The Evolution Story

The Evolution Story 🧬

Where do the millions of species on Earth come from? The answer took centuries of argument to pin down — and it is one of Biology's richest sources of extended-response marks. This module runs the whole story: Darwin, Mendel, fossils, extinction and the superbugs evolving in hospitals right now. (All of 4.6.3 is **Biology-only**.)

Darwin's big idea 🐦

**Charles Darwin** proposed **evolution by natural selection**: 1. Individuals in a species show **variation**. 2. There is **competition** to survive, and organisms with characteristics best suited to the environment are **more likely to survive** and breed. 3. These **advantageous characteristics** are passed on to their **offspring**. 4. Over **many generations**, the beneficial characteristics become more common — the species evolves.

How selection works

In natural selection, why do advantageous characteristics become more common over time?

• Organisms with them are more likely to survive, breed, and pass them on
• Organisms choose to develop them because they are useful
• Organisms grow them during life because they need them
• Purely by random chance, with no advantage involved

A slow acceptance 📜

Darwin published *On the Origin of Species* in 1859, but his theory was only **gradually** accepted. Why? • It **challenged the religious belief** that God made all the animals and plants. • There was **not enough evidence** yet to convince many scientists. • The **mechanism of inheritance** — genes — was not understood until about **50 years later**, so Darwin could not explain *how* characteristics were passed on.

Plan the answer

An interactive activity.

Mendel and the peas 🌱

Around the same era, the monk **Gregor Mendel** carried out breeding experiments on **pea plants**. He concluded that characteristics are passed on in separate **'units of inheritance'** (what we now call **genes**). His work was **not recognised until after his death** — the science that could make sense of it (**chromosomes, genes and DNA**) had not been discovered yet.

Ahead of his time

Why was the importance of Mendel's "units of inheritance" not recognised during his lifetime?

• Chromosomes, genes and DNA had not yet been discovered
• His experiments were later shown to be wrong
• He kept all of his results a secret
• Pea plants are nothing like humans

How new species form 🦋

**Speciation** is how brand-new species arise. If two populations of the same species become **separated** (say by a mountain range or an ocean), natural selection can push them in different directions. Given enough time, they can become **so different** that they can **no longer interbreed to produce fertile offspring** — at that point they are two separate species.

When is it a new species?

At exactly what point are two diverging populations considered **separate species**?

• When they can no longer interbreed to produce fertile offspring
• As soon as they look quite different from each other
• The moment they are physically separated
• When they live in different habitats

Reading the rocks 🪨

**Fossils** are the remains of organisms from millions of years ago, found in rocks. They form when: • parts of an organism **do not decay** because one or more conditions for decay are absent • hard parts are gradually **replaced by minerals** as they decay • organisms leave **casts, impressions or traces** (footprints, burrows, rootlet traces) The **fossil record is incomplete**: many early organisms were **soft-bodied**, so they left few traces, and many fossils have since been destroyed.

Gaps in the record

Why is the fossil record of early life forms incomplete?

• Many early organisms were soft-bodied and left few traces
• Because the Earth is too young for old fossils
• Because fossils cannot survive in rock at all
• Because humans have collected them all

Evolution we can watch 🦠

Some evidence for evolution is happening **right now**: **antibiotic-resistant bacteria** like **MRSA**. This is natural selection in fast-forward. A **random mutation** makes a few bacteria resistant. When an antibiotic is used, the non-resistant bacteria die, but the **resistant** ones **survive and reproduce** — so the resistant strain spreads. Key exam point: describe it as **natural selection**, never as bacteria "trying" or "choosing" to adapt.

Order the resistance

An interactive activity.

Slowing the superbugs

Pick the TWO actions that help reduce the rate at which antibiotic resistance develops.

• Doctors avoid over-prescribing antibiotics
• Patients always complete the full prescribed course
• Taking antibiotics for ordinary viral colds
• Stopping the course as soon as you feel better

Extinction, and the exam 🎓

Not every species survives. **Extinction** happens when a species can no longer adapt fast enough — causes include a **new disease**, **new predators or competitors**, an **environmental change**, a **single catastrophic event** (like an asteroid), or a new hazard. Grade-9 habits for this whole topic: • Darwin/Mendel questions: structure a **timeline** — idea → resistance → gradual acceptance as evidence built up. • Antibiotic resistance: argue it as **natural selection**, not "bacteria adapt to survive". • Speciation: state the criterion explicitly — **can no longer produce fertile offspring**.