1. What is Evolution???
Learning Target: I can explain Natural Selection and the 4 conditions that are required for Natural Selection to take place.

2. Change in the heritable (genetic) traits of a population over time
What is Evolution???
Change in the heritable (genetic) traits of a population over time
Note: when we discuss evolution, we are talking about
populations changing, not individuals

3. The main mechanism by which evolution occurs
Natural selection:
The main mechanism by which evolution occurs

4. 4 Conditions for Natural Selection:
Variation: Individuals in a population are not identical to each other.
Inheritance: Traits are passed to offspring; traits have a genetic basis
Environmental population limits: Environmental limiting factors prevent all individuals from surviving to reproduce; some die young.

5. Environmental selection:
Individuals in the population with more favorable (advantageous) traits are the ones that survive to reproduce.
Individuals without advantageous traits die before reproducing.
These factors result in a change in the average trait of the population… Biologists call this EVOLUTION!

6. 1. VARIATION
Members of a population have traits similar to the average trait of the entire population, but they are not identical.
YOUR TURN: Using height as an example,
sketch a graph to represent the statement above.
Mean (average) height
First clarify the statement, then reveal “your turn” assignment.
Discuss with students what type of graph would be best- (frequency distribution)
Reveal axes as a hint while students sketch a graph on their notes sheet.
Finally, reveal what the completed sketch should look like
Frequency
Height (cm)

7. 2. INHERITANCE DNA determines the traits of individuals
Individuals inherit DNA from their parents
This causes the traits of the offspring to resemble the traits of the parents
DNA
trait

8. 3. ENVIRONMENTAL POPULATION LIMITS
For all species, if every individual born into a population were to reproduce, the population would grow exponentially
Population
Make note to students that these graphs have different axes than the other graphs we have been examining.
Time

9. 3. ENVIRONMENTAL POPULATION LIMITS
Environmental factors (limiting factors) prevent the majority of individuals from surviving to reproduce
Population
Make note to students that these graphs have different axes than the other graphs we have been examining.
Time
What are some examples of limiting factors?

10. 4. SELECTION
Individuals without advantageous traits die before reproducing.
Individuals with advantageous traits survive to reproduce.
Again, make note the labeling of the axes
Frequency
die without reproducing
These individuals
These individuals
survive to reproduce
Characteristic

11. Populations change, not individuals
The “average” characteristic or other measure of the population changes over generations
average, 1st gen.
average, 2nd gen.
average, 3rd gen.
average, 50th gen.
Frequency
die without reproducing
These individuals
die without reproducing
These individuals
These individuals
survive to reproduce
Characteristic

12. The environment is the selective force behind evolution
The environment determines what characteristics are “favorable”
Characteristics are said to be “selected for”
Because the environment changes over time, the characteristic that is more favorable for a population changes
Therefore, characteristics of the population change, or evolution occurs

13. Artificial Selection Humans select for the desired trait
Example: plant & animal breeding
Not an example of evolution

14. Why is genetic variation in a population important?
A population without much variation limits a species’ ability to further evolve. (Variation is one of the 4 factors required for natural selection to cause evolution)
If populations do not adapt to their environment, they may become extinct.

15. Sources of genetic variation
SEXUAL REPRODUCTION
Meiosis – one copy of a gene is passed on from each parent, combination may be different than someone else
Random fertilization – only one of the millions of sperm involved in mating will fertilize the egg.
The randomness of sexual reproduction explains why siblings can look so different.

16. Sources of genetic variation
MUTATION
Mutation means a change in DNA sequence.
- Can happen when DNA is copied during cellular replication
A new DNA sequence can result in new variation (not always).
Many mutations produce traits that are harmful (e.g. Huntington’s disease)
Some mutations produce traits that are neutral (neither helpful nor harmful)
Very, very few mutations produce traits that are advantageous

17. Natural Selection
Individuals with advantageous genes survive to reproduce and pass on these genes to their offspring.
Individuals without advantages genes do not survive to reproduce, and these genes do not get passed on in the population.
“Survival of the Fittest” – best fit (able to survive and reproduce) in the environment
Natural Selection leads to adaptation!!!

18. Conclusion Mutation does NOT cause evolution
it is only a source of variation (just one of the factors required for natural selection)
Natural selection determines if the frequency of traits will change within a population.
Change in traits in a population = EVOLUTION