1. Summary – one summary at the end
Unit 3
Evolution
“Evolution as Genetic Change 3.6”
Name
Date
Period
EQ: Why does an advantageous trait increase in proportion within a species over time?
Take notes here
* Read Chapter 16 for additional support
LT:
Questions
25 Justify why an advantageous trait will increase in proportion from one generation to another when provided statistical evidence
Summary – one summary at the end

2. Natural selection never acts on genes directly
Natural selection never acts on genes directly. Natural selection acts on individuals who survive and reproduce If an individual dies without reproducing the individual does not contribute its alleles to the populations gene pool

3. Normal Distribution (of Finches) - starts as a bell-shaped curve
Smaller Larger
Beak Size
Number of
Birds
in Population

4. Because less small seeds, more big seeds
Directional Selection
When individuals at one end of the curve have higher fitness than individuals in the middle, or at the other end.
One extreme trait is favored
EX: Bigger beaks are favored
Because less small seeds, more big seeds
Selection against an extreme
Population after selection
Original population

5. Key Food becomes scarce. Directional Selection
Low mortality, high fitness
Directional Selection
High mortality, low fitness
Food becomes scarce.
Die Live
out
Die Live
out

6. Stabilizing Selection
When individuals near the center of the curve have higher fitness than individuals at either end of the curve.
Selection against both extremes
Original population
Population after selection

7. Stabilizing Selection
Average survives
Selection against both extremes keep curve narrow and in same place.
Selection against both extremes
Original population
Population after selection

8. Stabilizing Selection
Key
Low mortality, high fitness
High mortality, low fitness
Key
Selection against both extremes keep curve narrow and in same place.
Low mortality, high fitness
High mortality, low fitness
% of Population
Die Live Die
Out out
Birth Weight

9. Disruptive Selection
When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle.
Selection against the middle
Population after selection
Original population

10. Disruptive Selection
Opposite extreme traits are favored
EX: Small & large beaks
because only small & large seeds
Average (middle) trait is eliminated
EX: Less average size beaks
because less average sized seeds

11. Disruptive Selection Key Disruptive Selection
Low mortality, high fitness
High mortality, low fitness
Disruptive Selection
Large and small seeds become more common.
Live Die Live
out
Population splits into two subgroups specializing in different seeds.
Birds in Population
Live Die Live
out
Birds in Population
Beak Size
Beak Size

12. Organisms with an advantageous heritable trait tend to increase in proportion to organisms lacking this trait