Darwin's Theory of Evolution by Natural Selection

Evolution is the gradual change in the characteristics of a species (a population of organisms) over a very long time — usually many thousands or millions of years.

The big question scientists asked was: how does a species change? In 1859 Charles Darwin published his answer in a book called On the Origin of Species. His explanation is the theory of evolution by natural selection.

The word "theory" here does not mean "a guess". In science a theory is a well-tested explanation that is supported by a huge amount of evidence (fossils, the way living things are related, and modern DNA studies). Natural selection is the mechanism — the engine — that drives evolution.

The key idea is simple but powerful: nature "selects" which individuals survive and breed. Individuals that are better suited to their environment leave more offspring, so their useful features are passed on. Over many generations this slowly changes the whole population.

Exam tip. If a question asks who proposed natural selection, the answer is Charles Darwin. If it asks what natural selection produces, the answer is evolution (a change in the species over time).

Natural selection can only work if individuals are different from one another. These differences are called variation.

Look at any population — a herd of antelope, a field of plants, a colony of bacteria — and no two individuals are exactly alike. Some run faster, some are taller, some are a slightly different colour.

Where does this variation come from? The ultimate source is mutation — a random change in a gene (allele). Mutations happen all the time, by chance, when DNA is copied. Most mutations make no difference, some are harmful, but occasionally a mutation produces a new, useful characteristic.

Crucially, this variation is inherited because it is caused by a change in the genes — so it can be passed on to offspring. (Differences caused only by the environment, like getting bigger muscles from exercise, are not passed on.)

No variation → no natural selection. Without differences between individuals, there would be nothing for nature to "select" between.

Exam tip. The specification specifically wants you to know that the variation natural selection acts on comes from mutation. Writing "they were just born different" misses the marking point — name mutation as the source.

Living things produce far more offspring than can possibly survive. A single pair of rabbits could have dozens of young; a plant releases thousands of seeds. But food, water, space, light and mates are limited.

This means individuals must compete — there is a struggle for survival. They compete for:

• food and water,
• space / territory and shelter,
• mates (to reproduce),
• and they must avoid being eaten by predators.

Now bring in the variation. Some individuals, by chance, have advantageous characteristics that help them in this struggle — perhaps they can run faster, find food more easily, or hide from predators better.

These individuals are more likely to survive. Darwin called this "survival of the fittest". Here, "fittest" does not mean strongest or healthiest — it means best suited / best adapted to the environment. The fittest individuals survive long enough to reproduce.

Exam tip. Two common keywords examiners look for: a struggle for survival / competition because there are limited resources, and survival of the fittest meaning the best-adapted individuals survive and reproduce. Always link survival to reproduction — surviving alone is not enough; the individual must breed to pass its features on.

When the best-adapted individuals reproduce, they pass on their advantageous alleles (the versions of the genes that gave them the helpful characteristic) to their offspring.

So the next generation contains a higher proportion of individuals with the advantageous characteristic. The individuals without it are more likely to die before breeding, so their alleles become rarer.

Repeat this, generation after generation:

1. The advantageous allele becomes more and more common in the population.
2. The disadvantageous version becomes rarer.
3. After many generations, almost the whole population has the advantageous characteristic.

The result is that the population has changed — it has become better suited (adapted) to its environment. This change in the population over time is evolution.

We can sum up the whole theory as a chain of five steps:

variation (from mutation) → competition / struggle to survive →
individuals with advantageous features survive & reproduce (survival of the fittest) →
they pass on their advantageous alleles →
over many generations the population changes / adapts (evolution)

Here is the same idea shown as a diagram for camouflaged beetles being eaten by a bird:

Exam tip. The high-mark idea is the link between reproduction, passing on advantageous alleles, and many generations. A response that stops at "the fast ones survive" without saying they pass on their alleles over many generations will not reach the top marks.

Let us walk through a clear example: a population of gazelles is hunted by cheetahs. Over time the gazelles become faster. Here is the natural-selection story step by step.

1. Variation (from mutation). In the gazelle population there is variation in running speed — some are faster than others. This variation originally arose from mutation.
2. Competition / struggle to survive. Cheetahs hunt the gazelles, so there is a struggle to survive — only some gazelles escape.
3. Survival of the fittest. The faster gazelles are more likely to escape the cheetah and survive. They are the "fittest" (best adapted) for this environment.
4. Pass on advantageous alleles. The surviving faster gazelles reproduce and pass on the alleles for fast running to their offspring.
5. Change over many generations. The slow gazelles are more likely to be caught before they breed, so the allele for fast running becomes more common. After many generations, the whole population of gazelles is faster — it has evolved.

Notice how every step matters: remove the variation and there is nothing to select; remove the competition (no predator) and there is no pressure; remove the inheritance and the advantage dies with the individual.

Exam tip. When a question gives you a specific scenario (faster prey, longer necks, resistance, etc.), do not just retell the story in everyday words — map your answer onto the five steps and use the keywords variation, mutation, survive, reproduce, pass on alleles, many generations.