Darwin's contribution and natural selection

Darwin's contribution

Older belief held that all species were specially created as they are now and never change. Charles Darwin sailed round the world on HMS Beagle and studied many plants and animals. He concluded that living forms share similarities to varying degrees with each other and with forms that lived millions of years ago, many of which are now extinct. Different life forms lived in different periods. Alfred Wallace, working in the Malay Archipelago, reached the same idea. Darwin's two key concepts are branching descent (all forms come from common ancestors) and natural selection.

How natural selection works

1. 1
   Populations could grow very fast (exponentially) if all members reproduced fully, as a growing bacterial culture shows.
2. 2
   But resources are limited, and population sizes stay fairly stable except for seasonal changes.
3. 3
   So there is competition (struggle) for food, space and mates.
4. 4
   Members of a population vary; no two are exactly alike, and most variations are inherited.
5. 5★
   Individuals with heritable variations that help them use resources better survive and leave more progeny.
6. 6
   Over many generations, these variations become common, the population's features change, and new forms appear.

★ key step

Lamarck vs Darwin: the giraffe's long neck

Example: industrial melanism in England

1. 1
   Around 1850, before industrialisation, tree trunks were covered with almost white lichens. White-winged moths blended in; dark moths were seen and eaten by birds. White moths were more common.
2. 2
   After industrialisation (about 1920), smoke and soot darkened the trunks and the lichens died.
3. 3★
   Now dark-winged (melanised) moths were hidden, while white moths were picked off by predators.
4. 4
   So in polluted areas dark moths became more common. No moth changed colour; nature selected the variant that already existed.
5. 5
   Lichens do not grow in polluted areas, so they are used as indicators of industrial pollution.

★ key step

Evolution by human action (anthropogenic evolution)

• Heavy use of antibiotics and drugs has selected resistant microbes; heavy use of herbicides and pesticides has selected resistant weeds and insects.
• Resistance appears in months or years, not centuries, because microbes and insects have short life cycles.
• Example: a bacterial colony A varies in its ability to use a food. If the medium changes, only the variant B that can use the new food survives and soon outnumbers the rest. B is fitter in the new conditions.
• The same change in a fish or a fowl would take millions of years, as their life spans run into years.