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Types of Natural Selection

Types of Natural Selection. SBI3U. Sickle-Cell Anemia. Mutation + genetic variation + natural selection = evolution of a population Sickle- cell allele resulted from a single base mutation in the DNA coding for hemoglobin

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Types of Natural Selection

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  1. Types of Natural Selection SBI3U

  2. Sickle-Cell Anemia • Mutation + genetic variation + natural selection = evolution of a population • Sickle- cell allele resulted from a single base mutation in the DNA coding for hemoglobin • Heterozygous individuals, Ss, are resistant to malaria  better chance of survival • Sub-Sahara Africa- if you carry the allele, S, than you will survive to reproduce, and pass on the allele to the next generation • Therefore, an increase in the frequency of the allele

  3. Directional Selection • Occurs when selection favours an increase or decrease in the value of a trait from the current population average. • Favours individuals with a more extreme variation of a trait • Results in a shift away the average condition • Read Hummingbird example on the bottom of pg. 326

  4. Directional Selection Example • -The soil in the original environment was a medium brown • - After a volcano the soil becomes darker • Mouse population evolved to blend in with the darker surroundings • Therefore, environment favoured individuals with more extreme variations of the trait

  5. Stabilizing Selection • Selection against individuals exhibiting traits that deviate from the current population The average phenotype within a population is favoured by the environment • Read the human birth weight example on the bottom of page 327.

  6. Stabilizing Selection Example • The soil remains medium brown • Medium brown mice survive from predation • Medium brown mice pass of their allele for medium brown fur to their offspring • Light/ dark brown mice are eaten more -> those alleles are not passed on

  7. Disruptive Selection • Selection that favours two or more variations of a trait that differ from the current population average. • Favours opposite extremes of a trait over individuals with intermediate variations

  8. Disruptive Selection Example • Mouse move to a new environment with light brown rock and dark brown volcanic rock • Light brown and dark brown mice blend in with their environment, but medium brown mice do not • Medium brown mice are eaten by predators • Light/ dark brown mice pass on their genes to future generations

  9. Hardy- Weinberg Equilibrium • If allele frequencies change over time, the population is NOT in Hardy-Weinberg Equilibrium • Therefore, a ‘disturbing factor’ is acting on the population • Natural Selection • Small population size • Mutation • Immigration/ Emigration • Sexual selection

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