Evolution & Microevolution Tutorial

Evolution & Microevolution Tutorial Introduction Microevolution Hardy Weinberg Equilibrium Practice!

In this tutorial, you will learn: • The difference between macroevolution & microevolution. • How Hardy-Weinberg equilibrium works as well as factors that can upset this equilibrium. • How to use the equation, p2 + 2pq + q2 = 1, to calculate allele frequencies in a population. Credits: Figures and images by N. Wheat unless otherwise noted. Lesser ball python image used with permission from Tim Bailey, Bailey & Bailey Reptiles. Funded by Title V-STEM grant P031S090007.

Introduction • Evolution– includes all of the changes in the characteristics and diversity of life that occur throughout time. • Evolution can occur on both large and small scales.

Macroevolution – Evolution on a Large Scale • Macroevolution– evolutionary change on a grand scale. • Origin of novel designs • Evolutionary trends • Adaptive radiation

Microevolution – Evolution on a Small Scale • Microevolution- a change in the genetic composition of a population over time. • A change in the frequency of certain alleles in a population over several generations.

Polymorphism • Polymorphismoccurs when there are different allelic forms of a gene in a population. • Mojave (left) and Lesser (middle) are different alleles of the same gene. Wild type ball python is shown on the right. Photo courtesy of Bailey & Bailey Reptiles

Gene Pool • All of the alleles of all of the genes possessed by all of the members of the population are contained in the gene pool of the population. • We can measure the relative frequency of a particular allele in a population. • Allelic frequency

Population Genetics • Population Genetics – the study of how populations change over time. • Dependent on both Darwin’s theory of natural selection and Mendel’s laws of inheritance. • All heritable traits have a genetic basis, some are controlled by multiple genes – not as simple as in Mendel’s studies.

Genetic Equilibrium • According to Hardy-Weinberg equilibrium, the hereditary process alone does not produce evolutionary change. • Allelic frequency will remain constant generation to generation unless disturbed by mutation, natural selection, migration, nonrandom mating, or genetic drift. • These are sources of microevolutionary change.

Frequency of Alleles • Each allele has a frequency (proportion) in the population. • Example population of 500 wildflowers. • CRCR = red; CRCW = pink; CWCW = white • 250 red, 100 pink, 200 white • Frequency of CR = (250 x 2) + 100 / 1000 = 600/1000 =.6 = 60%

Frequency of Alleles (2) • p is the frequency of the most common allele (CR in this case). • p = 0.6 or 60% • q is the frequency of the less common allele (CW in this case). • p + q = 1 • q = 1- p = 1 – 0.6 = 0.4 or 40%

Hardy-Weinberg Theorem • Populations that are not evolving are said to be in Hardy-Weinberg equilibrium.

Hardy-Weinberg Theorem (2) • As long as Mendel’s laws are at work, the frequency of alleles will remain unchanged. Review Punnett squares in the genetics tutorial.

Hardy-Weinberg Theorem (3) • The Hardy-Weinberg theorem assumes random mating. • Generation after generation allele frequencies are the same.

Hardy-Weinberg Conditions • Conditions required for Hardy-Weinberg equilibrium to hold true: • Very large population • No gene flow into or out of the population • No mutations • Random mating • No natural selection

Departure Leads to Evolution • Departure from these conditions results in a change in allele frequencies in the population. • Evolution has occurred!

Practice with Hardy Weinberg • Frequency – the proportion of individuals in a category in relation to the total number of individuals. • 100 cats, 75 black, 25 white – frequency of black = 75/100 = 0.75, white =0.25. • Two alleles: p is common, q is less common. • p+q = 1

Question 1 The frequency of black cats is: • 0.75 • 75 • 0.25 • 25 • 100

Question 1 Sorry! • That is incorrect. • Try again!

Question 1 Congratulations! • You are correct!

Question 2 What would the frequency of black cats be if the population size was 80 instead of 100 (still 75 black)? • 0.75 • 75 • 0.94 (75/80) • 1

Hardy-Weinberg Equation • At a locus with two alleles, the three genotypes will appear in the following proportions: • (p + q) x (p + q) = p2 + 2pq + q2 = 1

Practice with Hardy Weinberg • (p + q)2 = p2 + 2pq + q2 Individuals homozygous for allele B Individuals heterozygous for alleles B & b Individuals homozygous for allele b

Practice with Hardy Weinberg (2) • We will use a population of 100 cats as a practice example. • 84 of the 100 cats are black. • 16 are white.

Practice with Hardy Weinberg (3) • We can use the equation and our color observations to calculate allele frequencies in our population of 100 cats. • p2 + 2pq + q2 = 1 • 100 = population size

Practice with Hardy Weinberg (4) • 84 of our 100 cats are black. • Black is the dominantphenotype. • Cats with the genotypeBb or BB will be black. • The frequency of black cats is 84/100, but we can’t yet say anything about the B allele. • See the genetics tutorial to review these terms.

Practice with Hardy Weinberg (5) • 16 of our 100 cats are white. • White is recessive(bb) and is represented by q2 in our equation: p2 + 2pq + q2 = 1 • So, q2 = 16/100 = 0.16 • q =square root of 0.16 = 0.40.

Practice with Hardy Weinberg (6) • q =square root of 0.16 = 0.40. • Sincep + q = 1; p = 1 – q = 0.60. • p2 = 0.36 • p2 represents the proportion of individuals in the population with the homozygous dominant phenotype (BB). • Remember population size = 100

Question 3 So, the number of cats in our population that have the BB genotype would be: • 0.36 cats • 0.36 x 100 = 36 cats • 0.16 x 100 =16 cats • 84 cats

Practice with Hardy Weinberg (7) • Now we know how many of our cats have the BB genotype and the bb genotype. • We can find the number of Bb cats using our equation: p2 + 2pq + q2 = 1. • 2pq represents the proportion of cats with Bb. • 2 x 0.6(p) x 0.4(q) = 0.48 • 0.48 x 100 = 48 cats with Bb genotype.

Question 4 Let’s try another! In our population of 100 cats, 75 are black & 25 are white. Where do we start? • 75 black cats = p2. • 75/100 = 0.75 black cats = p2. • 25 white cats = q2. • 25/100 = 0.25 white cats = q2. • Need more information.

Question 5 If q2 = 0.25, q= • 0.05 • 5 • 0.5 • 50

Question 6 If q=0.5, p= • 0.5 • 5 • 0.6 • 0.1

Question 7 So, if p=0.5, and p2=0.25, how many of our cats have the BB genotype? • 0.25 • 25 • 50 • 75

Question 8 Now, how many of the cats are heterozygous (Bb)? • 48 • 100 • .5 • 50

Question 9 If we measure allele frequency one year at p=0.8 & q=0.2 and then go back 5 generations later to find p=0.5 & q=0.5, what has happened? • The population has remained in Hardy-Weinberg equilibrium. • The population has doubled in size. • There has been a change in allele frequencies: evolution has occurred. • Nothing has changed.

Evolution & Microevolution Tutorial