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CHAPTER
2
Evolution I: Mutation and
Natural Selection
Objectives
In this lab we will introduce you to the concepts of genetic mutation and natural selection, the evolutionary mechanisms that produce adaptations. Charles Darwin first conceptualized the process of
evolution by natural selection in the 19th century. Today, scientists understand natural selection much
better because of their knowledge of genetics. Much of this course focuses on understanding adaptations (traits shaped by natural selection) and how they relate to the environment in which the organism lives. For natural selection to occur, traits must have a genetic basis (there must be genetic variation that can be inherited). Your goal in this lab is to learn the basic principles of natural selection,
and the underlying genetic processes that make natural selection possible. You will learn how genetic
variation occurs, the three necessary and sufficient conditions for natural selection (variation in traits,
inheritance of these traits, and differential Darwinian fitness), and the modes of natural selection.
You will also learn how two other types of selection (artificial selection and sexual selection) influence the outcome of evolution.
**As you read this lab, it may be helpful to refer to the illustrations in your textbook.**
Introduction
For thousands of years, people have been observing sexual reproduction among animals and plants and,
although offspring often strongly resemble their parents, they are never exact replicas. Evolution refers to
the process whereby the genetic materials, morphology (i.e., shape and size) and behavior of species change
over time. This is different from “evolution” as it is used in common parlance (e.g., the “evolution” of an idea
or the “evolution” of a car style). We sometimes use the phrase “organic evolution” to make it clear that we
are referring to the biological phenomenon that affects organisms.
Studies of animal and plant breeding, the fossil record, comparative morphology, biogeography (the spatial
and geographic distribution of species—which animals live where), embryology (the stages that developing
embryos pass through, and how they compare to embryos of other species), and comparative genetics all
provide evidence that the concept of organic evolution is real and affects every living thing in the world.
In the days before Charles Darwin, many biologists and philosophers tentatively accepted the concept of
organic evolution, but they lacked an acceptable explanation of its causes. Darwin’s great achievement was
to set forth a theory of “evolution by natural selection” that was logical and subject to testing using empirical
proof (proof by observable, reproducible evidence).
Organic evolution is the result of interacting causes. Mutation creates genetic variation in the biological
world. Natural selection determines what portion of this variation is “fit” to be passed on to future generations. Genetic drift and gene flow influence how the remaining variation is distributed within and between populations. This week, we will concentrate on understanding mutation and natural selection. Next
week, we’ll focus on gene flow and genetic drift and you will learn more about genetic inheritance and its
effect on populations.
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CAUSES OF ORGANIC EVOLUTION
I. Genetic Mutation
A mutation is any change in the genetic material (DNA) of an organism. Without mutation, there would be
no variation, and without variation evolution could not occur. Because mutation is the basis for evolution, you
can’t begin to understand evolution without a basic knowledge of what the genetic material is, what it does,
and how mutations occur.
DNA and the Genetic Code
All living things are made up of one or more cells. Most multi-celled organisms have different kinds of cells
with different functions. Mammals, for example, have skin cells, hair cells, liver cells, heart cells, and blood
cells, along with many other cell types. All of the cells that make up the body of a living organism are known
as somatic cells. Each somatic cell contains the exact same genetic information (all of the information required to make the entire individual), most of which is located on structures called chromosomes, found in
the nucleus of each cell. Sex cells or gametes (eggs and sperm) each contain one half of this information.
Chromosomes are made of paired strands of deoxyribonucleic acid (DNA). DNA strands have a sturdy “backbone” made of individual sugar-phosphate molecules. Each of these backbone molecules is attached to a
single “base molecule.” Base molecules are also known as nucleotides. There are four kinds of nucleotides:
adenine (A), cytosine (C), guanine (G), and thymine (T), and they can be arranged in any order along a
strand of DNA. These nucleotide bases form the primary units of the genetic code.
