Evolution and inheritance

Natural selection

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

为什么Individuals 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

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

为什么Each 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

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

为什么A 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

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

为什么Selection 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

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

为什么Both 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

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

为什么Darwin 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

提示It explains where new kinds of living thing come from.

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

Wallace

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

为什么Alfred 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

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

为什么Extinction 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

提示Evolution needs many generations; the weather did not wait.

为什么Adaptation 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

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

为什么A 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

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

为什么In 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.