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 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 flamingo s 6 different species of flamingo
The Hardy-Weinberg Theorem: a tool that describes a gene pool of a nonevolving population states that allele frequencies and genotypes in a population s gene pool remain constant over the generations unless acted upon by agents other than sexual recombination
for Hardy-Weinberg equilibrium to occur, the following conditions must be met: 1) Large population 2) No mutation 3) No gene flow (no immigration or emigration) 4) Random mating (no mating preference for particular phenotype) 5) No natural selection (all genotypes have an = chance of surviving & reproducing)
HOWEVER, in nature: 1) most populations are small & may mate with one another 2) there are always mutations (chance with every DNA replication) 3) gene flow often occurs between populations 4) mating is non-random 5) natural selection is always occurring **Therefore, in nature there will always be changes in populations ( microevolution )
**So why study population genetics? Why use the H-W Theorem? 1) shows how genetics is related to evolution; 2) provides a benchmark genetic equilibrium against which change can be noted; 3) permits an estimation of gene frequencies; especially useful in estimating the number of carriers of lethal alleles in human populations. Ex: Brachydactyly - fingers are abnormally short in heterozygotes; condition is fatal during infancy to homozygous recessive individuals due to major skeletal defects
Hardy-Weinberg Equation: p = frequency of dominant allele (A) q = frequency of recessive allele (a) p + q = 1 frequency of possible diploid combinations (AA, Aa, aa): p 2 + 2pq + q 2 = 1 (AA) (Aa) (aa)
Example Problem: If the frequency of a recessive allele is 35% in a population of 1500 people, how many people would you predict would be carriers of this allele, but would not express the recessive phenotype?
Solution: q = 35% = 0.35 p = 1 - q = 1-0.35 = 0.65 p 2 + 2pq + q 2 = 1 freq. of Aa genotype = 2pq = 2(0.65)(0.35) = 0.455 = 45.5% # of carriers = (0.455)(1500) = 683 people
Example Problem: In a population with 2 alleles for a particular locus, B and b, the allele frequency of B is 0.78. If the population consists of 172 individuals, how many individuals are heterozygous? How many will show the recessive phenotype?
Solution: p = 0.78 q = 1 - p = 1-0.78 = 0.22 p 2 + 2pq + q 2 = 1 freq. of Bb genotype = 2pq = 2(0.78)(0.22) = 0.343 = 34.3% # of heterozygotes = (0.343)(172) = 59 individuals
Solution: p 2 + 2pq + q 2 = 1 freq. of recessive phenotype = freq. of bb = q 2 = (0.22) 2 = 0.0484 = 4.84% # of recessive ind. = (0.0484)(172) = 8.3 individuals (8 ind.)
DEFINITIONS Microevolution = studies how pop s of organisms change from generation to generation; changes in allele frequencies in a population s gene pool 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 (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
Bottleneck Effect
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
Peppered Moth 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
Sexual Selection: INTRASEXUAL SELECTION = direct competition among individuals of one sex (males use antlers, aggressive behavior, etc.) INTERSEXUAL SELECTION = mate choice ; individuals of one sex are choosy (usually the females)