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How did the enormous variety of life on Earth come about, and how do living things come to be so well suited to where they live? The answer is evolution by natural selection — one of the most important ideas in all of biology, first set out in detail by Charles Darwin. This lesson, part of Topic B5 of your OCR Gateway Combined Science course, explains the theory step by step, shows it at work in a present-day example you can actually observe — antibiotic-resistant bacteria such as MRSA — and then examines the evidence for evolution (the fossil record), the causes of extinction, and how biologists classify and name living things. It builds directly on variation and mutation from the previous lesson.
By the end of this lesson you should be able to state the theory of evolution by natural selection, set out the sequence of steps, explain how antibiotic resistance arises, describe the fossil and antibiotic-resistance evidence for evolution, explain extinction, and describe classification and binomial naming.
This lesson develops AO1 (understanding the theory of evolution and classification) and AO3 (analysing fossil and antibiotic-resistance data as evidence for natural selection).
Evolution is the gradual change in the inherited characteristics of a species over many generations. The mechanism that drives it is natural selection, proposed by Charles Darwin (and independently by Alfred Russel Wallace) and published in Darwin's book On the Origin of Species in 1859.
Over a very long time, natural selection can change a species so much, or split a population into groups that change in different ways, that new species form. The modern theory also recognises that the variation natural selection acts on ultimately comes from mutations in DNA — something Darwin himself did not know about, because genes had not yet been discovered.
Exam Tip: Be careful with the word "evolution". It means a change in the inherited characteristics of a population over many generations, not a change within a single individual's lifetime. A frequent misconception is that an individual organism "evolves"; it does not — populations evolve over generations.
Natural selection follows a clear, logical sequence. Learn these steps in order, because exam questions very often ask you to put them together into a full explanation.
flowchart TD
A["Variation:<br/>individuals in a species show variation<br/>(caused by differences in alleles)"] --> B["Competition / struggle to survive:<br/>more offspring are produced than can survive;<br/>they compete for food, mates and space"]
B --> C["Survival of the fittest:<br/>individuals with advantageous characteristics<br/>are more likely to survive and reproduce"]
C --> D["Inheritance:<br/>survivors pass on the alleles for the<br/>advantageous characteristics to their offspring"]
D --> E["Over many generations:<br/>the advantageous alleles become more common,<br/>so the species changes (evolves)"]
In words:
A useful example is a population of rabbits in which a few, by chance, have slightly better hearing. If predators are common, the better-hearing rabbits are more likely to detect danger, survive and breed, passing on the alleles for good hearing. Over many generations, good hearing becomes the norm in the population.
Exam Tip: The phrase "survival of the fittest" does not mean the strongest. "Fittest" means best suited to the environment — which might mean best camouflaged, fastest, most resistant to a disease, or best at finding food. Use "better suited to their environment" or "have a survival advantage" for full marks.
One of the best pieces of evidence that natural selection really happens is that we can watch it occur in bacteria. Bacteria reproduce extremely quickly, so evolution that would take thousands of years in a large animal can happen in a matter of days. The rise of antibiotic-resistant bacteria — such as MRSA (methicillin-resistant Staphylococcus aureus) — is natural selection in action.
Antibiotics are medicines that kill bacteria, and they have saved countless lives. But bacteria are now becoming resistant to them, which is a serious problem for medicine. Here is how resistance arises and spreads, step by step — exactly the natural-selection sequence applied to bacteria:
flowchart TD
A["A random mutation occurs in one bacterium,<br/>making it resistant to the antibiotic"] --> B["The antibiotic is used;<br/>it kills the non-resistant bacteria"]
B --> C["The resistant bacterium survives<br/>(it has a survival advantage)"]
C --> D["The survivor reproduces rapidly,<br/>passing on the resistance allele"]
D --> E["Over time the whole population<br/>becomes resistant to the antibiotic"]
This is exactly why doctors are careful about prescribing antibiotics. To slow down the development of resistance, antibiotics should not be over-used or used for non-bacterial (for example, viral) infections, and patients should always complete the full course so that all the bacteria are killed and none of the more resistant ones are left to survive and reproduce. New strains like MRSA are difficult to treat because they have become resistant to several antibiotics.
Exam Tip: Antibiotic resistance is the classic exam example, and the marks come from the natural-selection sequence: a random mutation makes one bacterium resistant; the antibiotic kills the non-resistant ones; the resistant one survives and reproduces; resistance spreads through the population. Do not invent figures for how fast this happens — describe the mechanism.
A theory as important as evolution needs strong evidence, and there is plenty. The two pieces you must know are the fossil record and the antibiotic resistance you have just met.
A fossil is the preserved remains, or traces, of an organism that lived thousands or millions of years ago, found in rocks. By comparing fossils from different rock layers — older fossils lie in deeper layers — scientists can see how organisms have changed over time, which is powerful evidence for evolution. Fossils form in several ways:
| How the fossil forms | What happens |
|---|---|
| Hard parts replaced by minerals | Hard parts such as bones, teeth or shells are slowly replaced by minerals as they decay, forming a rock-like copy of the original |
| Casts and impressions | An organism leaves a mark — for example a footprint, a burrow or the impression of a leaf — which is then filled in or hardened into rock |
| Preservation where decay is prevented | Where conditions stop microbes decaying the organism — too cold, too little oxygen or too acidic — remains can be preserved, for example whole mammoths frozen in ice or insects trapped in amber |
A complication is that the fossil record is incomplete — there are gaps in it. This is because many early organisms were soft-bodied and decayed before they could fossilise, and because fossils only form in particular conditions and many have been destroyed by movements of the rocks or have simply not yet been found. The gaps are one reason it took time for the fossil evidence for evolution to become convincing.
The second key piece of evidence is the antibiotic resistance described above. Because bacteria reproduce so quickly, we can actually observe evolution by natural selection happening over days and weeks: a random mutation makes a bacterium resistant, the antibiotic kills the non-resistant bacteria, and the resistant survivors reproduce so resistance spreads. This is direct, present-day evidence that natural selection really does change populations.
Exam Tip: When asked for evidence for evolution, the two reliable answers are the fossil record (showing change over time) and antibiotic-resistant bacteria (natural selection observed today). For fossils, be ready to explain the gaps: many early organisms were soft-bodied and decayed before fossilising.
Extinction is the permanent loss of all the members of a species — when the last individual of a species dies, the species is extinct and gone forever. Extinction is a natural part of the history of life (most species that have ever lived are now extinct), but it can be caused by several factors:
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