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18.3) Selection

18.3) Selection

 

Natural selection:

  • Variation within populations.
  • Production of many offspring.
  • Competition for resources.
  • Struggle for survival.
  • Reproduction by individuals that are better adapted to the environment than others.
  • Passing on their alleles to the next generation.
  • The Variations have to be heritable for natural selection to be effective.
  • Both genes and the environment can cause variation, but only genetic variation can be passed on to the next generation.

 

Selective breeding:

  • Selection by humans of individuals with desirable features.
  • Cross-breeding these individuals to produce the next generation.
  • Offspring with the most desirable features are chosen to continue the breeding programme and the process is repeated over a number of generations.
  • The largest fruit on a tomato plant might be picked and its seeds planted next year. In the next generation, once again only seeds from the largest tomatoes are planted. Eventually it is possible to produce a true-breeding variety of tomato plant that forms large fruits.
  • Similar principles can be applied to farm animals.

 

Evolution: is the change in adaptive features of a population over time as a result of natural selection.

Adaptation: is the process, resulting from natural selection, by which populations become more suited to their environment over many generations.

 

Antibiotic-resistant bacteria:

  • Over time, bacteria can become resistant due to random mutations in the genes to certain antibiotics (such as penicillin). This is an example of natural selection.
  • In a large population of bacteria, there may be some that are not affected by an antibiotic. These survive and reproduce – producing more bacteria that are not affected by the antibiotic.
  • The number of strains of antibiotic-resistant bacteria has increased, partly due to the misuse of antibiotics.

 

Comparing natural and artificial selection:

  • Natural selection occurs in groups of living organisms through the passing on of genes to the next generation by the best adapted organisms, without human interference.
  • Those with genes that provide an advantage, to cope with changes in environmental conditions for example, are more likely to survive, while others die before they can breed and pass on their genes.
  • However, variation within the population remains.
  • Artificial selection is used by humans to produce varieties of animals and plants that have an increased economic importance.
  • It is considered a safe way of developing new strains of organisms, compared with genetic engineering, and is much faster process than natural selection.
  • However, artificial selection removes variation from a population, leaving it susceptible to disease and unable to cope with changes in environmental condition.

Potentially, therefore, artificial selection puts a species at risk of extinction.

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