Each type of base molecule has a unique shape (and chemical composition) that makes it ideally suited to
pairing with one of the other base types. Because of this, A pairs with T and G pairs with C. Each DNA strand
has a “partner strand,” which is its exact opposite (the partner strand will have T instead of A, C instead of
G). The two strands “zip” together, through the pairing of complementary bases. In “zipped” form, the two
strands spiral around one another, in a structure that looks much like a spiral staircase. This structure is
known as the double helix. The double helix has two primary functions: (1) it is structurally strong, helping
to protect DNA strands from breakage; and (2) the pairs of complementary strands provide the necessary
information to replace missing bases or to build a new strand, if the two are separated.
The genetic code can be compared to a military code, used to keep information safe in wartime. The nucleotide bases form the “letters” or “symbols” of the genetic code. The genetic code on each DNA strand can
be broken down into smaller segments. Some of these segments provide the coded instructions for building
proteins. The segments that code for proteins are known as genes. The function of genes is to encode proteins. Genes are the “messages” of the genetic code. But how are the messages “read”?
Much like English, and many other languages, the letters of the genetic code only make sense when they
are divided into “words.” In the genetic code, every “word” is three “letters” (nucleotides) long. These threenucleotide “words” are known as codons. Two other molecules, made of ribonucleic acid (RNA) work together to decode DNA messages. Messenger RNA (mRNA) molecules are the “spies” of the genetics world.
The mRNA molecules copy portions of code from unzipped strands of DNA in the nucleus, and carry the
code out into the cytoplasm (the area between the nucleus and the outer cell membrane). Transfer RNA
(tRNA) molecules are the genetics “code breakers.” When presented with the copied segment of genetic
code, tRNAs “translate” it into a protein by substituting a specific amino acid (the building blocks of proteins) for each codon.
Proteins affect all of the physical and behavioral traits of an individual, including characteristics such as eye
color and aggressiveness. Structural proteins make up the physical structure of each cell in the body. Other
proteins, known as enzymes, conduct the body’s “daily business” by transporting nutrients and hormones,
or catalyzing chemical reactions. All of these proteins must be built correctly, in the right amounts, and at
the proper times, in order for the body to be healthy and function normally.
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Human Evolution Laboratory Manual
Genes form only a small percentage of the DNA found in an individual’s chromosomes. Long segments of
noncoding DNA are interspersed between most genes. As the name implies, these DNA segments do not
code for proteins. Some noncoding segments regulate when and how often certain genes are expressed by
attracting or repelling enzymes that help mRNA to copy the code. Noncoding DNA probably has many other
functions as well. This area of genetics is still being explored.
Mutation: Cause and Effect
Mutations can range from simple base substitutions (e.g., A substituted for C), to large-scale changes, such
as the deletion or insertion of a large segment of DNA, sometimes including one or many genes. The DNA
strand can be damaged by exposure to chemicals or radiation (including ultraviolet radiation from the sun),
or by accidents that occur during normal cell processes (such as replication). There are many kinds of DNA
repair mechanisms in the cell, which involve enzymes that recognize damage and replace lost nucleotides.
Mutations occur when these repair mechanisms fail . . . when the wrong nucleotide is put in place of a missing one, or when there is not enough information to replace a missing segment of DNA (e.g., when the
complementary strand is also damaged).
As you might expect, the impact of these mutations can vary. Base substitutions in coding DNA change individual codons, and this can result in the wrong amino acid being built into the protein. The outcome of
this change in the protein may be serious, if it prevents the protein from functioning properly. Large-scale
deletions can prevent important proteins from being produced at all, and large insertions can cause the protein to be produced twice as often, resulting in too much being produced. Mutations in regulatory areas of
noncoding DNA can have the same effect as large-scale deletions or insertions.
Mutations, Alleles, and Evolutionary Events
Most mutations occur in somatic cells during an individual’s lifetime. Skin cancer and lung cancer are good
examples of somatic cell mutations, caused by DNA damage due to exposure to the sun’s ultraviolet radiation, or inhaled chemicals, such as those found in cigarette smoke. Somatic cell mutations can be devastating to the individual, but cannot be passed on directly to his/her offspring. Only mutations present in the
gametes (eggs or sperm) are passed from one generation to the next.
