AQA GCSE Biology (8461)

4.6.3 The development of understanding of genetics and evolution

AQA 4.6.3 is a history-of-science topic. You need to explain WHY Darwin's theory of evolution by natural selection was only accepted slowly, what Mendel actually did with his peas and why his work was ignored for 35 years, how the later discovery of chromosomes and the 1953 DNA double helix supplied the mechanism of inheritance Darwin's theory had been missing, and how Carl Woese used RNA in the 1990s to redraw the tree of life into three domains. This page walks you through the timeline, the people, and the exact mark-scheme phrases AQA examiners reward.

From the 8461 specification

Understanding of genetics developed slowly because of religious objections, the slow accumulation of fossil evidence, and the absence of a mechanism of inheritance. Gregor Mendel carried out breeding experiments on plants in the mid-nineteenth century and proposed that inheritance was controlled by separately inherited 'units'. The importance of his work was not realised until after his death. In the late nineteenth and early twentieth centuries, scientists observed chromosomes and showed that genes were located on them. In the mid-twentieth century the structure of DNA was determined and the mechanism of gene function worked out, providing strong support for the theory of evolution. Evolutionary trees use current classification data and fossil data to show how scientists believe organisms are related.

Why this matters

Science does not arrive fully formed. Two of the biggest ideas in biology — evolution by natural selection, and the gene as the unit of inheritance — were proposed within seven years of each other in the 1850s and 1860s. They were both right. And they were both rejected or ignored for decades. Charles Darwin published On the Origin of Species in 1859. Alfred Russel Wallace had reached the same conclusion independently, and the two men presented joint papers to the Linnaean Society in 1858. Their theory could explain how species change, but it had a gaping hole — no one knew how characteristics were passed from parents to offspring. Gregor Mendel, a monk in what is now the Czech Republic, was at that very moment carrying out the breeding experiments on pea plants that would have plugged that hole. His 1866 paper proposed discrete inherited 'factors' (now called alleles) and laid out the laws of segregation and independent assortment. Almost no one read it. Mendel died in 1884 still unknown. His work was rediscovered around 1900 by three biologists working independently. Over the next fifty years, chromosomes were spotted under microscopes, genes were located on them, and finally, in 1953, Watson, Crick, Franklin and Wilkins worked out the double-helix structure of DNA. Suddenly biologists had a physical molecule that could store information, copy itself, and mutate — exactly what Darwin's theory needed. The story did not stop there. In the 1970s and 1990s Carl Woese sequenced ribosomal RNA from microorganisms that had been lumped together as 'bacteria' and discovered they were not one group but two — Bacteria and Archaea — that were as different from each other as either was from the rest of life. He proposed a three-domain system that displaced the old kingdoms-only view of classification.

How to learn this topic

Build on what you already know

GCSE 4.6.1: DNA, genes, alleles, chromosomes.
GCSE 4.6.2: variation, mutation, natural selection.
GCSE 4.6.4: classification (kingdoms, three-domain system).
KS3: scientific theories develop and change as new evidence emerges.

Place Darwin (1859) and Mendel (1866) side by side on a timeline.
Explain why Darwin was slow to be accepted: religion, fossil gaps, no inheritance mechanism.
Explain Mendel's pea experiments and the idea of discrete factors.
Explain why Mendel was ignored: no chromosomes, no DNA, before its time.
Bring in chromosomes (early 1900s) and the 1953 DNA structure as the missing mechanism.
Add Carl Woese's three-domain system (1990) using RNA.
Connect to evolutionary trees built from fossil and DNA evidence.

