Natural selection is the major mechanism of evolution that acts on phenotypic variations. It results from competition for limited resources that leads to differential survival of individuals with more favorable phenotypes, allowing them to produce more offspring and pass down their traits. Three types of natural selection are stabilizing selection, which selects for average traits; directional selection, which selects against one extreme of a trait; and disruptive selection, which selects against individuals in the middle of two trait extremes. Genetic variation within populations is important for adapting to changing environments through natural selection.

1. Evolution: Natural Selection
and Phenotypes
Big Idea 1: Process of evolution
drives the diversity of life

2. Essential Knowledge
• 1A1: Natural selection is a major mechanism
of
evolution.
• 1A2: Natural selection acts on phenotypic
variations.
• 1A3: Evolutionary change is also driven by
random processes

3. Evolution
• Change in genetic makeup of a population over
time.
• Natural selection is its major driving

5. What do
you
know
about
this guy?

6. Darwin
said…
• Competition for limited resources
results in differential survival.
What’s
wrong with
this picture
though???

8. Natural Selection
•Individs w/ more favorable phenotypes
(physical features) are more likely to survive
and produce more offspring, passing down
traits to subsequent generations!

9. Describe
Pandy’s
phenotype

10. Genetic Code:
DNA/RNA
Physical features
and Behaviors

11. Types of Natural Selection
1) Stabilizing
Selection:
Selective
pressures select
against the 2
extremes of a trait.
• Ex: Better to be
average!

12. 2) Directional
selection: 1
extreme of the
trait experiences
selection against
it.
• Ex: giraffes w/
short necks
Types of Natural
Selection

13. 3) Disruptive
selection: Selection
pressures act
against individuals in
middle of 2 extremes
of the trait.
• Ex: Don’t be
average
Types of Natural
Selection

14. Evolutionary Fitness
• To be fit, you must be able to reproduce!
• Measured by reproductive success.

15. Why is the Zorse Not Evolutionary Fit?

16. Genetic Variation is Good For You!
• A diverse gene pool is important for
survival of a species in a changing
environment.

17. Adaptation
• A genetic variation that is favored by selection and
provides an advantage to an org. in a environment.
• Occurs naturally through environmental
pressures.

18. • Chance and random events (genetic
drift) can also influence the
evolutionary process, especially for

19. What is Going to Happen to this Small
Population, thanks to Genetic Drift?

20. Natural Selection Acts on
Phenotype
• Environments may change and act as a
selective mechanism on populations.

21. Peppered Moth Video
Clip

22. If the environment changes, different genetic
variations can be selected in each
generation.

23. Phenotypic variations occur b/c of
random changes (mutations) and new
gene combinations (fertilization and
meiosis)

24. Some Phenotypic Variations
IncreaseFitnes
s
• Ex: Sickle cell anemia (Ss) makes you far less
likely to die from malaria. Heterozygote
advantage.

25. Whose
fitness will
be
decreased?

26. HumansImpact Variation
• Often times decreasing it…food crops
have been artificially selected to be
genetically similar.