Any mutation in the gametes (or in gamete-producing cells) can be an evolutionary event because there has
been a change in the genetic material that can be inherited by the offspring. Most of the time, mutations
have negative impacts on the organism, causing poor health or death. Other times, mutations have little
impact, producing proteins that work just as well. In rare instances, however, slight modifications in the structure or abundance of certain proteins turn out to be beneficial, especially as changes in the environment
cause an individual’s needs to change.
When mutation creates multiple forms of a gene, then we call these variants “alleles.” For example, we all
have a gene for eye-color (everyone has irises, of some color), but some of us have an allele that produces
brown eyes, others have a blue-eye allele, others have an allele for green eyes. In each individual, the gene
is located in the same spot on the same chromosome, but slight changes in the code of the gene produce
different eye colors.
Genotype, Phenotype, and Norm of Reaction
The term “genotype” refers to the genetic code carried by an individual. In contrast, the “phenotype” includes the observable morphological, physiological (body chemistry), and behavioral characteristics of an
organism. Using the example of eye color, your eye color genotype is the specific sequence of nucleotides
in your eye color gene. Your eye color phenotype is a certain shade of blue, green, brown, etc.
Chapter 2: Evolution I: Mutation and Natural Selection
13
There is not necessarily a one-to-one correlation between genotype and phenotype, however. A single genotype can produce different phenotypes given different environmental conditions. The term “norm of reaction” means the range of all of the possible phenotypes for a particular genotype under the influence of varying developmental and environmental conditions. Thus, norm of reaction refers to variation of traits that is
not determined by the genes (genotype). For example, humans have a broad norm of reaction for body
weight. Under different environmental conditions, the same person (a constant genotype) could express
wildly different body weights. Certain individual humans have higher norms of reaction for body weight than
others—that is, their weight can vary to a larger degree than other peoples’ weight can. Humans, as a group,
have a narrower norm of reaction for body height than for body weight, and virtually no norm of reaction for
number of fingers per hand.
II. Natural Selection
Natural selection involves traits in individuals being selected for or against by environmental conditions. It
is the process by which individuals with particular heritable alleles (or sets of alleles) reproduce either more
or less than other individuals.
Evolution by natural selection is based on three necessary and sufficient conditions: variation in a trait, genetic inheritance of traits, and differential fitness. All three must be present for natural selection to occur (they
are necessary) and when all three are present, natural selection is occurring (they are sufficient).
1. VARIATION IN A TRAIT
A trait is a feature of an organism’s phenotype (for example, hair color, testosterone levels, behaviors such as the tendency to migrate, and height are traits). Variation in a trait means that for
any given trait there is more than one version present in the genotype, which is expressed in the
phenotype (brown and blond hair, blue and brown eyes, and a continuous range of heights).
2. GENETIC INHERITANCE OF THE TRAIT
Genetic inheritance means that the trait is produced by an allele that is present in the gametes
(i.e., the allele can be passed on genetically from parent to offspring). Traits produced by mutations in the somatic cells do not contribute to natural selection.
3. DIFFERENTIAL FITNESS
If one version of a trait allows a particular individual, compared to the average in the population, to have more offspring that survive to have their own offspring, then that version will become more common in subsequent generations. This trait thus possesses a higher differential
fitness. The word “differential” here means that the fitness is not the same for each version of
a trait in a population.
Darwinian Fitness vs Reproductive Success
In order to understand how natural selection works, we need to understand fitness. An often misused and
misunderstood phrase is “the survival of the fittest,” which was originally coined by Herbert Spencer, a 19th
century social theorist. In biology, “fitness” is a very specific term, which refers to the increase in the frequency of an allele resulting directly from the inherent properties of that allele in preceding generations.
“Survival of the fittest” actually refers to the inheritance of the most beneficial alleles for survival in a given
environment. In biology, it is the “survival” of alleles, from one generation to the next, that matters.