Key terms

natural selection: The process by which individuals with characteristics best suited to their environment are more likely to survive and reproduce, passing on advantageous alleles. Proposed independently by Darwin and Wallace in 1858. (Always credit BOTH Darwin and Wallace as co-discoverers in 4.6.3-style history questions.)
On the Origin of Species: Charles Darwin's 1859 book setting out the theory of evolution by natural selection and arguing that all life shares common ancestors. (Get the date (1859) and author (Darwin) correct — both are markable details.)
special creation: The religious belief that God created each species in its current form. Widely held in nineteenth-century Britain and one of the main reasons Darwin's theory was resisted. (Examiners accept 'contradicted religious beliefs' as a marking point for why Darwin was opposed.)
Gregor Mendel: Augustinian monk (1822-1884) who carried out breeding experiments on pea plants in the mid-1800s. Proposed that inheritance involves discrete 'factors' passed from parents to offspring. His work was ignored until rediscovered around 1900. (Two markable facts: 'carried out breeding experiments' and 'discrete units / factors passed from parents to offspring'.)
factors (Mendel's): Mendel's name for the discrete units of inheritance. We now call these genes; the alternative versions of a factor are alleles. Each individual carries two factors per characteristic, one from each parent. ('Units' and 'factors' are both accepted by AQA. Linking to 'now called genes' picks up the mark.)
chromosome: A long molecule of DNA, packaged with proteins, found in the nucleus of a cell. Chromosomes were identified under the microscope in the late 1800s and shown to behave in the same way as Mendel's factors. (Identification of chromosomes is the bridge between Mendel and DNA in the historical timeline.)
DNA double helix: The twisted-ladder structure of DNA worked out in 1953 by Watson and Crick, using X-ray crystallography data from Rosalind Franklin and Maurice Wilkins. Explained how genes can be stored, copied and mutated. (Naming all four scientists is safest; AQA mark schemes credit Franklin and Wilkins as well as Watson and Crick.)
Alfred Russel Wallace: British naturalist (1823-1913) who independently arrived at the theory of natural selection while working in South-East Asia and presented joint papers with Darwin in 1858. (AQA history questions often credit Wallace alongside Darwin — do not omit him.)
Carl Woese: American microbiologist (1928-2012) who used ribosomal RNA sequencing to show that prokaryotes split into two domains (Bacteria and Archaea), proposing the three-domain classification system in 1990. (Phrase to use: 'compared RNA sequences' and 'three-domain system'.)
three-domain system: Classification system proposed by Carl Woese (1990) dividing all life into three domains: Archaea, Bacteria and Eukaryota. Based on RNA sequence differences rather than physical features alone. (List all three domain names (Archaea, Bacteria, Eukaryota) for full marks.)
fossil record: The collected fossils of dead organisms preserved in rocks, providing evidence of how life has changed over geological time. Incomplete because soft-bodied organisms decay before they can fossilise. (AQA marking points: gradual change over time, soft tissues decay, gaps in record.)
evolutionary tree: A branching diagram showing how scientists believe different species are related, built using fossil data and modern DNA / RNA evidence. (Modern trees use BOTH fossil and molecular (DNA) evidence — say so explicitly.)

Notes

Two big ideas, both ignored at first

In the middle of the nineteenth century, two ideas arrived that between them would later explain almost the whole of biology. Both were rejected or ignored for decades.

1859 — Charles Darwin publishes On the Origin of Species, proposing that species change over time by natural selection. Alfred Russel Wallace had reached the same conclusion independently, and the two had presented joint papers in 1858.
1866 — Gregor Mendel, an Augustinian monk, publishes the results of his pea-plant breeding experiments, proposing that inheritance involves discrete 'factors' passed from parents to offspring.

These two ideas needed each other. Mendel's factors were exactly the mechanism Darwin's theory was missing. Yet neither author lived to see them put together.

Why was Darwin's theory only accepted slowly?

AQA asks this directly. There are four standard reasons.

