Chapter 23 Notes: Population Genetics
DEFINITIONS • POPULATION: a localized group of individuals belonging to the same species • SPECIES: a group of populations whose individuals have the potential to interbreed and produce fertile offspring(viable) • GENE POOL: all alleles at all gene loci in all individuals in a population
We all belong to the same gene pool!!! A population of flamingos 6 different species of flamingo
DEFINITIONS • Microevolution = studies how pop’s of organisms change from generation to generation and how new species originate • Macroevolution = studies changes in groups of related species over long periods of geologic time; determines evolutionary relationships among species
Causes of Microevolution: 1) Natural Selection 2) Genetic Drift reduces genetic diversity (changes in the gene pool of a small population due to chance) Examples: -Bottleneck Effect: results from drastic decrease in population size -Founder Effect: few individuals in a population colonize a new habitat
3) Gene Flow (migration of fertile individuals between populations) 4) Mutation (introduces new alleles into a population) 5) Nonrandom Mating (individuals choose mates based upon their traits)
Ways Natural Selection Acts on a Population: 1) Stabilizing Selection: eliminates individuals with extreme or unusual traits; existing population frequencies of common traits are maintained
*Example of Stabilizing Selection in humans: *human babies most commonly weigh 3-4 kg; babies much smaller or larger have higher infant mortality rates.
2) Directional Selection: favors traits at one extreme of a range of traits; common during periods of environmental change Examples: -insecticide resistance -peppered moth (Industrial Melanism)
Peppered Moth (Industrial Melanism) example: • 100 years after the first dark moth was discovered in 1848, 90% of moths were dark; • the light variety continued to dominate in unpolluted areas outside of London.
3) Diversifying (a.k.a. Disruptive) Selection: occurs when environment favors extreme or unusual traits while selecting against common traits
4) Sexual Selection: differential mating of males in a population; leads to sexual dimorphisms -females tend to increase their fitness by increasing the quality of their offspring by choosing superior male mates (and are therefore “choosier” or more selective when finding a mate)
Sexual Selection (cont.) -males increase their fitness by maximizing the quantity of offspring produced **as a result, in vertebrate species, the male is typically the “showier” sex -colorful plumage -lion’s mane -antlers