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Human Evolution Laboratory Manual
Fitness, in the evolutionary sense (often referred to as “Darwinian fitness,” for clarity), does not refer to athletic ability or good health, the common meanings of the term “fitness”; these two traits might or might not
be inherited in a given instance. Athletic ability and good health also differ from Darwinian fitness because
these traits might or might not affect reproductive success in a given instance.
Darwinian fitness is related to reproductive success (RS) but is not exactly the same thing. Reproductive
success refers to the number of offspring an individual produces. The difference between RS and differential fitness is that the former is a direct measure of achieved reproduction at the moment, whereas the latter
is the potential genetic contribution of an individual to future generations, given the beneficial or deleterious
nature of the trait in question. Fitness is a quality of the trait while RS is a quality of the individual.
For example, you may have the best variant of a trait that should allow you to produce 20 viable (able to
survive) offspring that produce their own offspring, whereas everyone else only produces on average 5 viable offspring that produce their own offspring. If, however, you happen to slip on ice and break your neck
before you have had a chance to reproduce, your (fittest) variant will not become more common in the next
generation because you had no reproductive success. In this way, RS may be said to equal fitness plus or
minus the effects of random chance.
In the context of a typical biological population, fitness is often correlated with RS. There are situations, however, where producing the maximum number of offspring is not the best way to increase the proportion of
your genes in the next generation of the population. If you have 20 offspring, you have a higher RS than other
members of the population if they only have an average of 5 offspring in their lifetime. If, however, your 20
offspring are all sterile or die before they themselves reproduce due to the fact that you could not feed them,
then you have no fitness despite your high RS. Thus, natural selection optimizes RS in order to maximize
fitness. To account for this distinction, when measuring the fitness of a trait, biologists often measure the
number of grandchildren of an individual who has the trait rather than simply the number of children.
There can be different degrees of fitness based on how an individual uses the difference between optimization and maximization. Those individuals who leave the most surviving descendants in future generations
generally have more “fit” versions of traits in their genotype, and individuals who survive a long time but raise
few or no offspring to maturity have versions of traits with relatively low fitness. Individuals with a high rate
of reproduction but a life span so short that they leave few or no surviving offspring also have versions of
traits with low fitness.
Natural Selection and Adaptations
Natural selection is the force that shapes adaptations. In biology, an adaptation is a version of a trait that is
inherited (it has a genetic component), and is likely to increase its bearer’s fitness (or, among several bearers, such as relatives, will increase average fitness). However, environments rarely stay constant, and the fitness advantages of a particular trait may change if the environment changes. For example, a warm, furry
coat that evolved in a species during an ice age may become a disadvantage if the climate warms.
Not only is there fairly regular global climatic change affecting the organism’s environment, but other important things change as well, including the types, diversity, and numbers of prey or vegetation on which an
organism may feed, or predators that may feed on it, population density, and so on. The versions of traits
that are adaptive for an organism may be constantly shifting, so natural selection may select for different
versions of those traits from one generation to the next.
Units of Natural Selection
Natural selection contains three fundamental units: the individual (whose reproductive success is at stake),
the phenotypic (i.e., morphological, behavioral, or physiological) expression of a trait (which affects the
individual’s fitness), and the alleles (genotype) associated with the production of traits. An individual refers
to an individual organism, NOT the population or species. As you will see, individuals do not behave for
the good of the species or group, but for their own benefit.
Chapter 2: Evolution I: Mutation and Natural Selection
15
Modes of Selection
So far, we’ve been discussing how natural selection works on the individual, the phenotype, or the allele. We
can also consider natural selection in terms of the patterns of population change it produces when it affects
many individuals for a period of time. When a morphological trait exhibits a continuous distribution (e.g.,
height or weight), we can characterize these resulting patterns of population change as one of three modes
of selection: stabilizing selection, directional selection, and disruptive selection. Natural selection works on
the individual, but populations evolve: the modes of selection are descriptions of evolution at the population
level.
