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Natural Selection

Biology

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Objectives

  • Students will be able to explain and describe natural selection
  • Students will be able to discuss examples and make predictions about natural selection

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Allele Frequencies

  • Allele frequency tells us how common a certain allele is in a population. (it’s a percentage!)
  • Allele frequencies change because of births and deaths
  • We define evolution in Biology as a change in allele frequencies in a population
  • Evolution can only occur for a population over time, as new individuals are born and other individuals die

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Biological Fitness

  • How likely an individual is to survive in a particular environment.

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  • More fit organisms are likely to have more offspring (due to their increased survival)

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Fitness and Environment

  • Fitness depends on the environment in which the organism lives

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  • The fittest organism during an ice age, for example, is probably not the fittest organism once the ice age is over

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Natural Selection

  • Some organisms in a population are more likely to survive or reproduce.
  • Their offspring are more likely to have the alleles that contribute to their fitness.
  • Some organisms will tend to have more offspring caused by biological fitness. (Natural Selection)
  • Natural selection results in evolution – an increase in some allele frequencies and a corresponding decrease in others

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Ingredients of Natural Selection

  • Natural selection requires variation – different alleles that cause some individuals to be more or less fit
  • Natural selection requires “selection pressure” – the environment or other conditions must determine which alleles increase fitness

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What Natural Selection is NOT!

  • First, natural selection is not all-powerful; it does not produce perfection

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  • Second, it is mindless and mechanistic - it has no goals; it’s not striving to produce “progress” or a balanced ecosystem

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What are the implications?

  • Alleles that allow organisms to be more fit will become more abundant over time
  • This is evolution: a change in the allele frequency of a population over time
  • Therefore, natural selection can cause evolution (it is a “mechanism” for evolution)

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