It contradicted religious teaching. Most people in Victorian Britain believed in special creation — that God created all species in their current form. Common ancestry and gradual change challenged that belief head-on.
There was insufficient evidence. The fossil record at the time was patchy and full of gaps. Critics could point to missing intermediates and say the theory was not supported.
There was no mechanism for inheritance. Darwin could describe variation, selection and reproduction, but he could not explain HOW favourable traits were passed on. Without genes, chromosomes or DNA, his theory looked incomplete.
It was a new and radical idea. Many scientists were simply unwilling to accept a theory that overturned the framework they had been trained in. New scientific ideas take time to be accepted.

Mendel — the right answer, ignored

Gregor Mendel worked in a monastery garden in Brno. Between 1856 and 1863 he carried out thousands of controlled breeding experiments on pea plants. He picked seven simple, clear-cut characteristics — plant height, flower colour, pea shape, pea colour, pod shape, pod colour and flower position — and crossed pure-breeding lines.

His key findings:

The F1 generation always showed one of the two parent characteristics — never a blend.
When F1 plants self-fertilised, the F2 generation showed a 3 : 1 ratio of the two characteristics.
He concluded that each plant carries two factors for each characteristic, one inherited from each parent. These factors are discrete (they don't blend) and one can be dominant over the other.

Mendel published his results in 1866. His paper was read by almost no one. He died in 1884 still unrecognised.

### Why was Mendel ignored?

No knowledge of genes, chromosomes or DNA existed to support his ideas. He had no physical thing to point to — his 'factors' were a mathematical abstraction.
He was an obscure monk publishing in a regional journal, not a major scientist at a famous university.
The dominant view of inheritance at the time was 'blending' (offspring are a mix of their parents), which Mendel's data clearly contradicted but which most biologists still believed.

Mendel's work was rediscovered around 1900 when three biologists — de Vries, Correns and Tschermak — independently reached the same conclusions and went looking for prior literature. We now call these inherited factors genes, and the alternative versions of a gene alleles.

Filling in the mechanism — chromosomes and DNA

Over the next fifty years, three discoveries closed the gap between Darwin and Mendel.

Early 1900s — chromosomes identified. Improved microscopes let scientists see chromosomes in dividing cells. They noticed that chromosomes behave in exactly the way Mendel's factors should — they come in pairs, separate during gamete formation, and recombine at fertilisation.
By the 1920s scientists had shown that genes are sections of chromosomes — Mendel's 'factors' had a physical home.
1953 — the structure of DNA. James Watson and Francis Crick, building on the X-ray crystallography images produced by Rosalind Franklin and Maurice Wilkins at King's College London, worked out that DNA is a double helix of two complementary strands. This explained how genetic information could be stored (in the sequence of bases), copied (the strands separate and each templates a new partner) and mutated (a wrong base inserted during copying).

DNA was the missing mechanism. It gave Darwin's theory the molecular foundation it had lacked, and it explained why Mendel's factors behaved the way they did. Evolution, genetics and chemistry merged into a single picture.

Wallace — the forgotten co-discoverer

Alfred Russel Wallace independently arrived at the theory of natural selection while working in the Malay Archipelago. He sent his manuscript to Darwin in 1858. Friends of Darwin arranged for both men's work to be presented jointly at the Linnaean Society that summer. Wallace also did important early work on warning colouration in animals and on the geographical distribution of species (he proposed the 'Wallace Line' dividing Asian and Australian wildlife).

Carl Woese — RNA and the three domains

Classification used to be based entirely on physical features. In the 1970s and 1990s, microbiologist Carl Woese at the University of Illinois sequenced ribosomal RNA from a wide range of microorganisms. He found that organisms which had been grouped together as 'bacteria' actually split into two completely separate lineages — as different from each other as either was from animals, plants and fungi.

Woese proposed a three-domain system:

Archaea — primitive bacteria-like organisms, many living in extreme conditions (hot springs, salt lakes, deep ocean vents).
Bacteria — true bacteria, the ones we are familiar with as pathogens and gut microbes.
Eukaryota — all organisms whose cells contain a nucleus — protists, fungi, plants and animals.

Woese's system challenged the established Linnaean classification because it used molecular evidence (RNA sequences) rather than physical features alone. It is now widely accepted.