Stabilizing selection selects against individuals that have traits that fall on the extreme ends of the range of
variation. This form of selection increases the relative frequency of individuals with the average value of a
given trait. It occurs when the extreme values of a trait (the tallest and the shortest, the lightest and the darkest, etc.) are less fit than the average values. Take wing length in birds as an example. Long wings are good
for efficient flight, but birds with long wings cannot maneuver easily and are more easily caught by predators. Short wings are good for quick turns, but birds with very short wings spend too much energy on flight
and cannot forage efficiently. A particular species of bird will have an optimal wing length owing to the environmental context in which it lives (how far it has to fly to find food, what kinds of predators are a threat).
Despite the continuous introduction of mutations into a population over time for longer or shorter wings, the
average length remains the most common because individuals with too-short or too-long wings have lower
reproductive success than those with average length wings.
Relative frequency of trait in a
population
Pre-Selection Value
Stabilizing Selection
1
2
3
4
5
6
7
Value of Trait (i.e., small to large)
FIGURE 2-1. Stabilizing selection.
Directional selection changes the average value (the mean) of a trait over time. The result is that a new value,
either higher or lower than the original one, becomes the average for the trait in that population. So on the
representation on the graph, the mean actually moves in one direction. Using the example of bird wing
length, the selection pressures change if we remove the predators from the bird’s environment. In the absence of selection against long wings, the individual birds with longish wings might forage more efficiently
than the average bird in this population. Such individuals may leave more offspring than the average bird,
especially in years when the food supply is short. In this way, the population may change over time to become a long-winged form in comparison to the ancestral population.
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Human Evolution Laboratory Manual
Relative frequency of trait in a
population
Pre-Selection Value
Directional Selection
1
2
3
4
5
6
7
Value of Trait (i.e., small to large)
FIGURE 2-2. Directional selection.
Disruptive selection occurs when the average value of a trait is selected against, and individuals at the extreme ends of the distribution have relatively greater fitness and thus increase in relative frequency in the
population over time. Imagine a rodent population with variable, but generally a grayish, coat color that lives
in a heavily vegetated environment. Then imagine the environmental conditions change so that the vegetation becomes rare, and the denuded landscape now consists of open areas of white sand interspersed with
piles of very dark-colored boulders that cast heavy shadows. If this rodent manages to survive, the gray form
of rodent would be visible to predators on the sand and on the dark rocks. If the light-colored forms learned
to forage on the sand and the darkest-colored rodents foraged among the rocks, then both forms would have
behavior that lessened predators’ ability to catch them. Intermediate forms (gray rodents) would experience
increased predation in all contexts, and the gray fur variant of the trait would be less fit. Given enough time,
the ancestral population could split into two different forms—possibly two different species—one light, the
other dark.
Relative frequency of trait in a
population
Pre-Selection Value
Disruptive Selection
1
2
3
4
5
6
7
Value of Trait (i.e., small to large)
FIGURE 2-3. Disruptive selection.
Chapter 2: Evolution I: Mutation and Natural Selection
17
Other Types of Selection
In the research leading up to his famous book, The Origin of Species, Darwin spent many years carefully
observing his neighbors in the English countryside, who were engaged in experiments in artificial selection,
especially pigeon breeders. His observations served as one inspiration for his idea of natural selection (Darwin himself was an avid breeder of pigeons). Artificial selection is a subset of natural selection in which
humans are the agents that control the differential reproductive success of the organism. Artificial selection
is at work when humans manage to produce certain desired traits in livestock and crops by making sure that
individuals with the desired phenotypes mate with one another. In this sense all of our agricultural foods are
genetically modified organisms (GMOs)!
Darwin also noticed that many organisms in nature (those not subject to artificial selection by humans) possessed physical traits (or performed behaviors) that did not seem beneficial for their survival. At first, the
oversized antlers of some deer and the extravagant plumage (feathers) of some birds appear to be deleterious (harmful) traits because the antlers burden the deer’s movements and the plumage of the birds attracts
predators. So, why wouldn’t these traits be eliminated by natural selection?
