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Types of Natural Selection SBI3U. Sickle-Cell Anemia Mutation + genetic variation + natural selection = evolution of a population Sickle- cell allele.

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Presentation on theme: "Types of Natural Selection SBI3U. Sickle-Cell Anemia Mutation + genetic variation + natural selection = evolution of a population Sickle- cell allele."— Presentation transcript:

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

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4 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

5 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

6 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.

7 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

8 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

9 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

10 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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