1. Types of Natural Selection
Section 8.1

2. Types of Natural Selection
Learning Goal: To learn about the types of natural selection and explain how natural selection results in adaptations in organisms.
Success Criteria:
I know I am succeeding when I can…
describe the four types of natural selection
provide real life examples of natural selection
use the theory of Natural Selection to explain adaptations of living things
This lesson will present the many forms of evidence that convinced Darwin that species had changed over time and that species appeared to share a common ancestry. The basis of his theory on the evolution of species.

3. Natural Selection
evolution occurs when natural selection acts on the genetic variability within populations
genetic variation arises by chance through genetic mutations and recombination
the process of natural selection does not occur by chance, the environment favours certain individuals over others
ex. the prevalence of sickle cell allele in Africa

4. Types of Selection
selective pressures may result from any number of abiotic or biotic factors: diseases, climatic conditions, food availability, or predators—even your choice of mate!
these selective pressures can result in different patterns of natural selection

5. Types of Selection Cont’d…
Directional Selection:
occurs when selection favours individuals with a more extreme variation of a trait
the result is a shift away from the average condition
ex. in a new environment with longer flowers, selection will favour hummingbirds with longer bills

6. Types of Selection Cont’d…
Stabilizing Selection:
occurs when the average phenotype within a population is favoured by the environment
ex. hummingbirds with an average bill length are favoured by the environment

7. Types of Selection Cont’d…
Disruptive Selection:
favours individuals with variations at opposite extremes of a trait over individuals with intermediate variations
ex. the environment favours individuals with long and short bills over individuals with average bill lengths

8. Types of Selection Cont’d…
Sexual Selection:
the favouring of any trait that specifically enhances the mating success of an individual
results in sexual dimorphism, and mating and courtship behaviours (ex. male cardinals use brightly coloured plumage and song to attract females)
often a disadvantage when it comes to longevity (ex. more vulnerable to predators when brightly colored or have a distinctive song)

9. Types of Selection Cont’d…
sexual selection is not limited to animal populations
colourful flowers and scents maximizes plants chances of being pollinated, and thus have a greater likelihood of contributing more alleles to the next generation’s gene pool

10. Natural Selection in Action
natural selection results in evolutionary changes within populations
Tibetans living at high elevation have blood with a high capacity to carry oxygen
example of directional selection

11. Natural Selection in Action Cont’d…
A controlled experiment in mice suggests that some behaviours have a genetic component and can be influenced by directional selection
certain behaviours might have an inherited component and could be influenced by natural selection
researchers modelled directional selection in populations of mice by choosing individuals for breeding that spent the most time on exercise wheels
after only 10 generations, the populations descended from the chosen mice exhibited much higher running distances and average speeds when compared to control populations

12. Practice Questions
Identify which type of natural selection is at work in the following descriptions. Sketch a graph to show how the type of natural selection is working.
A population of sea stars are found in a range of sizes, from small (8 cm across) to large (16 cm across). Over time, however, the average size of the population shifts so that almost all sea stars in the population measure 12 cm across.
Frequency
Trait Value
Pattern of Natural Selection:
certain behaviours might have an inherited component and could be influenced by natural selection
researchers modelled directional selection in populations of mice by choosing individuals for breeding that spent the most time on exercise wheels
after only 10 generations, the populations descended from the chosen mice exhibited much higher running distances and average speeds when compared to control populations

13. Practice Questions Cont’d…
A population of flowers in a meadow show multiple colours of petals, from very pale pink to shades of darker pinks to a very dark burgundy. Over time, the population of shifts and only very pale pink and very dark burgundy petals are seen.
Frequency
Trait Value
Pattern of Natural Selection:
certain behaviours might have an inherited component and could be influenced by natural selection
researchers modelled directional selection in populations of mice by choosing individuals for breeding that spent the most time on exercise wheels
after only 10 generations, the populations descended from the chosen mice exhibited much higher running distances and average speeds when compared to control populations

14. Practice Questions Cont’d…
A population of scorpions is known for its toxic sting and its medium-to-large pincers. Over time, the population is made up of only scorpions with very large pincers.
Frequency
Trait Value
Pattern of Natural Selection:
certain behaviours might have an inherited component and could be influenced by natural selection
researchers modelled directional selection in populations of mice by choosing individuals for breeding that spent the most time on exercise wheels
after only 10 generations, the populations descended from the chosen mice exhibited much higher running distances and average speeds when compared to control populations

15. Types of Natural Selection
Learning Goal: To learn about the types of natural selection and explain how natural selection results in adaptations in organisms.
Success Criteria:
I know I am succeeding when I can…
describe the four types of natural selection
provide real life examples of natural selection
use the theory of Natural Selection to explain adaptations of living things
This lesson will present the many forms of evidence that convinced Darwin that species had changed over time and that species appeared to share a common ancestry. The basis of his theory on the evolution of species.

16. Homework
8.1 questions #1-8 on page 335