Darwin reconciled these traits with his theory of natural selection by proposing the additional concept of
sexual selection, as a special type of natural selection. Sexual selection refers to the selection for traits that
give individuals a better chance of gaining access to mate(s), even at the expense of their own survival.
Sexual selection most often involves two selection scenarios: (1) male-male competition or (2) female mate
choice. Some of the examples of sexual selection are rather extreme and the traits often seem to hinder rather
than promote survival of individuals in a population. In these kinds of scenarios, there is a trade-off between
the need to survive and the need to reproduce. Sexual selection also creates most of the differences in size,
physical details, and behaviors between males and females (sexual dimorphism), and understanding these
differences is crucial for our reconstructions of human evolution.
The Scientific Method
Darwin’s principle of “evolution by natural selection” is called a theory. In science, the word “theory” has a
very specific meaning. Like Darwin, modern scientists spend a great deal of time observing the world around
them. Based on their observations, scientists propose hypotheses (educated guesses) about the processes
that cause things they have observed to happen. The next step in the scientific method is to test the hypothesis by trying everything you can think of to prove it wrong. A scientific theory is a hypothesis that has
been tested and re-tested over time, has a great deal of supporting evidence, has proved to have a very strong
predictive value, and has not been disproved so far.
Evolution is a theory in the same sense that gravity is a theory. We can’t prove that gravity exists but, if you
look around, you do not see people floating off the earth and into space just because they do not believe in
gravity. All forms of life on this planet evolved from one single origin through the mechanisms of evolution
(mutation, natural selection, gene flow, and genetic drift). Evolution is always happening and will always continue to happen. As long as organisms reproduce, evolution will affect their populations.
Evidence of Evolution
There is a wide variety of evidence that supports the theory of evolution. People find evidence for evolution
in embryology; in the fossil record; in the biochemistry of living organisms (including the structures of DNA);
and by comparing the anatomies of living organisms.
A striking piece of evidence for the common ancestry of vertebrates (animals who have spinal cords enclosed
within the protection of vertebrae) is the development of embryos of different species. The study of the growth
and development of animals from conception until birth is called embryology. Important mutations are often
those that change the development of embryos; for example, turning off the growth of a tail or extending the
growth of the embryo’s brain for a longer period.
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Human Evolution Laboratory Manual
The more closely related two species are, the more similar their embryos look because their development is
more similar to each other than to other species. Yet, embryos of all vertebrates (fish, amphibians, reptiles,
birds, and mammals) resemble each other to a startling degree. Even those species that no longer need gills
(because they breathe with lungs) still pass through a stage where gill slits develop and the heart has two
chambers (like fish hearts still do). Species that lack tails (like humans) still form tail buds before the development of the tail is shut off. This supports the theory that all vertebrates carry some of the same ancestral
genes, and many of these genes are switched on, at least for a brief period, while the embryo first begins to
form.
HUMAN★
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The fossil record also provides excellent evidence of evolution. Consider the development of the horse, for
example. The earliest known mammals had five toes on each foot. Horses today have specialized toes: each
foot ends in a single toe encased in a hard covering (a hoof), but modern horses still have remnants of vestigial toes, now fused or partly fused to the central remaining toe. The earliest known horse ancestor, a small
animal called Hyracotherium that lived about 54 million years ago (mya), had four toes on its front feet and
three on its hind feet. The sequence of fossil species between this tiny browser (leaf-eater) and the large
grazing (grass-eating) horse that we see today is clear, and the form of horse-like creatures changes through
time.
Umbilicus
d
c
f
e
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f1
PIG★
d
c
a
e
f
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CHICKEN★
Yolk sac
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c
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breaker
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FIGURE 2-4. Embryology.
From The Human Evolution Coloring Book by Adrienne Zihlman. Copyright © 1982
by Coloring Concepts, Inc. Reprinted by permission of HarperCollins Publishers, Inc.
