Download 1 Microevolution in Action Lab: Ferrets and Finches In this lab, you`ll

Microevolution in Action Lab: Ferrets and Finches
In this lab, you’ll explore two different mechanisms of microevolution – natural selection and genetic drift.
This handout provides important background on basic concepts of evolutionary change, genetic drift, and
natural selection. Since this lab runs the same week that you’ll cover microevolution in class, this background
information may or may not be review. Importantly, this handout also describes how you’ll be calculating allele
frequencies and the two populations you’ll study, black-footed ferrets and finches. PLEASE MAKE SURE
YOU READ THIS HANDOUT CAREFULLY (and watch the required video) BEFORE LAB!
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 within a population.
 Use what you know about the effects of genetic drift on allele frequencies and genetic diversity to design an
effective conservation strategy for a small population.
 Experience how natural selection can alter 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. Microevolution specifically
refers to changes that occur within populations. Microevolutionary change is the underlying force for many
other levels of evolution, including speciation. As you know, heritable traits are traits that are determined by
genes, and therefore can be passed on from generation to generation. Scientists frequently assess
microevolutionary change by looking at changes in allele frequencies, which is described further in the next
section, as well as examining changes in trait distribution. This is because many traits are subjective;
however, it’s important to keep in mind that the relative frequencies of alleles in a population don’t always (or
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 (e.g. complete dominance versus incomplete
dominance versus codominance). Although the definition of evolution may seem simple, it’s a challenging
concept to grasp. Watch the video below that describes some common misconceptions regarding evolutionary
change. IMPORTANT: Yes, the concepts in the video are fair game for your pre-lab quiz!
http://ed.ted.com/lessons/myths-and-misconceptions-about-evolution-alex-gendler
A common misconception regarding evolutionary change, that was eluded to but not specifically spelled out
in the video, is that the terms “evolution” and “natural selection” mean the same thing – this is NOT true!
Natural selection is only one of the multiple things that can cause evolutionary change. The other 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. 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 chance. 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. Two scenarios in which
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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. It’s also important to note
that ALL populations are always under the influence of genetic drift, however, the allele frequencies of small
populations are more likely to be altered by genetic drift compared to large populations. Just imagine tossing a
coin. If you toss a coin four times, you could certainly imagine getting only four heads (i.e. only passing on
“heads” alleles to the next generation). However, if you tossed a coin fifty times, it would be pretty unlikely to
get fifty heads! You’d most likely get something closer to 50% heads and 50% tails.
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. For example, imagine a population of 100 diploid individuals. Each
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. Just how genetic drift and natural
selection alter allele frequencies in the gene pool is what you’ll be exploring in this lab.
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 have a chance to conduct experiments to determine the best way to allocate limited
conservation resources to best preserve what remains of the ferret’s genetic heritage.
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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.
For many decades, Peter and Rosemary Grant, spent part of each year in a tent on a tiny, barren volcanic
island in the Galapagos called Daphne major. Each year, they caught, weighed, measured, and identified
hundreds of small birds and recorded their diets of seeds. The Grants and their assistants specifically focused
on the struggle for survival among individuals in two species of finch. The struggle is mainly about food;
different types of seeds, and the availability of that food is dramatically influenced by year-to-year weather
changes. For finches, the size and shape of their beaks significantly 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 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 seed 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, the struggle to survive favored birds with deep, strong beaks
that were better able to crack open the hard seeds. Finches with less powerful beaks perished. The bigbeaked 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. Small-beaked birds best-suited
to eat the smaller beaks 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.
REFERENCES
1) Adapted from SimBio® Virtual Labs Genetic Drift and Bottlenecked Ferrets
2) Adapted from http://www.pbs.org/wgbh/evolution/library/01/6/l_016_01.html
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