4.6: Evolutionary Detectives (Worksheet)

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Introduction

Evolutionary biologists reconstruct evolutionary processes like detectives. They compare the frequency of alleles in a population over time, and use their understanding of evolutionary forces to work out what’s happened to cause any shifts in allele frequencies. In this activity, you will do the same!

Change in a population over time is the result of any combination of the four “forces'' of evolution. But which forces are occurring? We can figure this out by considering the order in which the “forces” typically occur, and work out a narrative - a story - by which this happens.

You can remember the forces of evolution as a sequence of stages:

1. Genes Mutate,
2. Individuals are Selected,
3. Populations Evolve.

Mutations are the only “force” of evolution that adds completely new variation to a population, so it comes first. The other “forces”- Natural Selection, Genetic Drift, and Gene Flow/Admixture - move this variation around in a population.

Instructions

For each of the following cases:

• Identify which forces of evolution are occurring,
• Identify if natural selection is present, and if so, how it is operating, and,
• Put this information into a coherent narrative (story) that explains how, exactly, evolution is taking place. For this step, you will need to identify the sequence by which the forces that you identified are actually occurring.

Quick Recap: Distinguishing Between the Forces

First, consider if the case study involves the introduction of new alleles - either through mutation, or through gene flow/admixture (the transfer of alleles between populations). New alleles means new traits, which can provide natural selection more variation to act upon.

If natural selection is occurring, then individuals with traits that are a better “fit” with the environment will be selected for, or favored. That means individuals with advantageous traits will have an easier time finding food, avoiding predators, surviving to reproductive age, and/or reproducing successfully and sending their traits (and the alleles that control those traits) into the next generation.

If genetic drift is occurring, we will see traits (and the alleles that control them) either increasing or decreasing in frequency, perhaps even being eliminated from a population, due to chance. This may correlate with events like natural disasters (floods, earthquakes, etc.), but also with events like overhunting by humans, or migration into a new area. With genetic drift there is no interaction between traits and the environment: it does not matter if the organism has traits that are a good “fit” with the environment. Those who survive and pass on their genes are the “lucky” ones, not the ones with the best traits.

 

Case #1: The Story of the Peppered Moth

In Great Britain, prior to the 1800s, most peppered moths had wings that were white with black specks. In 1848, a new ‘species’ was spotted: a completely black moth! By the 1950s, one hundred years later, more than 90% of moths were black.

1. What forces of evolution are at work? Tick all that apply.

___ Mutation

___ Natural Selection

___ Genetic Drift

___ Gene Flow

2. Is natural selection involved? If it is, then how, specifically is it operating?

    

    

    

    

    

3. Explain how the forces of evolution shift allele frequencies in the population over time. Your answer should include all of the ‘forces’ of evolution that you identified as being employed (above). Put this information into a narrative to explain how individuals are selected, and how alleles shift over time.

 

Case #2: Sickle-Cell Anemia

As many as 20-30% of people living in equatorial Africa have at least one allele on Chromosome 11 that codes for sickle-cell anemia (they have an, “S” rather than an, “A” allele). This is odd because usually, 80% of people who have two S alleles die before they can reproduce - so why is the allele still around?

1. What forces of evolution are at work? Tick all that apply.

___ Mutation

___ Natural Selection

___ Genetic Drift

___ Gene Flow

2. Is natural selection involved? If it is, then how, specifically is it operating?

    

    

    

    

    

3. Explain how the forces of evolution shift allele frequencies in the population over time. Your answer should include all of the ‘forces’ of evolution that you identified as being employed (above). Put this information into a narrative to explain how new traits are introduced, how individuals are selected (if present), and how alleles in the population shift over time.

 

Case #3: Lactase Persistence

Being able to digest lactose after the age of four is common / uncommon (circle one) around the world. 77% of European Americans and 14% of African Americans have the LCT gene on Chromosome 2, which codes to produce the enzyme lactase, which allows people to digest lactose in adulthood.

