Due today – Phylogenetic Trees & Variation & Gene Pools AP Bio Learning Target – Illustrate gene pool dynamics & microevolution Due today – Phylogenetic Trees & Variation & Gene Pools Chapter 21 Reading Guide due Monday

There are 5 agents of evolutionary change Mutation Gene Flow Non-random mating Genetic Drift Selection

Populations & gene pools a population is a localized group of interbreeding individuals gene pool is a collection of alleles in a particular population (remember difference between alleles & genes!) allele frequency is how common that allele is in the population (how many of A or a in whole population)

Evolution of populations Evolution is a change in allele frequencies in a population What conditions would cause allele frequencies not to change? In a non-evolving population you would need to remove all agents of evolutionary change - very large population size (no genetic drift) - no migration (no gene flow in or out) - no mutation (no genetic change) - random mating (no sexual selection) - no natural selection (everyone is equally fit)

Hardy-Weinberg equilibrium In a non-evolving population allele frequencies are preserved (they are said to be in Hardy-Weinberg equilibrium), but: natural populations are rarely in Hardy-Weinberg equilibrium However it provides a useful model to measure if evolutionary forces are acting on a population G.H. Hardy (the English mathematician) and W. Weinberg (the German physician) independently worked out the mathematical basis of population genetics in 1908. Their formula predicts the expected genotype frequencies using the allele frequencies in a diploid Mendelian population. They were concerned with questions like "what happens to the frequencies of alleles in a population over time?" and "would you expect to see alleles disappear or become more frequent over time?" G.H. Hardy mathematician W. Weinberg physician

Hardy-Weinberg theory Counting Alleles assume 2 alleles = B, b frequency of dominant allele (B) = p frequency of recessive allele (b) = q frequencies must add up to 1 (100%), so: p + q = 1 BB Bb bb

Hardy-Weinberg theory Counting Genotypes frequency of homozygous dominant: p x p = p2 frequency of homozygous recessive: q x q = q2 frequency of heterozygotes: (p x q) + (q x p) = 2pq frequencies of all individuals must add to 1 (100%), so: p2 + 2pq + q2 = 1 BB Bb bb

H-W formulas Alleles: p + q = 1 Genotypes: p2 + 2pq + q2 = 1 B b BB Bb

Using Hardy-Weinberg equation population: 100 cats 84 black, 16 white How many of each genotype? First calculate frequency of b from the known number of bb genotypes. BB Bb bb

Using Hardy-Weinberg equation population: 100 cats 84 black, 16 white How many of each allele? q2 (bb)= 16/100 = .16 q (b): √.16 = 0.4 Now work out p p (B)= 1 - 0.4 = 0.6 BB Bb bb

Using Hardy-Weinberg equation population: 100 cats 84 black, 16 white How many of each genotype? p2 + 2pq + q2 = 1 q2 (bb): 16/100 = .16 q (b): √.16 = 0.4 p (B): 1 - 0.4 = 0.6 Now work out genotype frequencies p2=0.36 2pq=0.48 q2=0.16 BB Bb bb

How to Solve H-W Problems (p + q)2 = p2 + 2pq + q2 = 1 B b B b Frequency of allele types p = Frequency of allele B q = Frequency of allele b Frequency of allele combinations p2 = Frequency of BB (homozygous dominant) 2pq = Frequency of Bb (heterozygous) q2 = Frequency of bb (homozygous recessive) BB Bb bb Frequency of allele combination BB in the population = p2 Frequency of allele combination bb in the population = q2 Frequency of allele combination Bb in the population (add these together to get 2pq)

How to Solve H-W Problems Remember to use proportions in your calculations, not percentages! Examine question to determine what information is given. In most cases this is the frequency of the homozygous recessive phenotype q2 or the allele q Take the square root of q2 to find q or multiply q to find q2 Find p by subtracting q from 1 (p = 1 – q) Find p2 by multiplying it by itself (p2 = p x p) Find 2pq by multiplying p x q x 2 Check that your calculations are correct by adding values for p2 + q2 + 2pq (the sum should be 1)

A population of mice has a gene consisting of 90% B alleles (black fur) and 10% b alleles (gray fur). Determine the proportion of offspring that will be black and the proportion that will be gray. Recessive allele q = 0.1 Dominant allele p = Recessive phenotype q2 = Homozygous dominant p2 = Heterozygous 2pq =

A population of mice has a gene consisting of 90% B alleles (black fur) and 10% b alleles (gray fur). Determine the proportion of offspring that will be black and the proportion that will be gray. Recessive allele q = 0.1 Given Dominant allele p = 0.9 1 – q = p 1 – 0.1 = 0.9 Recessive phenotype q2 = 0.01 q x q = q2 0.1 x 0.1 = 0.01 Homozygous dominant p2 = 0.81 p x p = p2 0.9 x 0.9 = 0.81 Heterozygous 2pq = 0.18 2 x p x q = 2pq 2 x 0.9 x 0.1 = 0.18 Check q2 + p2 + 2pq = 1 0.01 + 0.81 + 0.18 = 1

1% of the population will be gray A population of mice has a gene consisting of 90% B alleles (black fur) and 10% b alleles (gray fur). Determine the proportion of offspring that will be black and the proportion that will be gray. Recessive allele q = 0.1 Dominant allele p = 0.9 Recessive phenotype q2 = 0.01 Homozygous dominant p2 = 0.81 Heterozygous 2pq = 0.18 1% of the population will be gray 99% of the population will be black

