Natural selection leads to evolution

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Key points

  • Evolution explains how better adapted organisms have an advantage and are more likely to have offspring with this same adaptation.
  • Over many years this leads to the formation of new species.
  • Evolutionary trees show how different species have evolved over time from common ancestors.

Video - How lemurs have evolved

Can you answer these questions based on the video?

1. What is evolution driven by?

2. What lengthened the middle finger of the lemur thousands of years ago?

  1. Natural selection
  2. A mutation

Evolution by natural selection

Evolution is the process by which small changes in organisms occur over long periods of time and new species are formed. A species is a group of organisms that can interbreed to have offspring. Natural selection drives evolution.

The steps in evolution

  1. In every population there is , some of which is inherited.
  2. Individual organisms with the best are most likely to survive and reproduce. This is natural selection.
  3. means these adaptations are likely to be passed to offspring.
  4. It also means that less well adapted organisms are less likely to pass on their adaptations.
  5. Over many generations these small differences add up to the formation of new species by evolution.

Variation can be caused by small changes in DNA called mutations. Most of these have no effect, some are advantageous and some are disadvantageous.

Charles Darwin

Charles Darwin was an English scientist who first proposed the theory of evolution by natural selection in a book called On the Origin of Species in 1859. He studied fossils and variation in plants and animals on a five year voyage around the world on a sailing ship called HMS Beagle.

During this trip, the ship visited the Galapagos Islands off the coast of Ecuador in the Pacific Ocean. There he saw some small birds called finches on the different islands which looked similar, but had differences in their beak shapes. Darwin saw that these changes allowed the birds to feed on different foods. He suggested that they originally all belonged to one population, but had slowly changed over time to suit the different habitats on the islands.

Darwin chose not to publish his theory. He correctly thought it would cause a lot of controversy. Twenty years later, Darwin was approached by another scientist called Alfred Wallace who had come up with the same theory. They published their scientific ideas jointly, but Darwin is remembered for devising the theory first.

At the time many people were upset by his ideas. Today some religious views about the creation of the world and the organisms in it still conflict with the scientific agreement about evolution.

A drawing by Darwin showing finches from four different islands

Evidence of evolution

Evidence of evolution is seen in the . Small changes that Darwin suggested can be seen in these fossils.

There are gaps in the fossil record because:

  • Not all fossils have been discovered yet.
  • Some fossils have been melted in magma during the rock cycle.
  • Not all parts of organisms become fossils.

can evolve quickly because they reproduce at a fast rate. Mutations of bacteria produce new strains. Some bacteria might become resistant to certain antibiotics, such as penicillin, and cannot be destroyed by the antibiotic. The evolution of the bacteria is an example of natural selection.

Evolutionary trees

Evolutionary trees are diagrams that show how different species have evolved from a common ancestor - an organism that lived thousands or millions of years ago.

Animals with a backbone are called . This evolutionary tree shows how different groups of vertebrates have evolved over time.

The oldest part of the diagram is the bottom where there was a common ancestor for all vertebrates. We are not sure what animal this is. Jawless fish first separated from everything else, followed by cartilaginous fish (like sharks), before bony fish and so on. The coloured short lines show where key features have evolved. You can see hair only on mammals because it is after the separation from amphibians and turtles.

An evolutionary tree showing vertabrates

Use the evolutionary tree to find the answer to these questions.

1. Which animal on the evolutionary tree is most closely related to snakes? 2. Why don’t fish have four limbs?

  1. Lizard
  2. This characteristic only evolved after the other groups of animals had separated from them.

Test your knowledge

Quiz

Test questions

Write a paragraph to answer the following question. Tap 'Show answer' to see seven points you could have included.

Describe how sharks have evolved the ability to detect blood from a long way away.

  • In every population, there is variation, some of which is inherited.
  • This means some sharks will have been able to detect blood a little better than others.
  • Individual organisms with the best adaptations are most likely to survive and reproduce. This is natural selection.
  • Inheritance means these adaptations are likely to be passed to offspring.
  • Therefore their offspring are likely to have been able to detect blood further away so could find more food.
  • Less well-adapted organisms are less likely to pass on their adaptations. So the sharks that couldn’t detect blood so far away probably got less food and were less likely to have offspring with this ability.
  • Over many adaptations, these small differences add up to the formation of new species by evolution.