Evolution

THEORY OF EVOLUTION: All of today's species have evolved from simple life forms that first started to develop over three billion years ago.

1) Darwin knew that organisms in a species show wide variation in their characteristics (phenotypic variation). He also knew that organisms have to compete for limited resources in an ecosystem.

2) Darwin concluded that the organisms with the most suitable characteristics for the environment would be more successful competitors and would be more likely to survive. The idea is called "survival of the fittest"

3) The successful organisms that survive are more likely to reproduce and pass on the genes for the characteristics that made them successful to their offspring.

4) The organisms that are less well adapted would be less likely to survive and reproduce, so they are less likely to pass on thier genes to the next generation.

5) Over time, beneficial characteristics become more common in the population and the species changes - it evolves.

1) Darwin's theory wasn't perfect. Because the relevant scientific knowledge wasn't available at the time, he couldn't give a good explanation for why new characteristics appeared or exactly how individual organisms passed on beneficial adaptations to their offspring.

2) The discovery of genetics supported Darwin's idea - provided an explanation of how organims born with beneficial characteristics can pass them on and showed that it is genetic variants that give rise to phenotypes that are suited to the environment. Evidence is found looking at fossils of different ages - this allows you to see how changes in organisms developed slowly over time.

The relatively recent discovery of how bacteria are able to evolve to become resistant to antibiotics also further supports evolution by natural selection.

Theory of evolution by natural selection is now widely accepted.

Over a long period of time, the phenotype of organisms can change so much because of natural selection that a completely new species is formed. This is called speciation.

Speciation happens when populations of the same species change enough to become reproductively isolated - this means that they can't interbreed to produce fertile offspring.

Extinction is when no individuals of a species remain.

The fossil record contains many species that don't exist any more - these sepcies are said to be extinct.

Species become extinct for these reasons:

-> The environment changes too quickly

-> A new predator kills them all

-> A new disease kills them all

-> They can't compete with another (new) species for food

-> A catastrophic event happens that kills them all