Evolutionary trees today

Modern evolutionary trees are built using both fossil data (where extinct species fit in time) and DNA / RNA sequence data (how similar living species are to each other). Species with similar DNA sequences are assumed to share a more recent common ancestor; species with very different DNA diverged long ago. This combination of evidence is now the gold standard for classifying organisms.

Exam tips

Memorise the four standard reasons Darwin was slow to be accepted: religion, fossil gaps, no inheritance mechanism, radical new idea. AQA wants ALL of them, not just one.
For Mendel questions, hit three marking points: he carried out breeding experiments; he proposed discrete factors / units passed from parents to offspring; no knowledge of genes / chromosomes / DNA existed to support him.
For DNA-structure questions, name all four scientists — Watson, Crick, Franklin, Wilkins — and give the date (1953).
Always credit Wallace alongside Darwin in natural-selection history questions.
For Carl Woese / classification questions, say: compared RNA sequences; proposed three-domain system; the three domains are Archaea, Bacteria and Eukaryota.
Fossil-evidence questions need four marks: fossils are remains of dead organisms in rocks; show gradual change over time; soft-bodied organisms decay before fossilising; gaps in the record make evolutionary relationships hard to establish.
Get your dates roughly right: 1859 Darwin, 1866 Mendel, ~1900 Mendel rediscovered, 1953 DNA structure, 1990 Woese.

Mark-scheme phrasing

Common misconceptions

Worked example

Question:

Answer:

Frequently asked questions

Did Darwin and Mendel know about each other's work?

No. Darwin's On the Origin of Species came out in 1859. Mendel's pea-plant paper came out in 1866 in an obscure regional journal in what is now the Czech Republic. There is no evidence Darwin ever read it, and Mendel died in 1884 without his work being noticed. Their ideas were only combined in the early twentieth century after Mendel was rediscovered around 1900. This is one of the great near-misses of science — they were the two halves of the same answer and never met.

Why was Rosalind Franklin not given a Nobel Prize for the structure of DNA?

Watson, Crick and Wilkins shared the Nobel Prize in 1962. Rosalind Franklin had died in 1958, aged 37, and Nobel Prizes are not awarded posthumously. Franklin's X-ray crystallography images, especially the famous 'Photograph 51', were crucial to working out the double helix. Modern accounts give her much more credit than the original 1953 papers did, and AQA mark schemes credit her alongside Watson, Crick and Wilkins.

How can DNA evidence change how we classify organisms?

Traditional classification looked at physical features — bones, leaves, body plans. Two organisms might look similar because they share an ancestor or because they evolved similar features independently (convergent evolution). DNA sequences cut through this: two organisms with very similar DNA almost certainly share a recent common ancestor, even if they look quite different. When Carl Woese compared RNA sequences from prokaryotes, he found that what we called 'bacteria' were actually two completely separate lineages, leading to the three-domain system.

If Mendel had explained Darwin's mechanism, why did no one combine their ideas at the time?

Three reasons. Mendel was unknown — he was a monk publishing in a regional journal, not a famous biologist at a major university. The dominant theory of inheritance at the time was 'blending' (offspring are a mix of parents), and most biologists were not looking for an alternative. And without microscopes good enough to see chromosomes in dividing cells, Mendel's 'factors' had no physical existence — they looked like mathematical abstractions. Once chromosomes were seen behaving exactly like Mendel's factors, around 1900, his work was rediscovered very quickly.

Is the three-domain system the final word on classification?

Probably not. Classification is still being revised as more DNA and RNA data come in. Some biologists now propose two domains (Archaea and Bacteria are grouped together) or even more refined trees that show eukaryotes nested within Archaea. But for GCSE, the three-domain system (Archaea, Bacteria, Eukaryota) proposed by Carl Woese in 1990 is the accepted answer, and the key point is that classification has moved from physical features to molecular evidence.