Download Lab 11 Microevolution Lab

Microevolution in Action: Ferrets and Finches
In this lab, you’ll explore two different mechanisms of microevolution – natural selection and genetic drift.
This handout reviews important concepts relating to evolutionary change, natural selection, and genetic drift
that were covered in lecture. Importantly, this handout also describes how to calculate allele frequencies, and
provides background on the two populations you’ll study, finches and black-footed ferrets.
LEARNING OBJECTIVES
 Gain a better understanding of how and why genetic drift and natural selection affect allele frequencies of
populations.
 Learn how to assess allele frequencies and genetic diversity of populations.
 Apply your understanding of genetic drift to design an effective conservation strategy for a small population.
 Apply your understanding of natural selection to explain its effects on allele frequencies in a population under
different environmental conditions.
INTRODUCTION
Evolution, Genetic Drift, & Natural Selection
Evolutionary change is defined as alterations in the distribution of heritable traits. As you know, heritable
traits are traits that are determined by genes, and therefore can be passed on from generation to generation.
Microevolution specifically refers to changes that occur within populations. Scientists studying microevolution
frequently assess changes in allele frequencies as well as examining changes in trait distribution. Assessing
both is important for multiple reasons. One reason is that the relative frequencies of alleles in a population
don’t often directly correspond to trait frequencies. Most traits are influenced by multiple genes, and the
influence of a person’s two alleles for a gene on a particular trait varies. Additionally, assessing traits can be
subjective, and traits can also be influenced by environmental factors.
The mechanisms of microevolutionary change include mutations, genetic drift, gene flow, sexual selection,
and artificial selection. All of these mechanisms are important, but for this lab, we’ll focus on just two – natural
selection and genetic drift. Natural selection favors alleles that improve an individual’s ability to survive and
reproduce in its current environment (evolutionary fitness). Since whether or not a trait is beneficial can
change if the environment changes, changes in environmental conditions can often lead to evolution via
natural selection. Genetic drift, on the other hand, refers to changes in the allele frequencies within a
population due to random sampling of alleles. In genetic drift, the frequency of an allele increases (or
decreases) simply because individuals with that allele happened to be lucky (or not), and some eggs and
sperm find each other, and some do not. ALL populations are always under the influence of genetic drift.
However, the allele frequencies of small populations are more likely to be significantly altered by genetic drift
compared to large populations. Two scenarios in which genetic drift can have significant effect on the allele
frequencies of a population are significant reductions in population size, and formation of a new population by
a small group of individuals. Both are situations where the new population is smaller than the initial population
and has only a small sample of the initial population’s alleles.
Allele Frequencies & Gene Pool
The frequency of an allele in a population is simply the total number of that allele in a population divided by
the total number of all alleles for that gene. Or, in other words, the percentage of that allele out of all the
alleles in the population for that gene. For example, imagine a population of 100 diploid individuals. Each
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Adapted from SimBio® Virtual Labs Genetic Drift and Bottlenecked Ferrets
Adapted from http://www.pbs.org/wgbh/evolution/library/01/6/l_016_01.html
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individual has two alleles for a particular gene, so there are 200 total alleles for every gene. To keep things
simple, the hypothetical genes that we’ll focus on in this lab have only two possible alleles. In this situation,
you can determine the frequency of an allele by counting the number of individuals in the population that are
homozygous for the allele and the number of heterozygotes. Each homozygote has two of copies of the allele,
and each heterozygote has one copy of the allele. Let’s say our population was composed of 30 individuals
that were homozygous for allele “A,” 60 heterozygotes, and 10 individuals that were homozygous for the other
allele, allele “B.” The frequency of the “A” allele would be 0.60 (120 copies of allele “A” divided by 200 total
alleles for the gene). The frequency of the “B” allele would be 0.40. By default, if there are only two alleles for
a gene, the frequencies of the two alleles add up to one.
The overall composition of alleles for a particular gene within a population is called the gene pool. In our
hypothetical population, the gene pool would be 120 “A” alleles and 80 “B” alleles. This represents the pool of
alleles that can potentially be passed on to the next generation. How many of each allele makes it to the next
generation depends on many factors including which individuals survive and reproduce (natural and sexual
selection), and which eggs get paired up with which sperm (genetic drift). These factors can change the
frequency of an allele within a population from generation to generation.
Black-Footed Ferrets1
The black-footed ferret is the cutest vicious killer in the world. Also known as the American polecat, this
member of the weasel family relies almost exclusively on prairie dogs for food. Black-footed ferrets were once
found in many parts of North America. However, due to habitat loss and loss of prairie dog populations, the
number of black-footed ferrets declined so severely that in 1979 it was declared extinct. In 1981, a lone
surviving population was found near Meeteetse, Wyoming with about forty individuals. Unfortunately, the
population was hit by both canine distemper and sylvatic plague, two diseases that can kill black-footed ferrets.