Chapter 2: Evolution I: Mutation and Natural Selection
19
Biochemistry provides comparisons of organisms on the molecular level. For example, proteins whose structure is determined by a specific gene can often be built using one of several alternative amino acids to
achieve the same final structure, shape and function. If the structure of a protein in two species is absolutely identical, however, this means the underlying gene is identical. If two species have a very high frequency of identical proteins, they are very similar at a genetic level. Chimpanzees and humans, for example,
share over 98% of their genes. The most likely explanation for the extremely high level of similarity between
organisms at this molecular level is that they descended from a shared ancestor.
We will use comparative anatomy extensively in this course to consider evolutionary relationships. Simply
focusing on the anatomy of our closest relatives (both living and dead), however, may hide the bigger picture of evolution that a wider look at the animal world reveals. The ancestral form of the forelimb of fossil
fish can be sorted into component bones that made up this limb, and the presence of those same bones—
modified in shape and function, but still recognizably a version of the same set of bones—can be seen in all
descendants of that ancestral species.
Bat
Bird
Early Reptile
Human
Porpoise
FIGURE 2-5. Comparative anatomy. Forelimbs of five tetrapods.
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Human Evolution Laboratory Manual
IN-LAB EXERCISES
Exercise I: Natural Selection Simulation
In this exercise, you will simulate the process of evolution by natural selection. Each group should elect
one person to play the role of the Selector. Each Selector will be provided with a card, indicating which
trait will be under selection for their group (only the Selector should have this information). Your TA will
then provide the drawing of an animal for everyone but the Selectors to copy. When all students at your
table have finished their drawings, the Selector will choose the one that best represents the trait under
selection.
You will then repeat the process, by copying the selected drawing. Each round represents a new generation. Stop after four generations. Compare the “selected” animal from the fourth generation with the
original image provided by the TA (the “ancestral” animal). Can you tell which trait was under selection?
What mode of selection was operating in this case? The Selector can now tell you if you are right. Write
your answers on the worksheet (at the end of the lab). You will hand this in before you leave.
Finally, each group should show their fourth-generation “selected” animal to the rest of the class, and
explain which trait was under selection, and which mode of selection each example represents.
Exercise II: Discussion Questions
Your instructor will assign the following discussion questions for you to work on in groups. Each group will
receive a different question. Write your answers on the worksheet, to hand in at the end of class. As a
group, you will then explain both the assigned question and your answers and conclusions to the rest of
the class.
1. Stevan Arnold has studied garter snakes (Thamnophis elegans) that live in California. The snakes live
both on the coast and at inland sites. Slugs live on the coast, but not inland. In laboratory tests Arnold
found that 68–85% of garter snakes that lived on the coast would eat slugs. Only 17% of inland
snakes would eat slugs. Garter snakes in inland California eat primarily frogs and fish. Arnold collected
pregnant snakes from coastal and inland sites, and the snakes gave birth in the lab. He offered
newborn snakes pieces of slug and recorded the responses. Newborn snakes whose mothers were
from the coast usually attacked the slugs the first time they were exposed to them. Newborns whose
mothers were from inland sites usually did not attack the slugs and starved to death if not given
another type of food.
Based on this information, do you think that this geographic difference in food preference is due to
natural selection? Be sure to consider the three necessary and sufficient conditions for natural selection.
2. Species in which the males and females differ in traits other than those directly concerned with
reproduction are said to be sexually dimorphic. Often, this difference is the result of sexual selection.
In the African swallowtail butterfly, a species perfectly edible to birds, females exhibit three distinct
morphological types, all of which mimic (look like) distasteful species. The male retains its original
coloration because mate choice on the part of females is thought to be so conservative that it selects
against any color change in males.
Chapter 2: Evolution I: Mutation and Natural Selection
21
If a male swallowtail butterfly that mimicked an inedible species arose through mutation, its survivorship would be superior to non-mimetic males (it would be less likely to be eaten by birds). Would the
mutant male’s fitness increase with the improvement in survivorship? Why or why not?
