Types of Selection and Sources of Variation IB Biology Evolution

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]

Types of Selection Disruptive selection

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

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

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

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)

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)

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)

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

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

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

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?

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

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

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)

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

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