Random Genetic Drift : Chance as an Evolutionary Force Random Genetic Drift is the random change in allele frequencies from one generation to the next that is caused by the finite size of the breeding population of parents. By chance, some parents have more offspring than other parents. By chance, some parents have fewer offspring or no offspring at all.

Random Genetic Drift occurs in ALL countable, finite populations. RGD is STRONGER in Small Populations (0 < N < 500). RGD is WEAKER in Large Populations (500 < N). The Strength of RGD is proportional to (1/2N) in a diploid population and to (1/N) in a haploid population.

The Strength of RGD is proportional to (1/2N) in a diploid population. RGD is STRONGER in Small Populations (0 < N < 500). If N = 20 diploids, then (1/2N) = 0.025; If N = 20 haploids, then (1/N) = 0.050. RGD is always TWICE as strong for a haploid population of N individuals than a diploid population of the same size. RGD is WEAKER in Large Populations (500 < N). If N = 2,000, then (1/2N) = 0.00025.

RGD and its Evolutionary Consequences 1) WITHIN populations RGD DECREASES GENETIC VARIATION :: Random Genetic Drift makes a population less genetically less variable and makes the individuals in the population more Homozygous. The ultimate outcome: ALL HERITABLE VARIATION IS LOST!! The population becomes either p* = 0.0 (allele is lost) or p* = 1.0 (allele is FIXED). 2) BETWEEN populations RGD INCREASES GENETIC VARIATION: Random Genetic Drift makes two populations become genetically different from one another.

Generation 1 Generation 2 5 5 No Chance Events 5 5 5 5 5 7 Frequencies Stay Exactly the Same 5 5 5 7 Chance Events 5 4 Random Genetic Drift 5 4

Generation 1 Generation 2 Chance Events Outcome 2 5 5 5 6 5 4

Generation 1 Generation 2 Chance Events Outcome 3 5 3 5 6 5 6

Many Possible Outcomes Generation 2 Random Genetic Drift Different Chance Events Lead to Different Outcomes 7 4 Generation 1 4 5 3 5 6 5 6 5 One Starting Point 6 4 Many Possible Outcomes

On Allele Frequencies = ZERO Generation 1 Generation 2 Average of MANY Independent Chance Events 5 5 5 5 5 5 Average Effect of Random Genetic Drift On Allele Frequencies = ZERO Average DP = 0 On AVERAGE Frequencies Stay the Same

Generation 1 Generation 2 5 5 No Chance Events 5 5 5 5 5 Frequencies Stay Exactly the Same 5 5 5 Random Genetic Drift 7 5 4 On AVERAGE Frequencies Stay the Same 5 4

Q: If the frequency of an allele, B, at time 0, equals 0 Q: If the frequency of an allele, B, at time 0, equals 0.65, what is probability that RGD will result in LOSS of the B allele? A) 0.00 B) 0.50 C) 0.65 D) 1.00 E) None of the above

http://www.biology.arizona.edu/evolution/act/drift/frame.html

RGD INCREASES Variation AMONG Populations RGD for A Few Generations With NO Migration START: All Populations Genetically Identical END: Populations Genetically Different from one another

RGD DECREASES Variation WITHIN Populations RGD for Many Generations With NO Migration All Populations Genetically Identical Populations Genetically Different from one another

All Populations Genetically Genetically Different Migration opposes RGD and LIMITS its effects RGD and Migration for Many Generations All Populations Genetically Identical Populations Somewhat Genetically Different

Random Genetic Drift : Effects on Populations 1. On average WITHIN a population, RGD DECREASES genetic variation: A) Random Genetic Drift makes individuals more homozygous, more genetically similar. B) the population becomes genetically less variable. C) RGD causes FIXATION (Pa = 1) or LOSS (Pa = 0) of alleles. D) RGD diminishes the heritable variation within a population for natural selection. 2. On average AMONG populations, RGD INCREASES genetic variation: A) RGD makes different populations genetically more different from one another. 3. GENETIC FIXITY of SPECIES is NOT possible. There is continual change of allele frequencies in finite populations by chance, RGD.

Many Possible Outcomes Generation 2 Random Genetic Drift 7 Different Chance Events 4 Generation 1 4 5 3 5 6 5 6 5 One Starting Point 6 4 Many Possible Outcomes