Assignment: Forces of Evolution

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Assignment: Forces of Evolution

Assignment: Forces of Evolution

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Purpose: To illustrate how multiple forces of evolution act on populations through time.

Learning Objectives:

· Define the driving forces of evolution and identify their effects at the population level

· Understand the necessary conditions of evolutionary (Hardy-Weinberg) equilibrium

· Predict how different forces of evolution influence diversity within a population through time

Lab Activity: Population Genetics & Evolutionary Forces Simulation

The “Modern Synthesis” was a term coined in 1942 by Julian Huxley in a book of the same name. Huxley used the phrase to summarize the tremendous explosion of biological theory and research that began to occur in the 1930’s as Darwin’s theory of Natural Selection was merged with Mendelian principles of heredity. The synthesis is represented by a series of collaborative seminal works from scientists who have since become legends: geneticists such as Haldane, Dobzhansky, and Wright, mathematicians such as Fisher, taxonomists such as Mayr, and paleontologists such as Simpson. At that time, their publications laid the foundation for the basic theories of evolution that are still accepted and applied today, and prompted the famous precept by Theodosius Dobzhansky that “Nothing in biology makes sense except in the light of evolution.”

In the previous lab, we discussed the basic principles of genetic inheritance of traits for individuals. This week’s lab will expand upon your understanding of how genetics works at the population level. In the following exercises, we will do a group activity and use a computer simulation to evaluate the effects of evolutionary forces on populations over multiple generations.

In the context of population genetics, evolution is defined as a change in allele frequency in the gene pool of a population over time. Alleles are separate copies of genes. In most cases, each individual has 2 alleles for every gene, which are known together as the genotype. Evolution is only possible if there is variation in these alleles within a population. Since different alleles may have different effects on the body or behavior of an organism (the organism’s phenotype), changes in the frequencies of alleles can result in phenotypic changes of a population. Certain alleles may be selected out through time or disappear by chance, while others may become more commonplace. Changes in allele frequencies within populations are thought to be responsible for most major patterns of evolutionary change, when magnified by the accumulation of time over many generations.

The Hardy-Weinberg equilibrium is a theoretical model that states that allele frequency and genotype frequency (and by extension, phenotype frequency) will remain constant in a single population that is not subject to the forces of evolution. This model allows us to test how changes in any of these variables would influence the other variables.

The conditions of Hardy-Weinberg equilibrium are:

1. No mutation

2. No migration (gene flow)

3. No selection (all genes/traits equally viable)

4. Random mating is equally likely among all members of the population

5. Large population

Forces of Evolution

Modern evolutionary biologists recognize that four primary forces of microevolution can alter allele frequencies within populations. These include:

1. Mutation – the spontaneous change of one allele into another

2. Gene Flow – a.k.a. migration, the influx/outflow of alleles from/to other populations

3. Genetic Drift – the reduction of allele diversity via random chance (such as genetic bottlenecks and the founder effect)

· Genetic Bottleneck = sudden reduction in population size (e.g., due to natural disasters)

· Founder Effect = a small, random sample of a larger population breaks off to establish a new population (for instance, a small group from the mainland colonizes an island)

4. Selection – a.k.a. natural selection as proposed by Darwin. Conditions of the environment (physical, social, etc.) cause nonrandom elimination of some individuals’ alleles in the next generation, either because they were unable to successfully reproduce, or had no offspring that were able to successfully reproduce.

Exercise 1: Population Genetics (2 pts)

You come upon a tribe of humans that was previously undiscovered. The tribe is called Bretep. It has approximately 30 members and is governed by one chief, Chief Sakon. In the midst of learning everything you can about the groups culture and behavior, you gather some information on the evolutionary history of the group. Chief Sakon tells you that his second son, Ulan, will become the next chief because his first born, Lyzij, decided to leave the tribe and create his own group about 10 years ago. Lyzij took a small subset of the original tribe with him when he left. Chief Sakon hasn’t heard from him since and hopes he is doing well.

1. By leaving the original tribe, Lyzij’s group will be genetically affected by a _____________________ [evolutionary force]. How will the gene pool of this new group likely compare to the gene pool of the original tribe? Why do you expect that? (.75 pts)

You ask Chief Sakon if losing those members of his tribe has hurt the dynamics of the group. He describes a period of time in which the group struggled because their best potters when with Lyzij. However, the past few years have been fine because they found another tribe that was willing to trade goods with them. They meet often and even regularly choose members from this tribe to marry and bring into the Bretep. You are told that Ulan’s wife was originally from this other tribe. Chief Sakon very happily informs you that they have had many strong, healthy children that will help the Bretep prosper in the future.

