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Evolution, Part II 1 Microevolution • Change in allele frequency of a population – Populations evolve, individuals do not – Happens over time 2 Terms • Allele – Member of a paired gene • Dominant allele – Allele that is expressed when combined with a recessive allele • Recessive allele – Allele that is NOT expressed when combined with a dominant allele • Homozygous – Both alleles the same, AA or aa • Heterozygous – Alleles are different, Aa 3 Terms • Codominance – Both alleles are dominant, AB blood type • Gene Pool – All the alleles in a population 4 Math Explains Allele Frequencies • p +q=1 • p = percent of dominant alleles in a population • q = percent of recessive alleles in a population • If 70% of alleles in a population are dominant then 30% must be recessive 5 Genotype Frequencies • Square the equation p + q = 1 • p2 + 2pq + q2 = 1 • Correlation between genotypes and variables in the equation are: • p2 = AA • 2pq = Aa • q2 = aa 6 Hardy-Weinberg Equations • p +q=1 – Frequency of dominant alleles plus frequency of recessive alleles is 100% ( or 1) • p2 + 2pq + q2 = 1 – AA plus 2Aa plus aa add up to 100% (or 1) • Applies to populations that are not changing – They are in equilibrium 7 Hardy-Weinberg Example • • • • • • • • Normal pigmentation (not albino) = A Albinism recessive = a AA = (p2) = normal Aa = (2pq) = normal aa = (q2) = Albinism 1 in 20,000 people have albinism aa = 1/20,000 = 0.00005 a = 1/141 = 0.00707 8 First Equation • p +q=1 – p is the frequency of the dominant allele, A – q is the frequency of the recessive allele a • p + 0.00707 = 1 • p = 1- q = .9929 9 Second Equation • p2 + 2pq + q2 = 1 • p2 = AA – .9929 x .9929 = .9858 • 2pq = Aa – 2 x .9929 x 0.00707 = .0140 • q2 = aa – .00005 • .9858 + .0140 + .00005 = 0.99985 or 1 10 Practical Application of HardyWeinberg Equations • If you know the frequency of the recessive phenotype (aa) you can calculate the percent of the population that are carriers (Aa) and that are AA. 11 Populations are rarely in HardyWeinberg equilibrium • Most populations are evolving • Factors that cause allele frequencies to change – Nonrandom mating – Genetic drift – Gene flow – Mutation – Natural selection 12 Nonrandom Mating • Most people choose their mates based on – – – – Physical appearance Ethnic background Intelligence Shared interests • One-third of marriages are between people born less than 10 miles apart 13 Nonrandom Mating • Religious & Cultural Influences – Many people will only marry within their own religion or culture – Consanguineous marriages increase risk of birth defects by 2.5 times 14 Genetic Drift • Change in gene frequency when small a group of individuals leave or are separated from a larger population – Founder Effect – Bottleneck 15 Founder Effect Original Population 10 1% has allele A Founders New Population 10% with A allele • 10 people leave to found a new population • 1 of the founders has allele A • 10% of new population will have allele A 16 Ellis-van Creveld Syndrome • • • • Dwarfism Extra fingers Heart defects High frequency in Amish population of Pennsylvania • A founder of the population had allele for the syndrome 17 Bottleneck • Population almost dies out • Survivors genes are at a higher frequency in the descendants than the original population 18 Cheetah Bottleneck • 2 major bottlenecks – 10,000 years ago – 1800’s • Present cheetah are more alike genetically than inbred lab mice 19 Gene Flow • Immigration- when genes move from one population to another • Genes flow between the two populations below OKC Dallas 20 Gene Flow • Can change the frequency of genes in a population • If gene flow stops for a long period of time the two populations may change enough from each other to become new species. 21 No Gene Flow 22 Mutations • Introduces new alleles into a population • Most mutations are lethal – Mutation for no heart would be lethal • Some mutations are beneficial 23 Beneficial Mutation • Mutation for albinism beneficial for bears who live on the ice and snow • Polar bears were once part of a population of brown & black bears • Now polar bears are a separate species 24 Natural Selection • Some individuals are more likely to survive and pass on their genes than others • Nature selects against gene for black fur in the arctic – Black fur does not enable bears in that environment to survive as well • Nature selects against gene for white fur in Oklahoma – White fur is not as advantageous in Oklahoma 25 Tuberculosis • • • • Number 1 killer in 1900 Antibiotics decreased cases dramatically 1980 very few cases Bacterium that causes TB is constantly mutating • Mutant strains resistant to antibiotics are naturally selected to survive 26 Evolution of Tuberculosis Cases of TB 1900 1980 2000 27 Sickle Cell Anemia Frequency • Sickle cell anemia is most common in parts of Africa with malaria • Carriers who live in an environment with malaria have an advantage – Immune to malaria 28 29 Timeline of Evolution • • • • • • • • • 4500 mya- Earth forms 3800 mya- Prokaryotes form 2500- Eukaryotes (algea) form 543- Marine life and vertebrates 408- Amphibians 360- Reptiles 286- Insects 213- Mammals and Birds 5- Early humans 30