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 EQ: Distinguish genetic drift from gene flow and genetic drift in terms of (a) how they occur and (b) their implications for future genetic variation in a population.

Important Vocabulary Locus – (plural = loci) – location of a specific gene on a chromosome– you can kind of 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) at all loci in all individuals 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

The Smallest Unit of Evolution Natural Selection acts directly on individuals  but individuals themselves do not evolve  only populations change overtime Example from book: drought 1,200  180 overtime population had larger beaks  but individual birds beaks did not grow themselves Popular Example: Giraffe’s neck

Microevolution Microevolution: change in allele frequency in a population over time Three main mechanisms cause allele frequency to change: – Natural Selection – aka Adaptive evolution – Genetic Drift – chance events that alter allele frequency change – Genetic Flow – transfer of alleles between populations Only Natural Selection consistently improves match of organisms to the environment (non- random)

Mutation and Sexual Reproduction Natural Selection can only happen if individuals differ in their inherited characteristics Two processes that produce genetic differences: 1.Mutation – must lead to a phenotype change; must be in gamete to be passed along 2.Sexual reproduction – recombines alleles Sexual Reproduction has a more significant impact on creating variation in organisms

Sexual Reproduction Three mechanisms in sexual reproduction lead to shuffling of genes: 1.Crossing Over 2.Independent Assortment of Homologous Chromosomes 3.Fertilization

Mutation Rates Low in plants and animals Even lower in bacteria and viruses but because generation span is so low (2 days for HIV) we see quicker change Viruses w/RNA genome experience even more mutations because they lack RNA repair mechanisms in the lost

H-W used to determine if a population is evolving at a particular locus Variation is needed for evolution, but just because differences exists does not mean evolution automatically occurs H-W Equation can be used to determine if evolution is happening. One way to assess if evolution is occurring is to determine what the genetic make up of a population would be if it were not evolving at a particular locus. – Compare scenarios from real data If there is NO difference in the data then evolution is NOT occurring (H-W Equilibrium) If there ARE differences than evolution IS occurring

Hardy Weinberg Principle Frequencies of alleles and genotypes in a population will remain constant from generation to generation if only Mendelian Segregation (Independent Assortment) and recombination of alleles (fertilization) happen – Such gene pool is said to be in equilibrium Deck of Cards Analogy – No matter how often you reshuffle to deal new hands, the deck itself does not change, more Kings do not appear – The act of reshuffling itself does not change allele frequencies

Hardy-Weinberg Conventions (locus w/2 alleles and 3 genotypes) p represents the frequency of the dominant allele q represents the frequency of the recessive allele Because there are only two alleles p + q = 1 p 2 = homozygous dominant (HH) genotype freq. 2pq = heterozygous (Hh) genotype freq. q 2 = homozygous recessive (hh) genotype freq. Because there are only three genotypes: – p 2 + 2pq + q 2 = 1

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

Conditions for H-W’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 Common for one locus to be evolving while others are not Process can be so SLOW its not detectable

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 Must be in a gamete to be past along 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 no 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 (allele frequency) 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 Individuals become isolated from a larger population  establish new population  likely gene pool differs from original population Accounts for a number of relative high frequency of certain inherited disorders among isolated human populations Bottleneck Effect Sudden reduction in population size due to a change in environment; resulting gene pop 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. Can prevent populations from fully adapting to environment More likely than mutation to alter allele frequency directly

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.Be able to explain the difference between natural selection and genetic drift in the way they occur and the impact they have on the gene pool.

Learning Objectives Learning objective 1.1The student is able to convert a data set from a table of numbers that reflect a change in the genetic makeup of a population over time and to apply mathematical methods and conceptual understandings to investigate the cause(s) and effect(s) of this change. [See SP 1.5, 2.2; Essential knowledge 1.A.1] Learning objective 1.2The student is able to evaluate evidence provided by data to qualitatively and/or quantitatively investigate the role of natural selection in evolution. [See SP 2.2, 5.3; Essential knowledge 1.A.1] Learning objective 1.3The student is able to apply mathematical methods to data from a real or simulated population to predict what will happen to the population in the future. [See SP 2.2; Essential knowledge 1.A.1] Learning objective 1.6The student is able to use data from mathematical models based on the Hardy- Weinberg equilibrium to analyze genetic drift and effects of selection in the evolution of specific populations. [See SP 1.4, 2.1; Essential knowledge 1.A.3] Learning objective 1.7The student is able to justify the selection of data from mathematical models based on the Hardy-Weinberg equilibrium to analyze genetic drift and the effects of selection in the evolution of specific populations. [See SP 2.1, 4.1; Essential knowledge 1.A.3] Learning objective 1.8The student is able to make predictions about the effects of genetic drift, migration and artificial selection on the genetic makeup of a population. [See SP 6.4; Essential knowledge 1.A.3]