Natural selection



All individuals of any species tend to vary from each other. There are processes in an organism's life cycle that can generate this genetic variation:

  • Meiosis, through the events of 'independent assortment' and 'crossing over' produces haploid cells each of which is different from any other.
  • random mixing of chromosomes at fertilisation

Genteic variation can also be due to a mutation (a spontaneous change in the coding of DNA)

Therefore, within most species, there is considerable variation among individuals. Some individuals are better adapted than others for the particular environment within which they live.

Fitness- The adaptiveness of organisims to survive and reproduce

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Survival of the Fittest

As a population increases in size, many factors in its environment become limiting. There may be competition for food in animals and for light, water and minerals in plants.

The best adapted individuals(fittest for their environment) are more likely to survive and pass their alleles on to the next generation. While the less well adapted organisms are more likely to die.

This will produce a change in the proportion of alleles in the gene pool and may result in evolution (over many generations.)

Note: natural selction DOES NOT create useful adaptions, but rather edits genetically inherited features in a population, increasing the frequency of some adaptions, while decreasing the frequency of others over time.

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Directional selection

  • Directional selection is where average individuals are not the best adapted, but the best adapted individuals lie closer to one of the extremes of variation.
  • This will occur when the environmental conditions change.
  • example of directional selection - the peppered moth
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Stabilising Selection

  • This selection occurs in a constant environment and tends to eliminate extremes in a population.
  • It reduces the variability of a population and so reduces the opportunity for evolutionary change
  • example of stabilising selection- human birth weight
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