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