How does evolution work?

Video presented by Chris Lyal

How does evolution work?
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Giraffe killed by lions

The constant threat of predation makes it difficult for many organisms to survive long enough to reproduce.

Evolution is the process by which successive generations of organisms change. It is the reason we have many different species on the planet.  

Evolution is driven by the fact all organisms have to struggle to survive long enough to reproduce in a world filled with predators, disease, and competition for resources. 

To survive, an organism may have many offspring that are equally successful in reproduction. Others must avoid being eaten or killed by disease. It must compete for food and perhaps for a mate. Having mated, it must then ensure its offspring survive. Some organisms live long enough to reproduce, and some of these produce fewer offspring, or even die before they can reproduce. Those that reproduce and create successful offspring are more ‘fit’ for survival than those that don’t. The characteristics that helped this ‘fitness’ will be more common in the next generation than those of its less ‘fit’ relatives. This process is called natural selection.

DNA double helix.

DNA found in the chromosomes of all living things occasionally becomes randomly altered, creating a mutation.

Natural selection is a critical aspect of the evolutionary process, but it is not the whole story. Evolution depends on there being a diversity of living things for natural selection to act on. The force that creates this diversity is called mutation.

Mutations are random alterations in our genes, the result of genes failing to copy themselves properly or exposure to radiation or other chemicals (called mutagens). 

Mutations can be minor, causing almost undetectable changes in the organism, such as a slightly longer neck, or they can be obvious, such as an extra limb. Most mutations hinder organisms, killing them before they get a chance to reproduce and pass on their genes. 

Snake showing vestigial claw.

Mutations that led to reduced limbs in snakes are among the rare mutations that created an advantage for the organism. The legs have been reduced so much that all that can be seen outside of the snake's skin is a single tiny claw.

However, occasionally a mutation improves an organism’s chances of survival or reproduction. In these rare situations, if the organism survives long enough to breed the mutation is passed down to the next generation. 

Because beneficial mutations provide organisms with a survival advantage, they tend to spread quickly through the population. When the mutation shifts from being rare to common the population can be said to have ‘evolved’.


  • Presented by: Chris Lyal
  • Date: 10 May 2005
  • Duration: 5:38

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