1. Evolution as Genetic Change
16-2
Evolution as Genetic Change

2. Natural Selection acts on Phenotypes:
Natural Selection on Single-Gene Traits
Can lead to changes in allele frequencies and thus evolution.
A change in environment that would favor a particular phenotype will likely increase the frequency of its allele.
Natural Selection and Polygenic Traits
Natural selection of traits controlled by two or more genes will likely lead to a normal bell-shaped distribution.

3. Normal Distribution of Finches
Beak Size
Number of
Birds
in Population

4. Directional Selection
When individuals at one end of the curve have higher fitness than individuals in the middle, or at the other end.
Key
Directional Selection
Low mortality, high fitness
High mortality, low fitness
Food becomes scarce.

5. Stabilizing Selection
When individuals near the center of the curve have higher fitness than individuals at either end of the curve.
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

6. Disruptive Selection
When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle.
Disruptive Selection
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

7. Genetic Drift
In small populations an allele can become more or less common merely by chance
Remember, the smaller the group the less likely it will reflect the laws of probability
Genetic drift: a random change in allele frequency due to chance
Usually natural disasters
If many chance occurrences favor an individual with a particular allele to leave more offspring, then the allele may become more common in the population
Founder effect: a situation in which allele frequencies change as a result of the migration of a small subgroup of a population.

8. Sample of
Original Population
Descendants
Founding Population A
Founding Population B

9. Sample of
Original Population
Descendants
Founding Population A
Founding Population B

10. Sample of
Original Population
Descendants
Founding Population A
Founding Population B

11. Evolution Versus Genetic Equilibrium
Hardy-Weinberg principle: the allele frequency in a population will remain constant unless one or more factors cause those frequencies to change
A way to measure if evolution may be occuring
1. random mating
all members of a population must have equal opportunity to produce offspring
2. large population
the population must be large enough to avoid genetic drift
3. no movement into or out of the
population
no new alleles may leave or come into the population

12. 4. no mutations 5. no natural selection
no new alleles can be introduced into the population through mutation
5. no natural selection
all genotypes in the population must have equal probabilities for survival and reproduction
genetic equilibrium: if all the above factors are maintained in the population then the existing allele frequencies will remain constant
No evolution is taking place.

13. Isolating Mechanisms speciation: the formation of new species
As new species evolve, populations become reproductively isolated from each other
reproductive isolation: when members of two populations cannot interbreed and produce fertile offspring (3 types)
behavioral isolation
capable of interbreeding but have differences in courtship rituals and other behaviors
geographic isolation
two populations are separated by geographic barriers (rivers, mountains, or bodies of water)
temporal isolation
two or more species reproduce at different times