Evolution of populations Ch 21

I. Background Individuals do not adapt or evolve Populations adapt and evolve Microevolution = change in allele frequencies in a population Requires inherited traits : genetic variability

II Sources of Genetic Variation A. gene mutation….only gamete mutations passed on 1. point mutation : sickle cell anemia 2. may be silent….no Δ amino acid 3. may be neutral…. No change in phenotype a. introns b. amino acid Δ makes no difference 4. may be beneficial or detrimental 5. may be lethal

B. Chromosomal Mutations– change many loci 1. duplication of genes (cross over error) 2. transposable elements a. transposons just move b. retrotransposons leave a copy at original position and move = more copies of gene 3. large changes often harmful, but a few extra copies often good 4. extra copy can take on new function

C. Sexual Reproduction 1. crossing over 2. independent assortment 3. fertilization 2n possible gamete chromosome combinations D. rapid reproduction = higher rate of mutation

III. Hardy-Weinberg Principle A. detects microevolution of particular genes 1. determines what gene frequencies would be if there were NO evolution (null hypothesis) 2. compare null hypothesis with data collected from population 3. null hypothesis supported = no evolution 4. data differs from null hypothesis = evolution

B. Based on Population Gene Pools 1. population = all individuals of a species in an area that reproduce together 2. gene pool = all copies of every allele at every locus in all individuals of a population 3. if all alleles of a gene are the same in the gene pool the gene is said to be fixed 4. frequency of that allele would be 100%

C. Genotype Frequencies 1. If you know the genotypes….. 2. And there are 500 individuals in population 3. # individuals/ total population = % of population with that genotype

D. Allele Frequencies (ƒ) 1. Number of dominant alleles = a. # homozygous dominant x2 plus b. # heterozygous 2. do same for Recessive alleles 3. total frequency = 100% p = dominant alleleƒ q = recessive alleleƒ p + q = 1 (100%)

E. Null Hypothesis : no evolution 1. frequencies of alleles will remain constant from generation to generation determined only by segregation and recombination of alleles 2. so in next generation each allele is equally likely to join with any other allele in offspring 3. mating must be completely random and all allele combinations must survive equally well 4. so we use multiplication rule of probability to predict % of genotypes of offspring

F. Rule of Multiplication (probability 11.9) 1. A is 80% of gene pool so each egg has 80% chance to get an A 2. sperm has same 80% chance so.. Chance of AA indiv. = .8 x .8 or p x p = p2

3. a is 20% of gene pool so each egg has 20% chance to get an a 4. sperm has same 20% chance so.. Chance of aa indiv. = .2 x .2 or q x q = q2

5. The remainder of population is heterozygous 6. There are 2 ways for a zygote to get one of each allele Egg A & sperm a = .8 x .2 (p x q) Egg a & sperm A = .2 x .8 (q x p) So heterozygotes = 2(.2 x .8) or 2pq

G. Hardy-Weinberg Formula = p2 + 2pq + q2 = 1 p2 = frequency of homo dominant q2 = frequency of homo recessive 2pq = frequency of hetero 1 = 100 % of population

H. conditions for Hardy-Weinberg Equilibrium 1. no mutations 2. random mating 3. no natural selection 4. extremely large population 5. no gene flow into/out of population a. immigration b. emigration

IV. Causes of Evolution A. Natural Selection 1. Differential fitness of one allele 2. example a. 0% of fruit flies had DDT resistance in 1930 b. 37% had allele for resistance in 1960 c. null hypothesis rejected d. explanation : new mutation then natural selection favored files with DDT resistance allele.

B. Genetic Drift : Chance events cause random change in allele frequency 1. Small population size maximizes effect a. may decrease genetic variation & fix alleles 2. Founder Effect a. new population started by few individuals b. fewer alleles, different %s than main pop. 3. Bottleneck Effect a. Population nearly wiped out b. few remaining ind. have limited gene pool c. population may recover but genetic variation only recovers very slowly with out immigration

Florida Panther separate subspecies for 100 years Population dropped to under 50 in 1970 Inbreeding increased incidence of disorders kinked tail 88%, but only 27% in other pop.

Desert Bighorn Sheep Island population of desert bighorn sheep 20 founders in 1975 650 in 1999 Genetic variation significantly less than mainland

C. Gene Flow 1. alleles move into or out of population 2. individuals move a. emigration – out b. immigration – in 3. gametes move 4. increases genetic diversity 5. makes populations more similar 6. prevents speciation

Big horn sexual selection http://www.youtube.com/watch?v=E6Fx3CaJhgk