Natural Selection and Evolution: KS3 Biology Explained

Evolution is the change in inherited characteristics of a population over many generations. Natural selection is the mechanism that drives it: individuals with characteristics better suited to their environment survive longer, reproduce more, and pass those characteristics to offspring, gradually shifting what is typical for the species.

What did Charles Darwin discover?

Charles Darwin (1809–1882) is the biologist most associated with the theory of evolution by natural selection, which he published in On the Origin of Species (1859). Darwin did not work alone — Alfred Russel Wallace developed very similar ideas independently at the same time, and the two presented their findings jointly to the Linnean Society in 1858.

Darwin's key observations were:

Organisms produce more offspring than can survive (overproduction).

Individuals within a species vary — no two are identical (variation).

Some variation is inherited — offspring resemble their parents.

Resources are limited — there is competition for food, space, and mates.

From these observations, Darwin drew the logical conclusion: individuals with advantageous variants survive the competition and reproduce, passing those variants on. Over time, advantageous variants become more common in the population. This is natural selection.

How does natural selection work step by step?

Step | What happens | Example
1. Variation | Individuals in a population differ | Some moths are pale, some are dark
2. Selection pressure | Environmental challenge | Birds eat moths resting on tree bark
3. Survival of the fittest | Best-suited individuals survive | Dark moths on soot-covered bark are hidden
4. Reproduction | Survivors pass on their genes | Dark moths produce more offspring
5. Population shifts | Next generation has more survivors | Population becomes mostly dark moths

"Survival of the fittest" does not mean the strongest — it means best suited (most fit) to the current environment. A pale moth is not "inferior" in general; in a clean woodland it is perfectly camouflaged. Fitness is always relative to the environment.

What is the peppered moth example?

The peppered moth (Biston betularia) is the classic KS3 example of natural selection in action, observed in real time in England.

Before the Industrial Revolution, most peppered moths were pale, mottled white — camouflaged against pale lichen-covered tree bark. Dark (melanic) moths existed but were rare because birds could spot them easily.

During the Industrial Revolution, pollution killed lichens and coated tree bark with black soot. Suddenly, pale moths were conspicuous and dark moths were hidden. Bird predation wiped out pale moths disproportionately. Within decades, dark moths dominated industrial areas of northern England.

After the Clean Air Act (1956) reduced pollution and lichens returned, pale moths recovered. Today both forms co-exist, with local frequencies matching local bark colours — a direct demonstration of natural selection operating.

What is the difference between evolution and natural selection?

Natural selection is the process — the mechanism by which some variants survive and reproduce more than others.

Evolution is the result — the change in the inherited characteristics of a population over generations, caused (mainly) by natural selection.

Think of natural selection as the engine and evolution as the journey. Other mechanisms (such as genetic drift or mutation) can also drive evolution, but natural selection is the primary one and the only one required at KS3.

What evidence supports evolution?

Type of evidence | What it shows
Fossil record | Sequential changes in organisms over time; shows species appear, change, and go extinct
Comparative anatomy | Homologous structures (e.g. same bone arrangement in human arm, whale flipper, bat wing) suggest common ancestry
DNA evidence | Species with similar DNA sequences share a more recent common ancestor
Observable change | Antibiotic resistance in bacteria; pesticide resistance in insects — evolution watched live

Antibiotic resistance is particularly important in medicine. Bacteria reproduce rapidly (some divide every 20 minutes), so natural selection can shift a bacterial population to antibiotic resistance within days. This is why doctors stress completing a full course of antibiotics — stopping early leaves partially-resistant survivors to reproduce.

What is extinction and how does it relate to evolution?

Extinction occurs when the last individual of a species dies. It happens when a species cannot adapt quickly enough to a change in its environment — a new predator, disease, climate shift, or habitat loss.

Mass extinctions (such as the one that ended the dinosaurs 66 million years ago) clear ecological space, after which surviving lineages evolve rapidly to fill new niches. Most species that have ever existed are now extinct — extinction is the norm, not the exception.

Frequently asked questions

What is natural selection in simple terms for KS3?

Natural selection is nature's way of "choosing" which individuals survive to reproduce. Organisms that happen to have characteristics better suited to their environment live longer and have more offspring. Their offspring inherit those useful characteristics, so the population gradually changes over many generations.

Who came up with the theory of evolution?

Charles Darwin is most famous for the theory of evolution by natural selection, but Alfred Russel Wallace developed similar ideas at the same time. Both presented their findings in 1858. Darwin published his full account in On the Origin of Species in 1859. The theory has since been confirmed and extended by genetics, DNA evidence, and the fossil record.

Does evolution happen quickly or slowly?

It depends on the organism. Bacteria can evolve antibiotic resistance in days because they reproduce so quickly. Large, slow-reproducing organisms like elephants evolve much more slowly. Major changes in body plan (like the evolution of limbs from fins) take millions of generations. The key is that natural selection can only act on the variation that already exists in a population.

What does "survival of the fittest" mean?

"Survival of the fittest" means survival of those organisms that are best adapted (most fit) to their current environment — not necessarily the biggest or strongest. Fitness is defined by reproductive success in a specific environment. A pale moth is "fitter" in a clean woodland, while a dark moth is "fitter" in an industrial area.

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