and Darwin had no reply.

Gregor Mendel (1822–1884)

An Augustinian friar in Brno who failed his teaching exams twice and founded genetics anyway.

Mendel had trained in physics and mathematics, which is precisely why he succeeded where botanists had failed: he treated inheritance as a counting problem. Over eight years he grew and analysed around 28,000 pea plants in the monastery garden, tracking seven characteristics that came in two clear forms — tall or short, round or wrinkled, yellow or green.

The results were not blends. Cross a tall plant with a short one and you get tall plants — not medium ones. Breed those together and the shortness reappears in the next generation, in almost exactly one plant in four. Nothing had been diluted. The short factor had been carried, hidden and intact, through a generation that showed no sign of it.

Dominant and recessive: Each parent contributes one version — an allele — of each inherited factor. A dominant allele shows in the appearance whenever it is present; a recessive one shows only when both copies are recessive. That is why a characteristic can skip generations entirely and come back undiminished.

This is the exact answer Darwin needed. Inheritance is particulate: it comes in discrete units that are shuffled, not mixed. Nothing is diluted, so a rare advantageous variant can persist across generations until selection has time to work on it. Mendel published in 1866, in an obscure journal. He sent copies to leading botanists. It was cited a handful of times, never understood, and forgotten.

Mendel's paper sat unread for thirty-four years. In 1900 three separate botanists rediscovered the same rules independently and, searching the literature, all found that a Moravian friar had got there first and published it before any of them were doing serious work. He had been dead for sixteen years, having been promoted to abbot and largely given up research for administration.

There is a lingering statistical oddity: Mendel's published ratios are closer to the theoretical values than random chance comfortably allows. The most likely explanations are unconscious selection when classifying ambiguous plants, or stopping counts once the expected ratio appeared. The rules themselves have been confirmed thousands of times over, which is the test that matters.

What just happened

• Blending inheritance would have diluted any new variation away, undermining natural selection
• Mendel's pea experiments showed inheritance is particulate, with dominant and recessive alleles
• His 1866 paper was ignored until three botanists rediscovered the rules in 1900

Mendel supplied the missing half of Darwin's theory. Variation is generated and shuffled without being diluted, which is exactly what natural selection requires to work.

So inheritance came in units. But units made of what? Answering that took a photograph, taken in a basement laboratory in London.

5. Photograph 51

The image that gave away the structure of life — and the credit that went elsewhere.

By 1950 it was clear that DNA carried heredity, and completely unclear how. The molecule appeared far too simple to store the instructions for building an organism: just four bases, repeated endlessly. Many senior scientists thought proteins, with twenty amino acids, must be the real carrier. The question was structural — what shape is it, and how does that shape let it copy itself?

Rosalind Franklin (1920–1958)

The finest X-ray crystallographer of her generation, working in a laboratory that would not let her eat in the senior common room.

Franklin came to King's College London in 1951 having done outstanding work on the structure of coal in Paris. Her X-ray diffraction technique was meticulous — she controlled the humidity of the DNA fibres to obtain two distinct forms, which nobody else had managed. Her working relationship with her colleague Maurice Wilkins was poisonous almost from the start, partly because of a misunderstanding about which of them was leading the project.

May 1952 — The basement laboratory, King's College London

Franklin and her student Raymond Gosling expose a fibre of DNA to an X-ray beam for around one hundred hours. The plate that emerges — catalogued as Photograph 51 — shows a black X of smudges. To anyone who can read diffraction patterns, that X is not a decoration. It is the unmistakable signature of a helix, and the spacing of the marks gives its dimensions.

In January 1953 Wilkins showed the photograph to James Watson, who was working in Cambridge with Francis Crick. Franklin had not given permission and did not know. A report of her unpublished measurements also reached Crick through an administrative committee. Watson later wrote that the instant he saw the image, his mouth fell open and his pulse began to race. Within weeks he and Crick had built the double helix model.