3. Losos, Warheit and Schoner (in Nature, 1997) described the results of an experiment begun in 1977.
Anoles lizards were introduced onto 14 very small islands in the Bahamas that previously had no
native lizards. In their native habitats, anoles lizards that live in areas with large trees have long hind
limbs, and often climb on large branches and tree trunks. Lizards that live where there is smaller
vegetation and narrow branches are smaller and have shorter hind limbs. This is thought to be a
trade-off between speed (long limbs) and agility and maneuverability (short limbs).
Some of the islands onto which the lizards were introduced were so small that they had less than one
square meter (1 m2) of vegetated area; larger islands had as much as 6000 m2 of vegetation, but
none of the islands had tall trees. The experimenters placed 5 or 10 lizards on each island. After 10 to
14 years, the following was observed:
a. Hind limb length in the lizards had decreased in this short period of time relative to the original
population planted on each island.
Do you think that this is a case of natural selection? If yes, explain how the three necessary and
sufficient conditions were met. If no, what else could explain the outcome?
4. Predation may work through the mechanism of natural selection to modify the behavior of prey
species. Bosmina longirostris is a freshwater North American shellfish.
Two kinds of predators feed on B. longirostris:
a. Fish predators that are large enough to eat the eggs as well as the adults, and that locate their
prey by sight.
b. Crustacean predators (related to crayfish) that are small, and can only consume small prey. Soon
after hatching, B. longirostris grow too large to be eaten by crustaceans.
In Frains Lake, Michigan, both fish and crustacean predators are common in the summer (from April
through September). During the summer, B. longirostris females lay large clutches (groups of eggs)
but the eggs are very small and therefore not easily seen by the fish predators. By producing many
hard-to-see eggs, the negative effects of predation by fish are minimized.
During the winter, when crustaceans are the only significant predators, the B. longirostris females lay
fewer eggs per clutch but the eggs have significantly larger yolks. This allows their offspring to grow
quickly beyond the size range that can be handled by the crustaceans. Although fewer eggs are laid
per clutch by each female, each egg has a greater chance of survival due to its size.
Suppose that individual females in this population either lay clutches with many small eggs in the
summer or clutches with fewer, larger eggs in the winter. What mode of selection would this reflect?
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Human Evolution Laboratory Manual
What changes might be expected to occur in this pattern if fish predators somehow became extinct in
Frains Lake?
5. Jake and Tina have just had their first baby, a little girl named Olive. Olive seems healthy but, several
months after Olive’s birth, Tina discovers that she and her mother, Agnes, both have a rare form of
liver cancer. Doctors think that the disease was caused by exposure to a cancer-causing chemical in
the factory where both Tina and Agnes used to work. Tina quit working in the factory over a year
before she became pregnant.
Do you think that Olive will develop the same form of cancer as her mother and grandmother? Discuss
why or why not.
What would have to have happened for Olive to both have the cancer and pass it on to her children?
6. Harold has just bought his first house. The yard is a mess, so he decides to spruce it up by planting
some flowers. Harold is short on cash, so he buys a cheap “grab bag” of flower seed from the clearance rack at the local plant nursery. The bag doesn’t say what kind of seed it contains, but Harold
isn’t picky.
Harold plants the seeds around his house, and they soon sprout and flower. The plants all look the
same, but some have pink flowers and others have blue. Harold notices that the pink flowers are more
common in the shady parts of his yard, and blue is more common in sunny areas.
Develop a hypothesis to explain the difference in flower color, and describe how you would test your
hypothesis, using the scientific method (assume that you can’t get the information by asking someone, or by looking it up online or in a book).
Chapter 2: Evolution I: Mutation and Natural Selection
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Name
Section
LAB WORKSHEET
Evolution I
Exercise I: Natural Selection Simulation
Describe the evolutionary changes that occurred during the four generations. Which trait was under
selection, and which mode of selection was at work?
Exercise II: Discussion Questions
Use the space below to write out your answers to the questions asked in your assigned problem. Try to be
thorough in your answer!
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