Darwin’s theory

• Natural selection

  The process by which organisms with variations that are better adapted to their environment survive and reproduce, passing on their traits to future generations
• How natural selection leads to evolution

  1. Genetic variation

  2. Environmental change

  3. Natural selection

  4. Inheritance

  5. Evolution
• Darwin's theory is supported by evidence
• Evolution rarely happens in one neat line - there are usually many branches
• When conditions change, some individuals are better adapted to cope than others
• Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) came up with essentially the same idea about how evolution happens
• Woolly mammoths and elephants evolved from the same animal; they share a common ancestor
• An area in which the common ancestor lived started getting colder
• Due to genetic variation, some animals by chance had hairier skin
• The hairier animals were more likely to survive the cold than less hairy animals, especially when food was scarce
• More of the hairier individuals survived and bred
• Over time the animals became hairier and hairier, forming a new species
• In the 1940s and 1950s, a substance called warfarin was used to poison rats
• When warfarin was first used, most rats died, but within 10 years most rats were resistant to (not affected by) warfarin
• Due to genetic variation there had always been some rats that were resistant
• As the poison killed the non-resistant rats, the only ones left to breed were resistant
• The same thing has happened with bacteria and antibiotics
• In a population of bacteria, some bacteria are more resistant than others and take longer to be killed
• People who take an antibiotic to treat an infection often stop taking it too early, because they feel better
• This leaves resistant bacteria still alive. They reproduce and spread, causing infections that cannot be treated with the antibiotic because all the bacteria are now resistant
• This problem of resistance in bacteria was not present when antibiotics were first used
• Bacteria in a population show variation in the amount of resistance to an antibiotic
• With time, the antibiotic kills more and more of the bacteria
• The most resistant bacteria take the longest to die
• The resistant bacteria survive and reproduce
• The new population of bacteria are all now resistant to the antibiotic
• Stopping an antibiotic early can cause resistance to develop in a species of bacterium