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The Evolution Story

How we worked out where species come from: Darwin, Mendel, the fossil record and antibiotic-resistant superbugs, and how to argue it all like a natural selection.

⏱️ 15 min 🎯 15 activities
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What you'll cover

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**. Five ideas do all the work, and an exam answer that names them in order picks up the marks:

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

Two ideas ahead of their time

Darwin and Mendel were near-contemporaries. Both were right, and both waited a long time to be believed, because the science that could prove them did not exist yet.

Plan the answer

Extended-response practice. In about 40 words, explain why Darwin's theory of evolution was only gradually accepted. Structure your answer around:

  • the religious belief it challenged
  • the lack of evidence at the time
  • the fact that genes/inheritance were not yet understood

Mendel, in your own words

The monk Gregor _____ bred _____ plants and worked out that characteristics are passed on in separate units of inheritance, which we now call _____. His work was ignored in his lifetime, because the _____ that carry them had not yet been discovered.

Mendel pea genes chromosomes Darwin bean proteins ribosomes

How a new species forms

Put the stages of **speciation** into order.

  • Two populations of one species become separated, say by a mountain range or an ocean
  • Each population shows genetic variation
  • Conditions differ, so different characteristics are advantageous in each place
  • In each population the best-suited organisms survive and pass on their genes
  • Over many generations the two populations can no longer interbreed to produce fertile offspring

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

  • Parts that did not decay
  • Hard parts replaced by minerals
  • Casts, impressions and traces
  • Why the record is incomplete
  • One or more of the conditions needed for decay was absent
  • They slowly turned to rock as the rest of the organism decayed
  • Footprints, burrows and rootlet traces left in soft ground
  • Many early organisms were soft-bodied, so they left few remains

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. The trap that costs marks: describe it as **natural selection**. Never as bacteria "trying", "choosing" or "learning" to adapt.

Order the resistance

Put the steps by which a population of bacteria becomes antibiotic-resistant into order.

  • A random mutation makes one bacterium resistant to the antibiotic
  • The antibiotic is used and kills the non-resistant bacteria
  • The resistant bacterium survives
  • The resistant bacterium reproduces
  • The resistant population increases and spreads

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

Why species die out

  • A new disease
  • New predators or competitors
  • Environmental change
  • A single catastrophic event
  • An illness sweeps through a population with no resistance to it
  • A species arrives that hunts them, or takes their food and space
  • Conditions shift over time and the species cannot adapt fast enough
  • An asteroid collision or huge eruption destroys habitats all at once

Mark the answer

A student wrote this about MRSA. Two phrases describe evolution wrongly. Tap **both** of them.

  • When the antibiotic is used,
  • the bacteria try to adapt so that they can survive
  • . A random mutation means
  • a few bacteria are already resistant
  • , and the antibiotic kills the others, so
  • the survivors reproduce and pass the resistance on
  • . Over many generations
  • the whole population learns to resist the drug

In the exam

Whole story covered. Grade-9 habits for 4.6.3: • Darwin and Mendel questions: structure a **timeline**. Idea → resistance → gradual acceptance as the evidence built up. • Antibiotic resistance: argue it as **natural selection**. Never "bacteria adapt to survive", and never that they "learn". • Speciation: state the criterion explicitly. **Can no longer interbreed to produce fertile offspring**. • Extinction: name a **specific** cause rather than saying a species "could not cope".