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Read-to-Learn Study Guide - Evolution and Natural Selection
How did Organisms Become so Diverse?
Evolution is a change in the frequency of genes in a population over time.
Charles Darwin (1809–1882) was a British naturalist who is best known for his contributions to evolutionary theory.
Although others had noted that species evolved over time, Darwin first put forward the theory of natural selection to
explain the process of evolution. According to this theory, certain inherited characteristics give an organism a survival or
reproductive advantage. Organisms pass on these characteristics more often than they pass on other inherited traits.
Example: The species of primates called mandrills have evolved to have bright blue rear ends, because brightly colored
rumps help them attract mates and give them a reproductive advantage. Porcupines evolved to have quills, because
quills help them to avoid predators and reproduce. This gives porcupines a survival advantage.
A characteristic that gives a reproductive advantage helps an organism to mate successfully and pass on its genes to the
gene pool. A characteristic that gives a survival advantage helps an organism to live long enough to reproduce and pass
on its genes.
Reproduction of the Fittest: People often use the phrase “survival of the fittest” instead of “reproduction of the fittest,”
but according to evolutionary theory, survival alone isn’t enough. Creatures need to survive long enough to reproduce.
Reproductive success is measured by how many offspring a creature
produces.
An adaptation is an inherited characteristic that becomes prevalent in a
population because it provides a survival or reproductive advantage. Because
evolution occurs over a long period, an adaptation can remain in a population
even after it has stopped being useful.
Example: Human beings have a genetic preference for fatty foods, which explains
why fried chicken, French fries, and buttery popcorn are so popular. Evolutionary
psychologists say that the preference for fatty foods derives from the days when
people hunted and gathered and food was scarce. Eating high-fat foods was
important because fat gave people the calories they needed. In other words, the
preference for fat was adaptive. Today, in wealthy countries with abundant food and
sedentary lifestyles, the preference for fat remains, despite the fact that it no longer
has beneficial effects. In fact, consuming fatty foods can lead to health problems.
Mutations: Evolution relies on mutations, or small changes in genes.
Mutations happen because of two events that can occur during the
formation of egg and sperm cells:
•An error during copying of DNA
•Random rearrangement of small pieces of DNA in a chromosome pair
Sometimes, a mutation results in a new trait. If the individual with the
mutation reproduces successfully, the mutation will be passed on. If the new
trait proves advantageous, the mutated gene that caused the trait will
increase in the population over a long period and thus propel evolution.
Evidence for Evolution
Paleontology
Paleontology is the study of fossils—the bones, shells, teeth, and other remains of organisms, or evidence of ancient
organisms, that have survived over eons of time. Paleontology supports the theory of evolution because it shows a
descent of modern organisms from common ancestors. Paleontology indicates that fewer kinds of organisms existed in
past eras, and the organisms were probably less complex. As paleontologists descend deeper and deeper into layers of
rock, the variety and complexity of fossils decreases. The fossils from the uppermost rock layers are most like current
forms. Fossils from the deeper layers are the ancestors of modern forms.
Comparative anatomy
More evidence for evolution is offered by comparative anatomy. As
Darwin pointed out, the forelimbs of such animals as humans, whales,
bats, and other creatures are strikingly similar, even though the
forelimbs are used for different purposes (that is, lifting, swimming, and
flying). These similar structures are called homologous structures.
Darwin proposed that similar forelimbs have similar origins, and he used
this evidence to point to a common ancestor for modern forms. He
suggested that various modifications are nothing more than adaptations
to the special needs of modern organisms.
Darwin also observed that animals have structures they do not use.
These useless organs are called vestigial structures. In humans, they
include the appendix, the fused tail vertebrae, the wisdom teeth, and
muscles that move the ears and nose. Vestigial organs may represent structures that have not quite disappeared.
Perhaps an environmental change made the organ unnecessary for survival, and the organ gradually became
nonfunctional and reduced in size. For example, the appendix in human ancestors may have been an organ for digesting
certain foods, and the coccyx at the tip of the vertebral column may be the remnants of a tail possessed by an ancient
ancestor.
