Excerpts from Darwin's 'On the Origin of Species' describing 'natural selection' and 'survival of the fittest' as terms which denote that some animals survive to breed and some don't. Animals with a slight advantage have a better chance of survival. Footage shows a gazelle that is fast and strong enough to evade a cheetah.
Darwin suggested that natural selection is a constant process; a lot of favourable differences add up over time to produce a new species.
Natural selection and the survival of the fittest could be introduced as an extension topic that builds on and consolidates understanding of adaptations. Students could choose related species that have adapted to different environments and compare their adaptations. From this they may be able to step back in time to consider common ancestors that had not yet developed these specific adaptations. The class could then discuss how these changes came about.