1. In your own words, explain the significance of the diagram shown.

2. Evolution as Genetic Change in Populations
Read the lesson title aloud to students.

3. Learning Objectives
Explain how natural selection affects single-gene and polygenic traits.
Describe genetic drift.
Explain how different factors affect genetic equilibrium.
Click to reveal each of the three learning objectives.
Activate prior knowledge in students by reminding them of what they learned previously about Darwin’s theory of evolution. Have several students in turn state a fact about, or give an example of, natural selection.
Ask: If Darwin had known what you know about genes, how do you think he would have defined fitness?
Sample answers: possession of genes that enhance survivability; ability of an individual to pass copies of its genes to its offspring
Distribute the worksheets for this lesson and suggest that students use the table to take notes as information is presented in the lesson.
Let students know that at the end of this presentation, they will be able to explain how natural selection affects single-gene and polygenic traits, describe genetic drift, and explain how different factors affect genetic equilibrium.

4. How Natural Selection Works
evolutionary adaptation An
is any genetically controlled trait that increases an individual’s fitness.
Explain to students that when a pesticide is introduced, like the one shown in this picture, it typically kills most insects. In evolutionary terms, that pesticide becomes part of insects’ environment that dramatically reduces the fitness of most individuals. But usually, just by chance, some individuals in the insect population carry genetic variation that enables them to resist the pesticide.
Tell students: Pesticide resistance is an adaptation that enables individuals to survive and reproduce.
Ask: What happens when a pesticide is introduced into an environment?
Answer: The presence of the pesticide increases the relative fitness of the few individuals that carry the resistance adaptation. Resistant insects survive and reproduce, passing resistance genes to their offspring. Meanwhile, the pesticide eliminates reproduction by individuals that don’t carry the resistance genes.
Explain that over time, the frequency of resistant alleles increases in the population.
Tell students that this allows us to define “evolutionary fitness” in genetic terms as an individual’s success in passing genes to the next generation.
Ask for a volunteer to complete the sentence by filling in the correct words.
Click to reveal the correct answer.
Tell students: An evolutionary adaptation, in genetic terms, is any genetically controlled trait that increases an individual’s fitness.

5. Natural Selection on Single-Gene Traits
Natural selection on single-gene traits can produce changes in allele frequencies that may be reflected by simple changes in phenotype frequencies.
Explain that when natural selection produces differential reproductive success, the effects on phenotype can look different for single-gene and polygenic traits.
Ask: How does natural selection affect single-gene and polygenic traits?
Answer: When natural selection produces differential reproductive success, the effects on phenotype can look different for single-gene and polygenic traits.
Tell students: Imagine, for example, that a population of brown lizards experiences mutations in a gene that determines body color. The mutations produce red and black forms, as shown in the table.
Click to highlight these color mutations.
Ask: What happens to allele and phenotype frequencies?
Answer: If red lizards are more visible to predators, they might be less likely to survive and reproduce. Therefore, the allele for red coloring might not become common. Black coloration, on the other hand, might enable individuals to absorb more sunlight and warm up faster on cold days. If high body temperature allows individuals to move faster when feeding and avoiding predators, black coloration can act as an adaptation that increases fitness. In that case, the allele for black coloration and the black phenotype would both increase in frequency.
Point out that if black coloration has no effect on fitness, the phenotype produced by the mutant allele will not act as an adaptation. In that case, the allele will not be under pressure from natural selection.
Ask: What has happened to produce the population shown in Generation 30?
Answer: Red lizards have been selected against because they are more visible to predators, while black lizards have been selected for because they can move faster and avoid predators. As a result, red lizards have been eliminated and black lizards have become more common.

6. Natural Selection - Summary
VIST (Variation, Inheritance, Selection, Time)
1. There is variation in traits in a population.
2. There is differential survival / reproduction.
3. Selected traits are heritable.
4. The advantageous trait becomes more common in the population over time.

7. Natural Selection - Summary
Individuals
do not evolve!
Populations, and ultimately species, evolve due to differential survival and reproduction of individuals over time.

8. Natural selection leads to divergence and diversity
Environment 1
Environment 2
Time 1
Time 2
Picture from wikipedia commons
Time 3

9. Survival & Selective Pressure
“Struggle to Survive”
Competition for Food
Competition for Mates
Competition for Space
Survival & Selective Pressure
are interconnected

10. Host-parasite interactions and co-evolution

11. Natural selection leads to divergence and diversity
Differences among populations can accumulate leading to different species.
Picture from wikipedia commons

12. Natural selection leads to divergence and diversity
Diversification through millennia leads to the enormous diversity of species and forms of life extant and extinct.
Picture from wikipedia commons

13. Genetic Drift Genetic drift is a random change in allele frequency.
Genetic bottlenecks
The founder effect
Founding populations
Descendants
Explain to students that natural selection is not the only evolutionary mechanism that can change allele frequencies. In small populations, individuals that carry a particular allele may leave more descendants than other individuals leave, just by chance. Over time, a series of chance occurrences can cause an allele to become more or less common in a population.
Tell students: This kind of random change in allele frequency is called genetic drift.
Explain that sometimes, a natural disaster, such as a storm or flood, can kill many individuals in a population. The gene pool of the surviving population may have allele frequencies that differ from those of the original gene pool just by chance. If the reduced population later grows, its allele frequencies will also differ from the original population’s.
Tell students: The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population. A severe bottleneck effect can sharply reduce a population’s genetic diversity, and thus decrease diversity within the species.
Explain that genetic drift may also occur when a few individuals colonize a new habitat.
Click to show founding individuals.
Tell students: These founding individuals may carry alleles that differ in relative frequencies from those of the main population, just by chance. The new gene pool therefore starts out with allele frequencies that differ from those of the parent gene pool.
Explain that this situation, in which allele frequencies change as a result of the migration of a small subgroup of a population, is known as the founder effect.
Tell students: These two small groups from a large, diverse population could produce new populations that differ from the original group.
Click to reveal the descendants.
Point out that one example of the founder effect is the evolution of several hundred species of fruit flies on different Hawaiian islands. All those species descended from the same mainland fruit fly population. However, species on different islands have allele frequencies that are different from those of the original species.
Ask: Why are the two populations of descendants shown so different from each other?
Answer: The two populations of descendants are different because they were founded by small subgroups of the original population that were, by chance, genetically very different from each other.
Explain that the founder effect may be especially likely to occur following a major change in the environment, such as a forest fire, landslide, volcanic eruption, or human destruction of a habitat.
Ask: Can you describe a scenario in which an environmental change such as this leads to the founder effect?
Answer: A fire might isolate a few mice in a small remnant of forest.

14. Genetic Drift
A few “lucky” individuals leave behind more genes than others in the population due to random chance.
No natural selection or adaptation

15. Bottleneck:
In 1820 Northern elephant seals were hunted to near extinction, with only 20 individuals surviving. Today the population has rebounded to 30,000 but there is little variation in the population.

16. Founder Effect
Occurs when a new colony is started by a few members of an original population. Variation is decreased and high frequencies of certain alleles may appear in the population as a result.
EX: Afrikaner Dutch population in South Africa is descended from a few colonists. Today that population has a high frequency of the huntingtin gene

17. Classification reflects evolution
Cladistics: the process of grouping organisms descendent from a common ancestor. Clade: group of related organisms that share a common lineage.
Picture from wikipedia commons