A population of 134 lizards has 81 individuals with green skin and a gg genotype. The remaining 53 individuals have yellow skin and therefore have either the GG or Gg genotype. What proportion of the population are homozygous dominant? Recessive allele q = Dominant allele p = Recessive phenotype q2 = .60 81/134 = .60 Homozygous dominant p2 = Heterozygous 2pq =

A population of 134 lizards has 81 individuals with green skin and a gg genotype. The remaining 53 individuals have yellow skin and therefore have either the GG or Gg genotype. What proportion of the population are homozygous dominant? Recessive allele q = 0.77 √0.60 = 0.77 Dominant allele p = 0.23 1 - 0.77 = 0.23 Recessive phenotype q2 = 0.60 81/134 = 0.60 Homozygous dominant p2 = 0.05 0.23 x 0.23 = 0.05 Heterozygous 2pq = 0.35 2 x 0.77 x 0.23 = 0.35 0.60 + 0.05 + 0.35 = 1

Hardy Weinberg problems In humans, the ability to taste the chemical phenylthiocarbamide (PTC) is inherited as a simple dominant characteristic. You find that 360 out of 1000 college students could not taste the chemical. What is the frequency of the allele for tasting PTC? What percentage of students in this population are heterogynous?

Hardy Weinberg problems While working with pea plants you find that 24 plants out of 400 exhibit the recessive dwarf trait. What is the frequency of the tall gene? What percentage of the plants have the recessive allele?

Hardy Weinberg problems Albinism is recessive to normal pigmentation in humans. The frequency of the albino allele was 10% in a population. Determine the proportion of people that you would expect to be albino.

Gene Pool Dynamics & Microevolution Aa AA AA Aa Aa AA AA AA Aa AA Aa AA AA

Gene Pool Dynamics & Microevolution aa Aa AA AA Aa aa A’A aa Aa aa AA AA AA Aa AA Aa AA aa AA

Gene Pool Dynamics & Microevolution One aspect of gene flow is immigration & emigration – alleles may be gained from or lost to other gene pools aa Aa AA AA Aa aa aa Aa A’A aa AA AA AA Aa AA Aa AA aa AA Spontaneous mutations can alter allele frequencies and create new alleles. Important to evolution – original source of variation providing new material for natural selection What is the other source of variation in a population?

Gene Pool Dynamics & Microevolution One aspect of gene flow is immigration & emigration – alleles may be gained from or lost to other gene pools aa Aa AA AA Aa aa aa Aa A’A aa AA AA Aa AA Aa AA aa AA Selection pressure against certain allele combinations may reduce reproductive success or cause death. Natural selection accumulates and maintains favorable genotypes, reduces genetic diversity within gene pools, and increases differences between populations

Gene Pool Dynamics & Microevolution Deme 1 aa Aa AA AA Aa aa aa Aa A’A aa AA AA AA Aa AA Aa AA aa AA Deme describes a local population that is genetically isolated from other populations. Usually have clearly definable genetic or other character that sets them apart.

Gene Pool Dynamics & Microevolution Geographical barriers isolate the gene pool and prevent regular gene flow between populations Gene flow between populations can be the source of new genetic variation aa Aa AA aa Aa aa aa Aa aa AA Aa AA Aa AA aa aa AA

Gene Pool Dynamics & Microevolution aa Aa AA aa Aa aa aa Aa aa AA Aa AA Aa AA aa aa AA Mate choice (non-random mating): Individuals may not select mates randomly, seeking particular phenotypes, increasing the frequency of these “favored” alleles in the population.

Gene Pool Dynamics & Microevolution aa Aa AA aa Aa aa aa Aa aa AA Aa Genetic drift: Chance events can cause the allele frequency of small populations to change randomly from generation to generation. Can play a significant role in the microevolution of small populations. The founder effect (small population colonizes new area) and the bottleneck effect (population size dramatically reduced by catastrophic event) AA Aa AA aa AA

Founder effect

Bottleneck effect

Bottleneck effect Cheetahs ~ 20,000 Very little genetic diversity Nearly went extinct at end of last ice age Lack of variation creates problems – sperm abnormalities, decreased fecundity, high cub mortality, sensitivity to disease

Application of H-W principle Sickle cell anemia Caused by inheriting a mutation in the gene coding for haemoglobin oxygen-carrying blood protein recessive allele = HsHs normal allele = Hb low oxygen level causes RBC to sickle clogging small blood vessels depriving tissues of oxygen damage to organs The condition is often lethal

Sickle cell frequency High frequency of heterozygotes 1 in 5 in Central Africans = HbHs unusual for allele with severe detrimental effects in homozygotes 1 in 100 = HsHs usually die before reproductive age Sickle Cell: In tropical Africa, where malaria is common, the sickle-cell allele is both an advantage & disadvantage. Reduces infection by malaria parasite. Cystic fibrosis: Cystic fibrosis carriers are thought to be more resistant to cholera: 1:25, or 4% of Caucasians are carriers Cc Why is the Hs allele maintained at such high levels in African populations? Suggests some selective advantage of being heterozygous…

Malaria Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells 1 2 3

Heterozygote Advantage In tropical Africa, where malaria is common: homozygous dominant (normal) die or reduced reproduction from malaria: HbHb homozygous recessive die or reduced reproduction from sickle cell anemia: HsHs heterozygote carriers are relatively free of both: HbHs survive & reproduce more, more common in population Frequency of sickle cell allele & distribution of malaria

Any Questions??