Variation, natural selection and evidence for evolution – GCSE Combined Science Biology

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Look around your class. Nobody looks exactly the same. Over a very long time, differences like these can change a whole species. That slow change is called evolution.

This matters because it explains why living things are so well suited to where they live. It also explains how fossils and extinction fit into the story of life on Earth.

By the end you will be able to say why individuals differ and how natural selection works. You will describe how Darwin's idea was received, how fossils form, and why species die out.

Variation and mutation

Variation means differences between individuals. The causes are genes inherited from parents, the environment, or both. Eye colour is genetic. A scar is caused by the environment. Height depends on both genes and diet.

A mutation is a random change in the DNA of a gene. It can make a new allele, which is a different version of a gene. Most mutations change nothing. Some change a feature slightly. Very rarely one gives a feature that helps in a particular environment.

Think of copying a recipe by hand. Now and then you make a typo. Most typos do not matter.

There are two kinds of cause. Genetic variation comes from the different alleles inherited from the parents, and from the mixing of genes in sexual reproduction. Environmental variation comes from things such as diet, climate and accidents. Most features are controlled by many genes together, not by a single gene.

Most mutations have no effect on the phenotype. Some have a small effect. Very rarely, a mutation gives a new phenotype that is an advantage, and then natural selection can make it more common.

Natural selection

Evolution is a change in the inherited features of a population over many generations. Natural selection is how it happens.

Individuals vary. They make more offspring than can survive, so they compete for food, mates and space. Those best suited to the environment are more likely to survive and breed. They pass on their alleles.

So over many generations the helpful allele becomes more common. Individuals do not change on purpose. The population changes because of who survives and breeds.

Darwin and his theory

Charles Darwin proposed evolution by natural selection after observations on his travels and his own experiments. Many people did not accept it quickly.

Some thought it went against religious views that God made all living things. There was also not enough evidence at the time. Nobody knew how features were passed on.

How fossils form

Fossils are the remains of organisms from millions of years ago, found in rocks. Hard parts such as bones and shells do not decay easily. Minerals slowly replace them, and they turn to stone.

Some fossils form because decay could not happen. Decay needs certain conditions, and these are missing in ice, amber, tar pits and peat. Fossils can also be traces, such as footprints and burrows.

Think of a footprint pressed into wet cement. The shape stays long after the foot has gone. Older and younger fossils show how organisms have changed.

Gaps in the record, and extinction

The fossil record is incomplete. Early life was often soft-bodied and left few traces. Some fossils were destroyed by geological activity, and many have not been found.

Extinction is when no members of a species are left alive. Causes include new predators, new diseases or competitors, a changed environment, and hunting by humans. A single catastrophic event, such as a volcanic eruption or asteroid impact, can also cause it.

The dinosaurs are thought to have died out after an asteroid hit the Earth and changed the climate.

Fossils are not the only evidence. We can watch evolution happening in bacteria. A mutation can make a bacterium resistant to an antibiotic. When antibiotics are used, resistant bacteria survive and reproduce, so the resistant strain, such as MRSA, becomes common. This is natural selection.

Recap

Variation comes from genes and the environment. Mutations make new alleles.

Natural selection: variation, competition, survival and breeding, alleles passed on, allele becomes more common. Say that it takes many generations.

Fossils form from hard parts, from no decay, or as traces. The record is incomplete.

Extinction has causes such as predators, disease, environmental change and catastrophe.