04 April 2014

#135 Variation and antibiotic-resistance strains of bacteria

Variation is the slight individual differences within populations. All living things change and evolve from one generation to the next. As they do so, more variation is produced. 

Some variations is inherited (passed on from parents) and some is acquired (developed during life). 

Animals and plants produced by sexual reproduction will show variation from their parents, for example in the size of the muscles in the legs of lions.

When new organisms are produced, not all of them are likely to survive because of competition for resources such as food, water and shelter. The same is true for plants (they compete for resources such as nutrients, light, water and space).

The individuals with the most favourable characteristics are most likely to survive. 

The process of natural selection follows a sequence, as listed below.
  • Some of the variations within a population may give some individuals an advantage over others in the population. Bigger muscles in the legs of a lion would enable it to run more quickly and get food more successfully.
  • In an environment where there is food shortage, the lion with the biggest leg muscles is most likely to survive to adulthood.
  • The weaker individuals die before having the chance to breed, but the surviving adults breed and pass on the advantageous genes to their offspring.
  • More of the next generation carry the advantageous genes, resulting in a stronger population, better adapted to a changing environment.

Slow changes in the environment results in adaptation in a population to cope with the change. Failure to adapt could result in the species becoming extinct. This gradual change in the species through natural selection over time, in response to changes in the environment, is a possible mechanism for evolution.

Examples: antibiotic-resistance strains of bacteria

Bacteria reproduced rapidly - a new generation can be produced every 20 minutes by binary fission. Antibiotics are used to treat bacterial infections: an antibiotic is a chemical that kills bacteria by preventing bacterial cell wall formation. 

Mutations occur during reproduction, which produce some variation in the population of bacteria. 

Individual bacteria with the most favourable features are most likely to survive and reproduce.

A mutation may occur that enables a bacterium to resist being killed by antibiotic treatment, while the rest of the populating is killed when treated. This bacterium would survive the treatment and breed, passing on the antibiotic - resistant gene to its offspring. Future treatment of this population of bacteria using the antibiotic would be ineffective. 

Video GCSE BBC Science Bitesize - Variation, Inheritance and Evolution

Video The Animation of Antimicrobial Resistance

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