Embryology
Darwin noted the striking similarity among embryos of complex animals such as humans, chickens, frogs, reptiles, and
fish. He wrote that the uniformity is evidence for evolution. He pointed out that human embryos pass through a number
of embryonic stages inherited from their ancestors because they have inherited the developmental mechanisms from a
common ancestor. These mechanisms are modified in a way that is unique to an organism's way of life.
The similarities in comparative embryology are also evident in the early stages of development. For example, fish, bird,
rabbit, and human embryos are similar in appearance in the early stages. They all have gill slits, a two-chambered heart,
and a tail with muscles to move it. Later on, as the embryos grow and develop, they become less and less similar.
Comparative biochemistry
Although the biochemistry of organisms was not well known in Darwin's time, modern biochemistry indicates there is a
biochemical similarity in all living things. For example, the same mechanisms for trapping and transforming energy and
for building proteins from amino acids are nearly identical in almost all living systems. DNA and RNA are the mechanisms
for inheritance and gene activity in all living organisms. The structure of the genetic code is almost identical in all living
things. This uniformity in biochemical organization underlies the diversity of living things and points to evolutionary
relationships.
Early Earth and Evolution
Scientists hypothesize that the universe came into existence about 15 billion years ago with a colossal explosion often
referred to as the big bang. The gases and dust from that explosion produced the earliest generation of stars, and over
billions of years, the stars exploded, and their debris formed other stars and planets. Our solar system was presumably
formed in this way 4 to 5 billion years ago. During the next billion years, the molten earth cooled, forming a hardened,
outer crust.
About 3.8 billion years ago, earth's atmosphere consisted of such elements as nitrogen, hydrogen, sodium, sulfur, and
carbon. Some of these elements combined to form hydrogen sulfide, methane, water, and ammonia. Water vapor in this
mist probably caused millions of years of torrential rains, during which the oceans formed. Gas and water from the
earth's core came to the surface through volcanoes. Ultraviolet radiation bathed the earth, and the elements and
compounds interacted with one another to form complex molecules.
In 1953, Stanley Miller and Howard Urey performed a classic experiment in which they circulated methane, ammonia,
water vapor, and hydrogen gas in a closed environment and passed electric sparks through it. After several days, they
discovered that complex compounds of carbon had formed in the mixture. Their experiments indicated that in the
primitive earth atmosphere, complex organic molecules could form, including amino acids, carbohydrates, and nucleic
acids. The theory they expressed is the primordial soup theory. Recent theories about the origin of organic molecules
suggest that these molecules may have formed in hydrothermal vents deep in the oceans, where hot gases and
elements emerge from cracks in the earth's crust. Living organisms have been found near these vents, lending credence
to the theory.
Scientists estimate that at about 3.6 billion years ago, the first cells were in existence. Scientists believe that the first
cells lived within the organic environment of the earth and used organic foods for their energy and were therefore
heterotrophic. The evolution of a pigment system that could capture energy from sunlight and store it in chemical
bonds was an essential breakthrough in the evolution of living things. The organisms that possess these pigments are
commonly referred to as cyanobacteria (once called blue-green algae). These single-celled organisms produce
carbohydrates by the process of photosynthesis, and are said to be autotrophic. In doing so, they produce oxygen as a
waste product. For a period of about 1 billion years, photosynthesis provided oxygen to the atmosphere, which
gradually changed until it became oxygen rich, as it is today
_____ 1. Charles Darwin
_____ 2. Evolution
_____3. Natural Selection
_____4. Mutation
_____5. Homology
_____6. Vestigial Structure
_____7. Gene Pool
_____8. Prokaryote
_____9. Eukaryote
_____10. Heterotrophic
_____11. Autotrophic
_____12. Adaptation
_____13. Stanley Miller
_____14. Organic compounds
_____15. Speciation
_____16. Geographic Isolation
_____17. Reproductive Isolation
a. A change in DNA; results in variation among organisms
b. the inability of a species to breed successfully with related species due to
geographical, behavioral, physiological, or genetic barriers or differences
c. Trait that provides and organism with a survival or reproductive advantage
d. Describes organisms that cannot synthesize its own food; it must consume organic
compounds for nutrients.