The structure explained everything at once. Two strands wound around each other, with the four bases pairing specifically — A with T, C with G — so each strand carries the complete information needed to rebuild the other. Split them, and each half is a template for a new partner. Watson and Crick's paper contains one of the great understatements in science: a note that the specific pairing they had postulated immediately suggests a possible copying mechanism for the genetic material.

Franklin left King's shortly afterwards, did excellent work on virus structure, and died of ovarian cancer in 1958 at thirty-seven. The Nobel Prize went to Watson, Crick and Wilkins in 1962; it is not awarded posthumously, so she could not have shared it. What is fairly debated is whether she would have been offered it had she lived, and whether her contribution was properly acknowledged at the time. It was not: the 1953 paper mentions her data only in a brief note about having been stimulated by unpublished work.

What just happened

• DNA was known to carry heredity but its structure was unknown
• Franklin's Photograph 51 revealed the helix and its dimensions
• The image reached Watson without her knowledge, and the double helix followed within weeks

The structure explained replication instantly — which is why it is one of the few moments where a shape alone answers a biological question.

Which leaves the last question: how does a four-letter chemical alphabet turn into a living thing?

6. Life as Information

Four letters, one code, and every organism that has ever lived.

The letters are the four bases: A, T, C and G. Read in groups of three, each triplet specifies one amino acid, and a chain of amino acids folded into shape is a protein. Proteins do nearly everything — they form structures, catalyse reactions, carry oxygen, fight infection. A gene is a stretch of the sequence that specifies a protein or a functional RNA. That is the whole system.

The genetic code: The mapping from base triplets to amino acids. It is very nearly identical in bacteria, oak trees, fungi and humans — which is powerful evidence of common ancestry, since there is no chemical reason the same triplet had to mean the same amino acid in every lineage. It works out that way because we all inherited it from the same ancestor.

This universality has a practical consequence: human insulin can be manufactured by bacteria. The human gene is inserted into a bacterium, which reads it perfectly well because the code is the same, and produces exactly the protein a diabetic patient needs. Common ancestry is not a philosophical claim; it is the reason a medicine works.

Mutations — copying errors, radiation damage, chemical changes — supply the variation Darwin needed. Most do nothing, because much of the sequence is not coding, or because several triplets specify the same amino acid. Some are harmful. A few, rarely, are useful. That is the raw material, and it is generated at random with respect to what the organism might need.

Sequencing turned evolution from an inference into a measurement. We can now compare genomes directly and count the differences, and the family tree drawn from DNA matches the one drawn from fossils and anatomy a century earlier — an agreement between two entirely independent lines of evidence that nobody had to arrange. Humans and chimpanzees share around 98–99% of their coding sequence, depending on how you count.

And the story is not finished. We can now edit the code directly. Gene therapies have cured previously untreatable inherited diseases; CRISPR makes precise editing cheap enough for an ordinary laboratory. That raises questions Darwin, Mendel and Franklin never had to face: what should be corrected, who decides, and what happens when the edits are inheritable — passed to children who could not consent.

What just happened

• Base triplets specify amino acids; chains of amino acids fold into proteins
• The genetic code is nearly universal, which is strong evidence of common ancestry
• Mutation supplies random variation; sequencing has confirmed the evolutionary tree independently

This is where three separate stories — Darwin's mechanism, Mendel's units and Franklin's structure — turn out to have been the same story the whole time.

Now you know the story

The strangest thing about this story is how long the pieces sat around separately. Darwin had a mechanism and no theory of inheritance. Mendel had inheritance and no idea it explained evolution. Franklin had the structure and did not live to see what it became. Science is far more often this — a jigsaw assembled by people who never met — than a lone genius shouting eureka.

• Explain natural selection without any of the four common misconceptions
• Describe the evidence that convinced biologists, from fossils to DNA
• Explain Mendel's rules of inheritance and why they solved Darwin's worst problem
• Say what Photo 51 showed, who took it, and how the credit was handled

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The assessed treatment is Pearson Edexcel International GCSE Biology Topic 3 (Reproduction and inheritance), which requires genetic diagrams, ratios and specific terminology. This is the story behind it.