2. What evolutionary force is acting on the Bretep when they intermarry and reproduce with the other tribe? Will this increase or decrease the genetic diversity of the Bretep? Why? (.75 pts)

Exercise 2: Computer simulation, Evolutionary Forces

(Adapted from: Dr. Jonathan M. Brown, Grinnell College)

About the simulation program – Red Lynx is a browser-based population genetics simulator developed by Reed A. Cartwright, an evolutionary biologist at Arizona State University. It can be accessed at . Once the webpage has loaded, click “Start Red Lynx Simulator” to start the program.

Red Lynx allows you to investigate how the four evolutionary forces affect allele frequency over many generations. In this case, our allele of interest is called “A1”. Once the web page has been started, adjust the number of generations to 1000 (type it into the corresponding box to be exact), and run a number of simulations. If you select the “Help” hyperlink, descriptions for each variable will be displayed. You should see a variety of different outcomes in the frequency of A1. For the most part, the frequency will fluctuate through time, but A1 will still be present in the population – though in rare instances A1 will become either fixed (frequency = 100%) or completely lost (0%).

Getting started:

▪ Press “Clear Graph” and adjust the generations to 1000 by typing the number into the adjacent box.

▪ Play with the existing settings until your simulated population reaches Hardy-Weinberg equilibrium, in which no further evolution is occurring.

1. What happens to the allele frequency in the population once Hardy-Weinberg equilibrium is achieved? (0.5 pts)

2. What adjustments had to be made to the population size? How did changing the population size affect allele frequency equilibrium? (1 pts)

3. Once the population is in Hardy-Weinberg equilibrium, how does the allele frequency change if the initial frequency is adjusted? (0.5 pts)

4. Is Hardy-Weinberg likely to exist in nature? Why or why not? (0.5 pts)

5. Refresh the page and restart Red Lynx to return to the original settings. Mutations are the only source of novel genetic material in biological organisms. The average mutation rate among humans is 2.5e-8 per base per generation (very low!). Type this number into the box for A2 à A1 Mutation Rate.

▪ Be sure the population is set to 800 and the graph is clear.

▪ Set the number of generations to 10 (roughly 200 – 300 years for humans) and run 10 simulations.

What happened to the allele frequency? (0.25 pts)

6. Clear the graph, change the number of generations to 100, and run the simulation 10 times.

▪ Repeat the above instructions with 1000 generations (roughly 20,000 – 30,000 years).

What is happening as time increases? (0.5 pts)

7. What does this tell us about the effect of mutation alone on evolution? (0.5 pts)

8. Keeping the population size at 800 and the mutation rate at 2.5e-8, we will look at the effect of migration.

▪ Set the migration rate to 5% and the migrant allele frequency to 25%

▪ Change generations to 100, clear the graph, and run 5 simulations.

What happens to the allele frequency over time? (0.25 pts)

9. Is the overall variation within the population increasing or decreasing? (0.5 pts)

10. Clear the graph.

▪ Now we will investigate the effects of positive and negative selection. Return the migration rate to 0%.

▪ Set the selection strength to 0.006 and run 10 simulations.

What do you observe? Is the overall variation within the population increasing or

decreasing? (0.5 pts)

11. Now set the selection strength to -0.006 and run 10 simulations. How do the results differ from those in the previous 10 simulations? (0.5 pts)

12. Which one of these two selection scenarios would be more common in nature? Hint: are mutations that have an effect on the phenotype usually beneficial or harmful? Why? (0.5 pts)

 

READING ASSIGNMENT (2 pts): Stock, Jay, T. 2008. Are humans still evolving? EMBO reports Special Issues 9: S51-S54.

Remember that plagiarism will not be tolerated and may result in a score of 0 for the entire lab. If employing a phrase from the reading or elsewhere, you must place it in quotations!

1. Why do humans show less genetic diversity than other species? (.5 pts)

2. Provide two examples of recent human evolution from the article. (.5 pts)

3. Describe what ways animals can respond to environmental stress other than genetic adaptation? What possible future environmental stresses does the author predict? (1 pt)

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