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, p 2 + 2pq + q 2 = 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.Bailey & Bailey Reptiles Funded by Title V-STEM grant P031S

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  Polymorphism occurs 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.  C R C R = red; C R C W = pink; C W C W = white  250 red, 100 pink, 200 white  Frequency of C R = (250 x 2) / 1000 = 600/1000 =.6 = 60%

Frequency of Alleles  p is the frequency of the most common allele (C R in this case).  p = 0.6 or 60%  q is the frequency of the less common allele (C W 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  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  The Hardy-Weinberg theorem assumes random mating.  Generation after generation allele frequencies are the same.

Hardy-Weinberg Theorem  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

Hardy-Weinberg Theorem  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

The frequency of black cats is:      Question 1

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Congratulations!  You are correct! Question 1

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

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Congratulations!  You are correct! Question 2

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

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

Practice with Hardy Weinberg  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  We can use the equation and our color observations to calculate allele frequencies in our population of 100 cats.  p 2 + 2pq + q 2 = 1  100 = population size

Practice with Hardy Weinberg  84 of our 100 cats are black.  Black is the dominant phenotype.  Cats with the genotype Bb 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  16 of our 100 cats are white.  White is recessive (bb) and is represented by q 2 in our equation: p 2 + 2pq + q 2 = 1  So, q 2 = 16/100 = 0.16  q = square root of 0.16 = 0.40.

Practice with Hardy Weinberg  q = square root of 0.16 =  Since p + q = 1 ; p = 1 – q =  p 2 = 0.36  p 2 represents the proportion of individuals in the population with the homozygous dominant phenotype (BB).  Remember population size = 100

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

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Congratulations!  You are correct! Question 3

Practice with Hardy Weinberg  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: p 2 + 2pq + q 2 = 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.

Let’s try another! In our population of 100 cats, 75 are black & 25 are white. Where do we start?  75 black cats = p black cats = p 2.  75/100 = 0.75 black cats = p 2. 75/100 = 0.75 black cats = p 2.  25 white cats = q white cats = q 2.  25/100 = 0.25 white cats = q 2. 25/100 = 0.25 white cats = q 2.  Need more information. Need more information. Question 4

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If q 2 = 0.25, q=   5 5   Question 5

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If q=0.5, p=   5 5   Question 6

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So, if p=0.5, and p 2 =0.25, how many of our cats have the BB genotype?     Question 7

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Now, how many of the cats are heterozygous (Bb)?   .5.5  Question 8

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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 remained in Hardy-Weinberg equilibrium.  The population has doubled in size. The population has doubled in size.  There has been a change in allele frequencies: evolution has occurred. There has been a change in allele frequencies: evolution has occurred.  Nothing has changed. Nothing has changed. Question 9

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Congratulations!  You are correct! Question 9