1 1 Population Genetics

2 2 The Gene Pool Members of a species can interbreed & produce fertile offspring Species have a shared gene pool Gene pool – all of the alleles of all individuals in a population

3 The Gene Pool Allele – the options for a gene; for example - Eyes – blue/brown; Blood type – A/B/O Every organism has TWO alleles for EVERY gene! Dominant alleles (R, T, C, F, etc) – only need one copy of the allele to express the trait (brown eyes, brown hair) Recessive alleles (r, t, c, f, etc) – both alleles must code for the trait in order to have it (blue eyes, blonde hair, red hair) 3

4 4 The Gene Pool Different species do NOT exchange genes by interbreeding Different species that interbreed often produce sterile or less viable offspring e.g. Mule

5 5 Gene Pools Shuffling of alleles by sexual reproduction have no effect on the overall gene pool. gene pool A population’s gene pool is the total of all alleles in the population at any one time (2 x # of organisms).

6 6 Modern Evolutionary Theory GENES are responsible for the inheritance of characteristics POPULATIONS, not individuals, evolve due to natural selection & genetic drift Development of species (SPECIATION) usually is due to the gradual accumulation of small genetic changes

7 7 Microevolution Changes occur in gene pools due to mutation, natural selection, genetic drift, etc. Gene pool changes cause more VARIATION in individuals in the population Example: Bacteria becoming unaffected by antibiotics (resistant)

8 8 The Hardy-Weinberg Theorem non-evolving population. Used to describe a non-evolving population. Natural populations are NOT expected to actually be in Hardy- Weinberg equilibrium.

9 9. The Hardy-Weinberg Theorem Deviation from Hardy-Weinberg equilibrium indicates that evolution has occurred Understanding a non-evolving population helps us to understand how evolution occurs

10 Assumptions of the H-W Theorem 1.Large population size - small populations can have chance fluctuations in allele frequencies (e.g., fire, storm). 2.No migration - immigrants can change the frequency of an allele by bringing in new alleles to a population or by removing them 3.No net mutations - if alleles change from one to another, this will change the frequency of those alleles

11 Assumptions of the H-W Theorem 4. Random mating - If certain traits are more desirable, those organisms will be more desirable as mates 5. No natural selection - If some individuals survive and reproduce at a higher rate than others, then their offspring will carry those genes and the frequency will change for the next generation.

12 Hardy-Weinberg Equilibrium The Hardy-Weinberg Equation: 1.0 = p 2 + 2pq + q = p + q p 2 = frequency of AA genotype; 2pq = frequency of Aa/aA genotype q 2 = frequency of aa genotype P = frequency of A Q = frequency of a Genotype – the alleles of an organism Phenotype – the physical traits of an organism 12

13 Allele Frequencies Define Gene Pools There are 1000 copies of the genes for color, the allele frequencies are (in both males and females): 320x 2(RR)+160x 1(Rr) = 800R; 800/1000 = 0.8 (80%) R 160 x 1 (Rr)+20x 2(rr) = 200 r; 200/1000 = 0.2 (20%) r 500 flowering plants 480 red flowers20 white flowers 320 RR160 Rr20 rr

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15 1. Genetic Drift (Small population) Genetic drift: the change in the gene pool of a small population due to chance Two factors may cause genetic drift: a)Bottleneck effect: A large disturbance removes a large portion of the population so that the surviving population often does not match the allele frequency in the original population. b)Founder effect: A few individuals from a large population colonize an isolated habitat and may have allele frequencies that also do not match the original population 15

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19 3. Mutations Mutation is a change in an organism’s DNA Can be transmitted through sexual reproduction and immediately affect the composition of the gene pool. The original source of variation within a species In stable environments, mutations usually result in little or no benefit to an organism and are frequently harmful. Mutations are more beneficial (which is VERY rare) in changing environments. (Example: HIV resistance to antiviral drugs.) 19

20 4. Nonrandom mating Most organisms do not simply mate with the nearest organism of the opposite gender – nonrandom mating! Assortative mating – organisms tend to mate with others that look similar to themselves – blondes/blondes, no interracial marriages in the South Sexual selection – males compete for females and are chosen based on their ability to compete – peacock feathers, lions’ manes, deer antlers 20

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23 5. Natural selection Natural selection: Higher success in reproduction based on heritable traits that help an organism survive better – their genes get passed on more frequently The only method of change that results in adaptation to environment 23

24 Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resistant 0.00 resistant

25 Natural selection Generation 1: 1.00 not resistant 0.00 resistant Resistance to antibacterial soap

26 Natural selection Resistance to antibacterial soap mutation! Generation 1: 1.00 not resistant 0.00 resistant Generation 2: 0.96 not resistant 0.04 resistant

27 Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resistant 0.00 resistant Generation 2: 0.96 not resistant 0.04 resistant Generation 3: 0.76 not resistant 0.24 resistant

28 Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resistant 0.00 resistant Generation 2: 0.96 not resistant 0.04 resistant Generation 3: 0.76 not resistant 0.24 resistant Generation 4: 0.12 not resistant 0.88 resistant