AQA GCSE science B1B

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Darwin’s theory of evolution explains how species of living things have changed over geological time. The theory is supported by evidence from fossils, and by the rapid changes that can be seen to occur in microorganisms such as antibiotic-resistant bacteria. Many species have become extinct in the past and the extinction of species continues to happen.

Charles Darwin

Charles Darwin was an English naturalist. He studied variation in plants and animals during a five-year voyage around the world in the 19th century. He explained his ideas about evolution in a book called On the Origin of Species, which was published in 1859.

Darwin’s ideas caused a lot of controversy, and this continues to this day, because the ideas can be seen as conflicting with religious views about the creation of the world and creatures in it.

Darwin’s finches

Darwin studied the wildlife on the Galápagos Islands - a group of islands on the equator almost 1,000 kilometres west of Ecuador. He noticed that the finches - songbirds - on the different islands there were fundamentally similar to each other, but showed wide variations in their size, beaks and claws from island to island. For example, their beaks were different depending on the local food source. Darwin concluded that, because the islands are so distant from the mainland, the finches that had arrived there in the past had changed over time.


Darwin’s drawings of the different heads and beaks he found among the finches on the Galapagos Islands

Darwin studied hundreds more animal and plant species. After nearly 30 years of research, in 1858 he proposed his theory of evolution by natural selection.

Darwin's theory of evolution

The basic idea behind the theory of evolution is that all the different species have evolved from simple life forms. These simple life forms first developed more than three billion years ago - the Earth is about 4.5 billion years old. The timeline below shows some of the key events in the evolution of life forms on Earth, from the first bacteria to the first modern humans.

Natural selection

The theory of evolution states that evolution happens by natural selection. Here are the key points:

  • Individuals in a species show a wide range of variation.
  • This variation is because of differences in genes.
  • Individuals with characteristics most suited to the environment are more likely to survive and reproduce.
  • The genes that allowed the individuals to be successful are passed to the offspring in the next generation.

Individuals that are poorly adapted to their environment are less likely to survive and reproduce. This means that their genes are less likely to be passed to the next generation. Given enough time, a species will gradually evolve.

Evidence for evolution - fossils

Fossils of ammonites - sea creatures that became extinct about 65 million years ago

Most of the evidence for evolution comes from the fossil record. Fossils show how much, or how little, organisms have changed over time. One of the problems…

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