Eighteen individuals were taken into captivity for a managed breeding program. In 1986, no individuals were
known to remain in the wild. The captive breeding program was successful enough that wildlife managers
have been able to gradually re-introduce black-footed ferrets into the wild over the past twenty years. Selfsustaining populations have been established in several Southwestern states, and efforts continue to establish
more populations. Not surprisingly, ferrets do better where there are lots of prairie dogs to eat. However,
numbers and food are not the only challenges the black-footed ferret faces. Limited genetic diversity is also a
significant concern. All the members of the species alive today are descended from only seven individuals. If
a population has a high frequency of harmful alleles, or a high frequency of alleles that are not beneficial in
their current environment, this can prevent the population from growing and often cause it to shrink in size. In
lab, you will investigate why this population bottleneck in the black-footed ferret’s recent past may continue to
threaten its survival, and conduct experiments to determine the best way to allocate limited conservation
resources to best preserve what remains of the ferret’s genetic heritage.
Finches2
Galapagos Island finches are a classic example of the effects of natural selection on populations, and how
changes in environmental conditions lead to evolutionary change. Two scientists, Peter and Rosemary Grant,
are famous for their studies of the finch populations that live in the Galapagos. They spent years observing,
tagging, and measuring finches. During their analyses, they also documented environmental changes and
how these changes favored certain individuals within the population. Those individuals survived and passed
their characteristics on to the next generation, illustrating natural selection in action.
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Adapted from SimBio® Virtual Labs Genetic Drift and Bottlenecked Ferrets
Adapted from http://www.pbs.org/wgbh/evolution/library/01/6/l_016_01.html
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For many decades, the Grants and their assistants spent part of each year on the island called Daphne
Major. Each year, they caught, weighed, measured, and identified hundreds of finches, and recorded their
diets of seeds. The scientists focused on the struggle for survival among individuals in two species of finch.
Food type and availability, which is dramatically influenced by year-to-year weather changes, played a
significant role in determining the ability of individual finches to survive and reproduce. For finches, the size
and shape of their beaks affect what types of seeds they are best able to eat. Beak size and shape are
genetically determined and show variation within a population. Birds with the best-suited beaks for the
particular “seed” environment survive and reproduce better, and therefore pass along more of their alleles to
the next generation.
Natural selection had a powerful impact on the finch populations in 1977 when Daphne Major experienced
severe drought. That year, the vegetation withered and seeds of all kinds were scarce. Small, soft seeds were
quickly exhausted by the birds, leaving mainly large, tough seeds that the finches normally ignore because
they are difficult to open. Under these conditions, birds with deep, strong beaks were better able to crack open
the hard seeds. Therefore big-beaked birds lived to reproduce because they just happened to be the ones
favored by the particular set of environmental conditions that year. The Grants found that the offspring of the
birds that survived the 1977 drought tended to have bigger beaks. Thus, the environmental change (drought)
had led to a larger-beaked finch population in the following generation through the force of natural selection.
As you know, which alleles are favored by natural selection can change if the environment changes. Unusually
rainy weather in the mid-1980’s resulted in more small, soft seeds and fewer of the large, tough ones. Smallbeaked birds best-suited to eat the smaller seeds were the ones that survived and produced the most
offspring. Evolution had cycled back the other direction. In lab, you’ll take on the role of finches and see how
natural selection can alter the frequency of “beak gene” alleles within a population under different
environmental conditions.
LAB OUTLINE
Below is a general outline for today’s lab. Specific details for each activity and the Microevolution Lab
Questions will be provided for you in lab. Each person must turn in their OWN assignment before leaving lab.
I.
Quiz
To prepare for this quiz, read this handout carefully AND review your notes from our introduction to
evolution lecture and our discussions in class relating to natural selection and genetic drift.
II. Lab Discussion: Introduction & Overview
III. Genetic Drift and Bottlenecked Ferrets Simulation
Work with your partner to complete the simulation (instructions provided in lab) and answer the related
questions. Each person must turn in their OWN completed assignment.
IV. Natural Selection in Action – Finch Simulation
This is a hands-on activity, and instructions will be provided in lab. Answer the related lab questions as
you go through the activity. Remember that each person must turn in their OWN work.
Don’t forget to turn in your completed Microevolution Lab Questions
before you leave!
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Adapted from SimBio® Virtual Labs Genetic Drift and Bottlenecked Ferrets
Adapted from http://www.pbs.org/wgbh/evolution/library/01/6/l_016_01.html
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