Types of Selection and Sources of Variation IB Biology Evolution.

Types of Selection and Types of Selection and Sources of VariationSources of Variation

IB BiologyIB Biology

EvolutionEvolution

Types of SelectionTypes 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]

Types of SelectionTypes of Selection

Disruptive selection

Stabilizing SelectionStabilizing 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

Directional SelectionDirectional 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

Antibiotic Resistance ExampleAntibiotic 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

• 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)

Antibiotic Resistance ExampleAntibiotic Resistance Example

Disruptive SelectionDisruptive 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)

Sexual SelectionSexual 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)

Sexual SelectionSexual 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

Artificial SelectionArtificial 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

Sources of VariationSources 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

Sources of VariationSources 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?

Transient PolymorphismTransient 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

Balanced PolymorphismBalanced 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

Balanced PolymorphismBalanced 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)

Balanced PolymorphismBalanced 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

Neutral VariationNeutral 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