GCSE AQA Biology B1b 7.1-7.4 Evolution

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B1b 7.1 The Origins of Life on Earth

The fossil record provides insight into how much or how little organisms have changed.

Fossils are remain of organisms found in rocks, and can be formed in multiple ways:

  • Harder parts of animal are replaced by other minerals over time (most common form)
  • Organism not decaying due its conditions, e.g. a mammal preserved in ice due to lack of oxygen and very low temperature (very rare form)

Some fossils are not actually the animals- they are droppings or footprints.

However, fossil records are limited as full fossils are rarely found. Also, it is difficult to know exactly how life began due to no direct evidence.

Horses are an example of an animal with a full fossil record; they evolved from small, 4-toed, swamp-dwelling creatures to hooved, tall beings that run on open land.

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B1b 7.2 Theories of Evolution

The 'Theory of Evolution' states that all living organisms have evolved from simple lifeforms.

Lamarck, a French biologist, had a theory that all animals evolved from primative worms, and that diffrentiation occured due to inheritance of aquried characteristics.                                         His basic theory was that if your parents developed a certain characteristic, their offpsring would then inherit it. Applied to giraffes this meant that if a giraffe spent its life stretching for leaves, its offspring would be born with a slightly longer neck.

However, this theory was not accepted due to a lack of evidence. The generally accepted theory is that of Charles Darwin.

Darwin's theory in general is that a process of 'natural selection' has meant that only those organisms with characteristics most suited to their environent will survive and therefore produce offspring- passing on their successful genetic traits.

This proccess is also known as 'survival of the fittest.'

The fossil record has helped to prove this theory.

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B1b 7.3 Natural Selection

Individual organisms of any species show lots of variation due to different genes that are inherited by offspring (next generation) from their parents.

Only those with the genes best suited to an environment can survive to produce more offspring with their succesful characteristics.

New forms of genes are the result of 'genetic mutations' of existing genes. These occur naturally through mistakes when DNA is copied during cell division. However, some mutations have no effect on the characteristics of an organism and some are harmful.

The main points of natural selection:

  • mutation
  • variation
  • adaption
  • survival
  • genes passed onto offspring
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B1 7.4 Extinction

Extinction is the permenant loss of all members of a species (plants and animals).

Extinction can be caused by environmental changes. For example, when the Earth entered 'ice ages', many organisms would not have survived- however new organisms with better adaptions would have.

Other causes of extinction:

  • New predators that can wipe out unsuspecting prey (e.g. dodos became extinct due to human predators)
  • New diseases- more dangerous on islands where populations are closer together, and can push species to the point of extinction.
  • Successful competition of one species and can drive another to extinction (e.g. rabbits introduced into Australia eat so much and breed so fast causing other animals to die out because they cannot compete.)
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