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Evolution is the gradual change in species over time. It is a very slow process which takes millions of years.

Organisms need to keep changing because their environment is always changing, if they do not change to fit prevailing conditions, they may become extinct.

Theories of evolution/the origins of organisms:

  • Charles Darwin's theory
  • Jean-Baptiste Lamarck
  • Creationism- Christianity- God created the Heavens and the Earth then filled it with organisms.

1. Charles Darwin's theory: Evolution by Natural Selection

This is the most widely accepted theory in Science and it briefly says:

  1. Organisms within a species have variations (genetic variations and some due to random mutations)
  2. As they compete for resources, some variations are more favourable than others
  3. Organisms with favourable variation survive and reproduce (are naturally selected for) (those who can't compete die and don't pass on their genes)
  4. Favourable variations (genes) are passed on.
  5. With time sufficient changes occur due to the selection of only the favourable variations (genes) and a new species which is different from the original arises.

Evidence for Evolution

Fossils- earliest fossils show that life began around 4 Billion years ago.

Fossils are:

  • Parts of an organism that did not decay- could have been burried so oxygen couldn't reach them for microbes to survive, or temperature was too low for microbes e.g. the frozen baby mammoth found in Siberia.
  • Prints on rock or mud that has been turned to rock
  • Hard parts of an organism

Fossils show how organisms or their parts changed over time e.g. the pentadactyl limb (5 digits at the end of each limb).

The incomplete fossil record:

Some organisms are missing from the fossil record so it is incomplete (some may say it's evidence that the Evolution theory is wrong). This is expected as:

  • Soft bodied organisms may not have been fossilised
  • Conditions for fossilisation were not available
  • Geological activity may have destroyed some fossils

2. Lamarck's theory

Jean Baptiste Lamarck proposed that Giraffes developed long necks by constantly stretching them to reach for food on tall trees.

These long necks were then passed to their offspring.

In short, he thought that acquired characteristics like scars, language etc could be passed to offspring. This is not so as we now know that for charactristics to be inherited, they have to be in the genes.

Lamarck's Theory is no longer regarded as credible.

3. Creationism

Some people see religious Beliefs as in competition or war with scientific Theories: Evolution V Christianity, Hinduism, Muslims etc. No!

This is because of the following key ways in which the 2 work:

  • Religion is based on belief. You do not have to question or seek evidence; JUST BELIEVE!
  • Science works on evidence. Any concept such as a belief, value and feeling which cannot be scrutinised for scientific evidence is not possible to explain through science.

So, this does not mean competition or war, they are just 2 different dimensions of human existence.



Is the permanent loss of an organism- The organism no longer exists.

Mass extinction- Is the large-scale extinction of a species e.g. dinosaurs. Mass extinction may be due to asteroids, floods and volcanic activity. This is mainly possible for endemic species.

Endemic species are organisms mainly found in one geographical area.

Causes of extinction

  • Sudden environmental change- to rapid to allow organisms to adapt
  • New predators e.g. introduction of a new species by mistake
  • New diseases
  • New competitorse.g. introduction of a new species by mistake


Speciation- is the origin of a new species through evolution.

Examples of speciation

  • Geographical isolation- a natural feature, e.g. a volcano may build up and split a rainforest population in 2. The original species already has variations. The conditions in the new habitats may be very different such that they favour different genes, so the 2 populations may quickly become so different that they can no longer breed successfully. Speciation has occured.
  • Behavioural isolation- Species may develop different signals to attract mates
  • Habitat isolation- species may live in different habitats such that they fail to encounter each other
  • Temporal (time) isolation- The species may breed during different times of the year



1. Why was Darwin's Theory controversial when he started bit?

2. Why did it take a very long time to accept it?

3. Find out about other theories that explain the origin of life.



1. Make sure you can explain Darwin's Theory of Evolution step-by-step.

2. Always include the idea of Natural selection.