1. What forces of evolution are at work? Tick all that apply.

___ Mutation

___ Natural Selection

___ Genetic Drift

___ Gene Flow

2. Is natural selection involved? If it is, then how, specifically is it operating?

    

    

    

    

    

3. Explain how the forces of evolution shift allele frequencies in the population over time. Your answer should include all of the ‘forces’ of evolution that you identified as being employed (above). Put this information into a narrative to explain how new traits are introduced, how individuals are selected (if present), and how alleles in the population shift over time.

 

Case #4: Founding the Colony of Zygozia

Let’s pretend that there is a community called Zygozia. They say that they founded their small island colony in the South Pacific in 1959. When they arrived, there were no people living on the island, and so they became the only inhabitants. They came ‘from the west’, most likely Southeast Asia. The Zygozians of 2018 are unusual in that 80% of them have a rare genetic condition. Populations from Southeast Asia, whom the Zygozians descended from, only show this condition in 3% of their population. What happened?

1. What forces of evolution are at work? Tick all that apply.

___ Mutation

___ Natural Selection

___ Genetic Drift

___ Gene Flow

2. Is natural selection involved? If it is, then how, specifically is it operating?

    

    

    

    

    

3. Explain how the forces of evolution shift allele frequencies in the population over time. Your answer should include all of the ‘forces’ of evolution that you identified as being employed (above). Put this information into a narrative to explain how new traits are introduced, how individuals are selected (if present), and how alleles in the population shift over time.

 

Case #5: Native Americans and Type O Blood

Modern Native Americans have very high frequencies of Type O blood. In some places in North and South America, the frequency is as high as 100%. Anthropologists believe that early Native Americans arrived in North America by crossing over the Bering Land Bridge around 15,000 years ago, from East Asia. Modern East Asian populations, with whom modern Native Americans share ancestry, do not have high frequencies of Type O blood. Instead, they have some of the lowest frequencies of Type O blood in the world.

1. What forces of evolution are at work? Tick all that apply.

___ Mutation

___ Natural Selection

___ Genetic Drift

___ Gene Flow

2. Specifically, how is natural selection involved?

    

    

    

    

    

3. Explain how the forces of evolution shift allele frequencies in the population over time. Your answer should include all of the ‘forces’ of evolution that you identified as being employed (above). Put this information into a narrative to explain how new traits are introduced, how individuals are selected (if present), and how alleles in the population shift over time.

 

Case #6: Malaria in the Tropical Americas

Today, malaria appears throughout the tropical Americas. However, Native American populations only have “normal” hemoglobin (they are 100% homozygous for normal alleles and do not have any hemoglobin variations that protect them from malaria, as is seen in human populations in Africa, Asia and Europe). Why do Native Americans not exhibit any genetic diseases that make them better able to survive malaria?

1. What forces of evolution are at work? Tick all that apply.

___ Mutation

___ Natural Selection

___ Genetic Drift

___ Gene Flow

2. Specifically, how is natural selection involved?

    

    

    

    

    

3. Explain how the forces of evolution shift allele frequencies in the population over time. Your answer should include all of the ‘forces’ of evolution that you identified as being employed (above). Put this information into a narrative to explain how new traits are introduced, how individuals are selected (if present), and how alleles in the population shift over time.

 

Case #7: Skin Color Around the World

Around the world, populations near to the equator and in areas of high altitude have darker skin (their skin produces more melanin). Populations at northern latitudes have lighter skin- their skin produces less melanin.

1. What forces of evolution are at work? Tick all that apply.

___ Mutation

___ Natural Selection

___ Genetic Drift

___ Gene Flow

2. Specifically, how is natural selection involved?

    

    

    

    

    

3. Explain how the forces of evolution shift allele frequencies in the population over time. Your answer should include all of the ‘forces’ of evolution that you identified as being employed (above). Put this information into a narrative to explain how new traits are introduced, how individuals are selected (if present), and how alleles in the population shift over time.

 

The Forces of Evolution Cards

Please print encough cards so that each student group has one of each type: Natural Selection, Mutation, Genetic Drift, and Gene Flow. Print each type on a different colored paper (e.g. Natural Selection cards are all yellow, Gene Flow cards are all Blue).