1. Types of Selection and Sources of Variation
IB Biology
Evolution

2. Types of Selection
There are 4 major types of natural selection, and 1 type caused by humans (artificial selection)
Stabilizing Selection
Directional Selection
Disruptive Selection
Sexual Selection
[Artificial Selection]

3. Types of Selection
Disruptive selection

4. Stabilizing Selection
Eliminates individuals that have extreme or unusual traits
Most common traits are the best adapted
Individuals that differ are poorly adapted
Maintains existing population frequencies of common traits
Discourages species changes
Example: Height variation in humans

5. Directional Selection
Favors traits that are at one extreme of a range of traits
Selects against traits at the other extreme
Favored traits continue to become more extreme over many generations
Examples:
Insecticide resistance
Antibiotic resistance in bacteria

6. Antibiotic Resistance Example
Penicillin and other antibiotics are the chemical products of a living organism (fungus or another bacteria)…a product that cannot evolve
The bacteria we fight with them are evolving, so a resistant bacteria passes on resistant genes to its offspring…now the antibiotic won’t work on the population

7. Antibiotic Resistance Example
How do bacteria develop resistance to an antibiotic?
Mutation in individual bacterium
Conjugation and the sharing of plasmid DNA
What do we do that helps them evolve?
Overuse of antibiotics (lots of opportunity)
Improper use of antibiotics (incompletion of regiment)

8. Disruptive Selection Also called diversifying selection
Occurs when the environment favors extreme or unusual traits
Selects against common traits
Example: weeds
Tall weeds have advantage in wild (compete for sunlight better)
Short weeds have advantage on lawns (lawn mowers)

9. Sexual Selection
Differential mating of males (sometimes females) in a population
Females typically choose males carefully for their superior traits (quality)
Male traits that increase their mating frequency have a selective advantage (quantity)

10. Sexual Selection Two types:
Male Competition: leads to contests of strength that award mating opportunities to the strongest males
Evolution of antlers, horns, large stature, etc.
Female Choice: leads to traits or behaviors in males that are attractive to females
Colorful bird plumage (peacock), mating rituals/dances

11. Artificial Selection
A form of directional selection carried out by humans when they sow seeds or breed animals that possess desirable traits.
Examples:
Various dog breeds selected for a variety of traits that may not confer advantages in the wild
Brussels sprouts, broccoli, cabbage, and cauliflower have all originated from a single species of wild mustard

13. Sources of Variation
Mutations – raw material for new variation, can invent alleles that haven’t existed
Sexual Reproduction – new combinations
crossing over
independent assortment of alleles
Random joining of gametes in fertilization
Diploidy (polyploidy) – 2 or more alleles for each gene
Outbreeding – mating w/ unrelated partners

14. Sources of Variation
Transient Polymorphism – temporary changes in allelic frequencies due to some environmental change
Balanced Polymorphism – the maintenance of different phenotypes in a population
Alleles for advantageous traits increase in frequency in a population…so…
Why do we still have phenotypes in our population that are not the best adapted to our environment?

15. Transient Polymorphism
Peppered Moth Example: Industrial Melanism (Kettlewell)
1848 – pre-industrial revolution, most peppered moths were nearly white, matching the light-colored lichen (England)
By 1948 – post-industrial revolution, soot killed the lichen, revealing darker tree bark underneath…and the frequency of dark moths near London far exceeded the light variety

17. Balanced Polymorphism
Heterozygote Advantage – heterozygous condition results in an advantage over either homozygous condition
Both alleles and all 3 phenotypes are maintained
Example: sickle cell anemia (caused by homozygous recessive genotype), heterozygous genotype confers resistance to malaria…so both alleles are maintained in populations of humans

18. Balanced Polymorphism
Hybrid Vigor (heterosis): describes the superior quality of offspring resulting from crosses between two different inbred strains
Why? – heterozygous hybrids have fewer deleterious homozygous recessive conditions and more heterozygous advantages
Example: hybrid corn (artificial selection)

19. Balanced Polymorphism
Frequency-Dependent Selection (or minority advantage): occurs when the least common phenotypes have a selective advantage
When unusual features give an organism an advantage, it will eventually become common, but will then lose its advantage
Tend to fluctuate between low and high frequencies in a population

20. Neutral Variation
Most variations in a species do not confer selective advantages to individuals that possess them
Example: variations in fingerprint patterns in humans appear to give us no special advantages