Evolution and inheritance

Natural selection

HintThe environment does the choosing, the way a breeder picks which animals to mate.

The whyIndividuals in a species vary. Those whose features suit their surroundings are more likely to survive long enough to breed, so those features grow more common over many generations.

It is recessive

HintIt was hidden in both parents because each also carried the brown version, which shows up whenever it is present.

The whyEach parent carries two copies of the gene, called alleles. Both must be Bb: brown shows because B is dominant, yet each can pass on b. A child who inherits b from both is bb, so blue eyes appear even though neither parent has them. Real eye colour is shaped by several genes, so this is a simplified model, but the logic is exactly the one used at GCSE.

The environment

HintA plant grown in the dark ends up different from its twin on a windowsill.

The whyA scar, a suntan, the language you speak and much of your mass come from your surroundings rather than your genes. Most features are shaped by both together.

variation

HintIf every individual were the same, there would be nothing to choose between.

The whySelection cannot invent a new feature; it can only favour one that is already present. Sexual reproduction keeps supplying those differences by reshuffling genes.

They were better camouflaged, so more survived to breed

HintAgainst a blackened tree trunk, which colour is a hungry bird less likely to spot?

The whyBoth pale and dark forms already existed. Soot darkened the tree bark, birds ate more of the pale moths, and the dark form rose from rare to common within decades.

Beagle

HintA Royal Navy survey ship whose five-year voyage took him to the Galapagos Islands.

The whyDarwin spent nearly five years aboard as a naturalist. The finches and tortoises, each slightly different from island to island, later helped him explain how species change.

On the Origin of Species

HintIt explains where new kinds of living thing come from.

The whyDarwin had held back for twenty years, gathering evidence, and published only when a younger naturalist reached the same conclusion independently.

Wallace

HintA collector working in the islands of south-east Asia when he wrote to Darwin.

The whyAlfred Russel Wallace reached the idea independently while collecting specimens in the Malay Archipelago. Their papers were read together at the Linnean Society in July 1858, and Darwin published On the Origin of Species the following year.

Extinct

HintThe very last one has died, and there is no way back.

The whyExtinction is permanent. It follows when the surroundings change faster than a species can adapt, or when a new predator, disease or competitor arrives.

It cannot adapt quickly enough to survive

HintEvolution needs many generations; the weather did not wait.

The whyAdaptation depends on useful variations already being present and on enough generations passing for them to spread. A change over decades leaves no time for either.

Biodiversity

HintCount how many kinds of living thing a place holds, not just how many individuals.

The whyA native wood with fifty species is far more biodiverse than a field carrying one crop, even though the field may hold many more individual plants.

Other species can take over the lost role

HintIf only one kind of plant grows there and it dies, nothing remains.

The whyIn a varied ecosystem several species do similar jobs, so losing one rarely collapses the food web. A single-crop field has no spare, which is why crop disease can be devastating.