1. Natural Selection on Polygenic Traits

2. Mechanisms of Evolution
The main causes of evolutionary change are
genetic drift,
gene flow, and
natural selection.
Natural selection is the most important, because it is the only process that promotes adaptation.

3. Mechanisms of Evolution: Genetic Drift
Allele can become more or less common BY CHANCE
Random change in allele frequency is genetic drift (caused BY CHANCE!)
In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just BY CHANCE. Over time, a series of chance occurrences of this type can cause an allele to become common in a population = genetic drift

4. Genetic Drift (continued)
May occur when a small group of individuals colonies a new habitat. These individuals may carry alleles in different relative frequencies than the larger population that they came from. If so, the population that they create will be genetically different from the parent population

5. RR RR Rr rr RR Rr RR Rr RR Rr
Generation 1
p  0.7
q  0.3

6. Only 5 of 10 plants leave offspring p  0.7 q  0.3 p  0.5 q  0.5RR RR rr RR Rr Rr rr RR RR rr Rr Rr RR Rr rr RR RR Rr Rr Rr
Generation 1
p  0.7
q  0.3
p  0.5
q  0.5

7. Only 5 of 10 plants leave offspring Only 2 of 10 plants leaveRR RR rr RR RR Rr Rr RR RR rr RR RR rr RR RR Rr Rr RR RR RR Rr rr RR RR RR Rr Rr Rr RR RR
Generation 1
Generation 2
Generation 3
p  0.7
q  0.3
p  0.5
q  0.5
p  1.0
q  0.0

8. Genetic Drift: Bottleneck Effect
The bottleneck effect
is an example of genetic drift
results from a drastic reduction in population size
could be caused by earthquakes, floods, etc
Passing through a “bottleneck,” a severe reduction in population size,
decreases the overall genetic variability in a population because at least some alleles are lost from the gene pool, and
results in a loss of individual variation and hence adaptability.

9. Genetic Drift: Bottleneck Effect
Original
population
Bottleneck
event
Surviving
population

10. Genetic Drift: Bottleneck Effect
Cheetahs appear to have experienced at least two genetic bottlenecks:
during the last ice age, about 10,000 years ago, and
during the 1800s, when farmers hunted the animals to near extinction.
With so little variability, cheetahs today have a reduced capacity to adapt to environmental challenges.

11. Genetic Drift: The Founder Effect
Founder Effect – allele frequencies change as a result of the migration of a small subgroup of a population:
Sample of
Original Population
Descendants
Founding Population A
Founding Population B

12. Genetic Drift: The Founder Effect
Founder Effect – allele frequencies change as a result of the migration of a small subgroup of a population:
Sample of
Original Population
Descendants
Founding Population A
Founding Population B

13. Genetic Drift: The Founder Effect
Founder Effect – allele frequencies change as a result of the migration of a small subgroup of a population:
Sample of
Original Population
Descendants
Founding Population A
Founding Population B

14. Gene Flow Gene flow is another source of evolutionary change,
is separate from genetic drift,
is genetic exchange with another population,
may result in the gain or loss of alleles, and
tends to reduce genetic differences between populations.

15. Hardy-Weinberg Equilibrium (page 259)
Hardy and Weinberg – 2 scientists that asked: “Are there any conditions under which evolution will NOT occur?”
If allele frequencies stay the same – the population does NOT evolve

16. Hardy-Weinberg Equilibrium (page 259)
5 conditions are required to maintain genetic equilibrium from generation to generation:
Random mating – equal chance of passing genes
Large population – genetic drift has less effect
No movement into or out of the population – must maintain gene pool
No mutation – can’t have new alleles
No natural selection – no “survival of the fittest”

17. Hardy-Weinberg Equilibrium (page 259)
Meeting these five conditions is difficult to do and often cannot be met, so evolution will occur
Many organisms mate with chosen mates based on things like strength, color, etc
Not all populations are large
Migration occurs all the time
Mutations are bound to happen
Natural selection/survival of the fittest often cannot be avoided!

18. Natural Selection on Polygenic Traits
Effects of natural selection on polygenic traits are very complex
Can affect the distributions of phenotypes in any of three ways:
Directional selection
Stabilizing selection
Disruptive selection

19. Directional Selection
Individuals at one end of the curve have a higher fitness than individuals in the middle or at the other end
Entire curve moves as the trait changes
Ex. Finches and beak size – food became scarce; finches compete for food; bigger beaks are able to find food, survive and reproduce = beak size increases

20. Directional Selection
Key
Directional Selection
Low mortality, high fitness
High mortality, low fitness
Food becomes scarce.

21. Stabilizing Selection
Individuals near the center of the curve have higher fitness than individuals at either end of the curve
Keeps the center of the curve at its current position, but narrows the graph
Ex. Weight of human infants – small babies less likely to survive and large babies have difficulty being born = average-sized babies are favored

22. Stabilizing Selection
Key
Low mortality, high fitness
High mortality, low fitness
Selection against both extremes keep curve narrow and in same place.
Percentage of Population
Birth Weight

23. Disruptive Selection
Individuals at upper and lower ends of the curve have higher fitness than individuals near the middle
Can cause graph to split into two, creating two distinct phenotypes
Ex. Bird – 2 different seed sizes (small and large); birds with large beaks and birds with small beaks survive = two distinct beak sizes

24. Disruptive Selection Disruptive Selection Key
Largest and smallest seeds become more common.
Key
Low mortality, high fitness
Population splits into two subgroups specializing in different seeds.
Number of Birds in Population
Number of Birds in Population
High mortality, low fitness
Beak Size
Beak Size