1. Evolution of Populations

2. Evolution and Genetics
Variations are inherited from one generation to the next leading to natural selection
Differences that help organisms survive and reproduce become more common and differences that are not beneficial become less common
It changes the genes!!!

3. Darwin and Mendel?
Darwin did not know about how things are inherited yet (Mendel).
We can use our knowledge from Mendel’s study of heredity and combine it with Darwin’s study of evolution to explain how inheritable variation appears and how natural selection effects that variation.

4. Genes and Variation
Biologists studying evolution focus on a population = collection of individuals of the same species in a given area
A population shares a common group of genes (genetic information) = gene pool
Contain two or more alleles (forms of a gene) for each inheritable trait
Relative frequency (%) = the number of times an allele occurs in a gene pool compared to the number of times other alleles occur

5. Sources of Genetic Variation
Mutations = any change in a sequence of DNA
Remember: mutations result as a mistake during replication or toxin (chemicals/radiation)
Some mutations effect phenotypes (physical characteristics), which can effect an organism’s fitness (ability to survive)
Gene shuffling = different gene combinations inherited during gamete production creating different genotypes (genetic makeup), different phenotypes and more variation
Crossing over increases number of different genotypes
Does not change the relative frequency of alleles in a population
Think of a deck of cards – there are many possible combinations, but frequency remains same

6. Evolution as Genetic Change
Natural selection acts on phenotypes, survival and reproduction determine which alleles are inherited, changing relative frequencies of alleles in a population over time.
Thus evolution is any change in the relative frequencies of alleles in a population’s gene pool and acts on populations, not individuals.

7. Evolution of Polygenic Traits
Natural selection can affect the distributions of phenotypes in any of three ways:
Directional selection: When the entire bell moves left/right because there’s a higher fitness and increase in the number of individuals with the trait at one end of the curve
Stabilizing selection: When the bell becomes more narrow, because there’s a higher fitness and increase in the number of individuals with the trait in the center of the curve
Disruptive selection: The bell can split into two, because there’s a higher fitness and increase in the number of individuals at both ends of the curve

8. COPY THIS Graph of Directional Selection
Section 16-2
Key
Directional Selection
Low mortality, high fitness
High mortality, low fitness
Food becomes scarce.

9. COPY THIS Graph of Stabilizing Selection
Section 16-2
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

10. Copy this Graph of Disruptive Selection
Section 16-2
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

11. Genetic Drift Genetic drift = random change in allele frequencies
In small populations, individuals that carry a particular allele may leave more descendants than other individual, just by chance. Over time, a series of chance occurrences can cause an allele to become common in a population.
Genetic drift can happen when a small group of individuals colonize a new habitat carrying different relative frequencies that the larger population.
Founder effect = allele frequencies change as a result of the migration of a small subgroup of a population

12. Speciation Speciation = formation of new species
Species = group of organisms that breed with one another and produce fertile offspring (share a common gene pool)
As new species evolve, populations become reproductively isolated from each other: When 2 populations can’t breed and produce fertile offspring, resulting in separate gene pools
Behavioral isolation (sympatric): Capable of breeding, but have different courtship rituals or behaviors
Geographic isolation (allopatric): Separate by geographic barriers
Temporal isolation (sympatric): Reproduce at different times