IP5: Hardy-Weinberg/Genetic Drift/Gene Flow EK1A1: Natural Selection is a major mechanisms of natural selection EK1A3: Evolutionary change is also driven by random processes

Important Vocabulary Locus – (plural = loci) – specific location of a gene on a chromosome – you can think of it as a trait (but remember some traits…ie eye color…are controlled by multiple genes) Gene Pool – all the different alleles (letter used to represent a trait/gene) in a population – Human Blood – there are three different alleles (A, B, and O). Most traits have only two alleles. Population – group of organisms living in the same area that are able to reproduce futile offspring; they share the same gene pool

Hardy-Weinberg The Hardy-Weinberg equation is used to determine if a population is evolving at a particular locus – a population may be evolving at one locus but not another The Hardy-Weinberg principle states that the frequencies of alleles and genotypes in a population will remain constant from generation to generation if the gene pool is in equilibrium – If in equilibrium then evolution is not happening – If not in equilibrium then evolution is happening

Conditions for Hardy-Weinberg’s Equilibrium In order for a population to be in HW Eq the following conditions must be met: 1.No mutation 2.Random mating 3.No natural selection 4.Large Population (No Genetic Drift) 5.No gene flow These conditions are rarely met and therefore changes in gene pools are usually taking place

What is the HW Eq and What does it represent? P 2 + 2pq + q 2 = 1 Expected frequency of AA genotype + expected frequency of Aa + expected frequency of aa = 1 AA + Aa + aa = 1

How to Use H-W EQ For a locus with two alleles (A and a) in a population at risk from an infectious neurodegenerative disease, 16 people had genotype AA, 92 had genotype Aa, and 12 had genotype aa. Use the H-W eq to determine whether this population appears to be evolving. To answer this questions we will compare expected frequencies of the alleles in the population to actual frequencies

How to use H-W eq continued Before we can use the HW eq: P 2 +2pq+q 2 = 1 we need to determine p. p represents the total number of A alleles in the population Because 16 individuals are AA and 92 individuals are Aa there are a total of 124 A alleles in the population ( = 124) Remember p is the frequencies of A so to get the frequency we need to divide # of A/total # of A possible. Because there are 120 individuals total ( ) and each has 2 alleles, there are 240 alleles possible (2 X 120) Therefore p = 124/240 =.52 or 52% of the population has at least 1 A allele

We can use the same method to determine what % of the population would carry the a allele. Or because there are only two types of genotypes in the population we know: – p + q = 1 (frequency of A + frequency of a = 100 of the alleles in the population) Therefore if p =.52 then q =.48 How to use H-W eq continued

Now that we know the value of p and q we can plug them into the HW equation to determine our expected frequencies of genotypes: P 2 +2pq+q 2 = 1 – P 2 = (.52) 2 =.27 or 27% of the pop. = AA – 2pq = 2(.52)(.48) =.50 or 50% of the pop. = Aa – q 2 = (.48) 2 =.23 or 23% of the pop. = aa How to use H-W eq continued

Expected % of Genotype – P 2 = (.52) 2 =.27 or 27% of the pop. = AA – 2pq = 2(.52)(.48) =.50 or 50% of the pop. = Aa – q 2 = (.48) 2 =.23 or 23% of the pop. = aa To compare the expected data with the actual data we need to determine out of a population of 120 how many would be expected to have each genotype: – 27% of 120 = about 32 AA – 50% of 120 = about 60 Aa – 23% of 120 = about 28 aa

How to use H-W eq continued Now that we have our expected number of individuals who would have each genotype if the population was at H-W equilibrium we can compare it to the actual number of individuals who have each genotype What can we conclude? Is the population evolving?

Conclusion/Next Steps Due to the differences between the actual genotypes and expected genotypes we can conclude evolution is taking place… …but WHY?

Recall the Five Conditions of HW 1.No mutation 2.Random mating 3.No natural selection 4.Large Population (No Genetic Drift) 5.No gene flow When a population is not in equilibrium we turn to the 5 conditions to determine the reason for the change in gene pool

1. Mutations Mutation: an altered gene (point mutations, frame shift mutations, chromosomal mutations) – modify the gene pool New mutation can alter allele frequencies, but because they are rare, the change from one generation to the next is small – mutation can usually be ruled out as the main cause of the genetic change to the gene pool However, as we learned last week if the mutation provides a significant increase in fitness, due to natural selection that mutant allele can quickly increase in the population’s gene pool

2. Random Mating Nonrandom mating (for example – humans mating dogs that have particular traits together for a desired outcome) can affect the frequencies of homozygous and heterozygous genotypes but by itself usually has not effect on allele frequencies in the gene pool What is the different between genotype frequencies and allele frequencies – Genotype = both alleles (AA, Aa, aa) – Allele = only one letter (A or a)

3. Natural Selection - Nonrandom Adaptive evolution by acting on organisms phenotype; creates a better match between the organism and the environment Happens because a different proportion of genotypes are being passed to future generations due to differential fitness as a result of environmental change Yet another example: – Fruit flies have an allele that provides resistance to insecticides including DDT. Prior to the use of DDT in early 1930s the allele frequency of the resistant allele was 0%. In populations collected after the 1960s (following the use of DDT) the allele frequency was 37%.

4. Genetic Drift - Random Chance events that cause an unpredictable change in a populations gene pool typically effects a small population Tends to reduce genetic variation due to the loss of alleles C W C R C R C W C W C R Generation 1 C R C W C R p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3 Generation 2 C R C W p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 C R

Two main types of Genetic Drift Founders Effect Occurs when a few individuals become isolated from the larger population; allele frequencies in the small founder population can be different fro those in the larger parent population Bottleneck Effect Sudden reduction in population size due to a change in environment; resulting gene poop may no longer be reflective of the original population If population remains small further genetic drift can occur Original population Bottlenecking event Surviving population

Effects of Genetic Drift 1.Genetic drift is significant in small populations. 2.Genetic drift causes allele frequencies to change at random. 3.Genetic drift can lead to a loss of genetic variation within populations. 4.Genetic drift can cause harmful alleles to become fixed.

5. Gene Flow (Immigration and Emigration) - Random Consists of the movement of alleles among population – Insects bring pollen from a different population – Increase of travel – more human diversity Tends to reduce differences between populations over time. More likely than mutation to alter allele frequency directly

More on Natural Selection The natural selection phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals – reproductive success is generally more subtle and depends on many factors Relative fitness - the contribution an individual makes to the gene pool of the next generation Selection favors certain genotypes by acting on the phenotypes of certain organisms

Three modes of Natural Selection 1.Directional Selection: favors individuals at one end of the phenotypic range – Larger animals in extreme cold 2.Disruptive Selection: favors individuals at both extremes of the phenotypic range – Black-bellied seed cracker finches in Cameroon – beak size 3.Stabilizing selection: favors intermediate variants and acts against extreme phenotypes. – Birth weight in humans

Natural Selection Original population (c ) Stabilizing selection (b) Disruptive selection (a ) Directional selection Phenotypes (fur color) Frequency of individuals Original population Evolved population

You should now be able to: 1.Define the terms population, locus, gene pool, and relative fitness. 2.List the five conditions of Hardy-Weinberg equilibrium. 3.Apply the Hardy-Weinberg equation to a population genetics problem. 4.Explain why natural selection is the only mechanism that consistently produces adaptive change. 5.Explain the role of population size in genetic drift. 6.Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection.