Microevolution: Population Genetics and Hardy-Weinberg Equilibrium Chapter 16 Microevolution: Population Genetics and Hardy-Weinberg Equilibrium Mrs. Stewart AP Biology

Bell Work List the evidence (at least 3) that supports the theory of evolution.

Standard CLE 3210.5.3 Explain how genetic variation in a population and changing environmental conditions are associated with adaptation and the emergence of new species.

Objective List examples of the forces that act upon populations to alter the allele frequencies Explain how these forces affect allele frequencies in a population Calculate Hardy-Weinberg problems accurately

Variation Within a Population Populations show variations in their phenotypes: Different shaped beaks Different colors Different athletic abilities Different immune system responses

Sources of Variation Within a Population Variations in the genotypes of a population arise by: mutation – changes in genes that occur either naturally or influenced by environment Passed to offspring if occurs in gametes Recombination – the law of independent assortment (chromosomes) and crossing over during meiosis random pairing of gametes (sexual reproduction) – organisms produce large numbers of gametes, so the union of a particular pair is strictly by chance.

Population Genetics The study of evolution from a genetic point of view Microevolution: change in the allele frequencies of a population

Reminder: Phenotypes = physical appearance of traits Genotypes = Which “variations” (alleles) a person inherited Genes = the section of DNA that codes for a protein. Expression of these genes means those proteins are produced, which directly leads to the physical appearance of the trait (phenotype)

Review: Remember: Genes are alleles R = non-red r = red Question: What color hair would a Rr person have?

Review: Classify each of the following examples a phenotype or a genotype. Huntington’s disease Able to taste PTC (bitter) Carrier of Cystic Fibrosis

Genotype Expression Review: F = freckles, f = no freckles If a gene is inherited as complete dominance, and a person has FF, what is the only protein expressed? If a person has Ff, what protein(s) are expressed?

Genotype Expression Review Part II L = normal lactase; l = abnormal lactase Why is a person who is Ll still able to break down lactose?

Question: How do we know if a person showing the dominant phenotype has two dominant alleles or if they are heterozygous?

The Gene Pool A collection of all the alleles in a population is called the GENE POOL. In other words: All of the possible alleles (variations) that are present, for each gene, within a population Frogs in a pond Trees in the forest People in a town

Allele Frequency Allele frequency is the number of times a specific allele occurs in the gene pool This is in comparison to how often the other alleles occur too

Relative Allele Frequencies determined by dividing the total number of a certain allele by the total number of alleles of all types in the population Total number of a certain allele___ total number of all alleles in a population Expressed as a percentage or a decimal.

Calculate allele frequency

Hardy – Weinberg Allele Frequencies If all diploid organisms have 2 alleles, then we can calculate their frequencies as such: p = frequency of dominant allele q = frequency of recessive allele The frequency of all alleles in a population will add up to 1 p + q = 1

Microevolution is measured by any change in the relative frequency of alleles in a population. Remember: Populations, not individual organisms, evolve over time.

Hardy - Weinberg Genetic mutations Gene flow Genetic drift Allele frequencies in the gene pool do not change unless acted upon by certain forces. Genetic mutations Gene flow Genetic drift Nonrandom mating Natural selection

Hardy – Weinberg Principle The Hardy-Weinberg principle describes a population that is not evolving If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving

Five conditions that affect the relative frequency of alleles This is known as the Hardy Weinberg Genetic Equilibrium model used to determine and understand the forces that act upon genetic equilibrium

1. Mutations Mutations are changes in the DNA.

2. Gene Flow The flow of genes between populations Emigration and immigration cause gene flow between populations and can thus affect gene frequencies.

Immigration vs. Emigration Immigration: Gene flow INTO a population Emigration: Gene flow OUT of a population

Human Evolution from Gene Flow

3. Genetic Drift Genetic drift is a change in allele frequencies due to random events. Genetic drift operates most strongly in small populations.

Think – pair - share What are some other random events that could affect allele frequencies in a population?

Random Mating Do humans randomly mate? No. Random mating: happens more by chance and not by choice (has less effect on allele frequencies)

4. Nonrandom Mating: Sexual Selection Mating is nonrandom whenever individuals may choose partners. Sexual selection occurs when certain traits increase an individual’s success at mating. Sexual selection affects the allele frequencies of a population. Courtship ritual

5. Natural Selection The ongoing process in nature where the presence or absence of certain factors in the environment “select” which traits/variations within a population are most successful Most traits are polygenic = many variations

Without Natural Selection, polygenic traits maintain a bell curve

With Natural Selection… Three general patterns Stabilizing Selection favors the formation of average traits. Disruptive Selection favors extreme traits rather than average traits. Directional Selection favors the formation of one of the extreme traits.

Think – Pair – Share How does evolution by natural selection depend on variation within a population?

Bellwork – which natural selection pattern is represented?

HW Calculations If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then… p2 + 2pq + q2 = 1 p2 = # homozygous dominant individuals q2 = # homozygous recessive individuals 2pq = # heterozygous individuals

Applying the HW principle: We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that: The PKU gene mutation rate is low Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele

Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions The population is large Migration has no effect as many other populations have similar allele frequencies

PKU Problem If the number of babies born with PKU is 1 out of 10,000, then how many people in the population are heterozygous for this trait? p + q = 1 p2 + 2pq + q2 = 1

What do you know? The problem gave you a phenotype which we know is the recessive phenotype. So, 1 in 10,000 = the homozygous recessive = q2

Practice: The occurrence of PKU is 1 per 10,000 births q2 = 0.0001 The frequency of normal alleles is p = 1 – q = 1 – 0.01 = 0.99 The frequency of carriers is 2pq = 2 x 0.99 x 0.01 = 0.0198 or approximately 2% of the U.S. population

Red Hair Practice R = non red, r = red hair If 2% of humans on the planet have red hair, then what percent of humans are heterozygous for this trait?

Red Hair Red hair = q2 q2 = .02 … q = .14 p + q = 1 … p = 1 - .14 = .86 2pq = 2 (.86)(.14) = .24 or 24%

Practice problems

HW Goldfish Lab Each group should obtain the following items: 10 pretzel goldfish 10 cheddar goldfish

Activity Discover the allele frequency for brown (pretzel) fish. Add up the total number of all alleles in your population of fish Brown fish (pretzel) = BB Orange fish (regular) = Bb Yellow fish (parmesan) = bb Divide the number of “brown” alleles by the total number of alleles.

Activity Immigrate: 10 yellow fish move in from a neighboring pond Emigrate: 5 brown fish move out to another pond Recalculate the brown allele frequency Question: How does immigration or emigration affect allele frequencies in a gene pool?

Activity Genetic Drift: Perform a random act to your population Recalculate the brown allele frequency Question: How does genetic drift affect allele frequencies in a gene pool?

Activity What if every fish in your “pond” only wants to mate with an brown fish? What would that do to your allele frequencies?