e. Developed the theory of evolution through natural selection
f. A population that becomes divided by a physical barrier, allowing each of the
separated groups an independent evolutionary path.
g. Process of change over time by which modern organisms have descended from
ancient organisms
h. Body part in an organism that has lost all or most of its original function in the course
of evolution, such as human appendixes.
i. Describes organisms that can produce nutrients using energy from light
j. Describes a group of organisms that lack a cell nucleus, like bacteria
k. Gradual, nonrandom process by which biological traits become more or less common
in a population as a function of reproductive success; key mechanism of evolution
l. Describes organisms whose cells contain complex membrane-bound organelles and
nucleus, like Protista.
m. a lineage-splitting event that produces two or more separate species
n. Carbon-containing molecules that are essential for life, such as carbohydrates, amino
acids, and nucleic acids.
o. Describes body parts that are alike because the species in question share a common
ancestor, may serve the same or different functions.
p. Performed an experiment to model early earth and the creation of the first organic
compounds essential for life; worked alongside Howard Urey.
q. Complete set of unique alleles in a species’ population.
Putting Vocabulary to Use: Write a complete sentence that connects the meanings of the words below.
Mutation, Adaptation, Competition
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Natural Selection, Variation, Gene Pool
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1. According to the chart below, which species are most closely related? ___________________________
How do you know?_________________________
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2. Based on the chart, a gorilla and a chimpanzee are just as related as which two species? Explain.
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3. Create a phylogenic tree for the organisms listed in the cart below.
4. Why is the biochemical analysis of the proteins of different species such good evidence for the
evolutionary relatedness of organisms?___________________________________________________
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5. Identify each description of organisms as either vestigial, homologous, biochemical, or embryological
evidence for evolution.
a. Similar forelimbs of bats, penguins, lizards, and monkeys ______________________________
b. DNA and RNA comparisons lead to evolutionary (phylogenic) trees ______________________
c. Eyes in a blind fish _________________________________
d. In the earliest stages of development, a tail and pharyngeal pouches can be seen in fish, birds,
rabbits, and mammals. ____________________________________________
Researchers discovered four different species of finches on one of the Galapagos Islands. DNA analysis
showed that these four species, shown in the illustration below, are closely related even though they
vary in beak shape and size. It is thought that they share a common ancestor.
6. Describe how natural selection could have lead to these differences in beak
shapes. Use the words adaptation, survival, variation, competition, and
reproduction in your answer.
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7. A hawk has a genetic trait that gives it much better eyesight than other hawks of the same species in
the same area.
a. Explain how this could lead to competition within the hawk population.
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b. Explain how this could lead to survival of various individuals in the population.
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c. Sketch a graph that shows how the
frequency of the better-eyesight trait
would be expected to change over time
within the population. Be sure to label
your axes!
8. The Florida panther, a member of the cat family, has a population of fewer than 100 individuals and
has limited genetic variation. How will this lack of genetic variation affect these animals’ chance of
survival to a changing environment?
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9. Explain how brightly colored male birds would have a reproductive advantage over not brightly colored
males, and how this could lead to a population with mostly brightly colored males.
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Speciation is a lineage-splitting event that produces two or more separate species.
Imagine that you are looking at a tip of the tree of life that constitutes a species of
fruit fly. Move down the phylogeny to where your fruit fly twig is connected to the
rest of the tree. That branching point, and every other branching point on the tree,
is a speciation event. At that point genetic changes resulted in two separate fruit fly
lineages, where previously there had just been one lineage. But why and how did it
happen?
The branching points on this partial Drosophila phylogeny represent long past
speciation events. Here is one scenario that exemplifies how speciation can
happen:
 The scene: a population of wild fruit flies minding its own
business on several bunches of rotting bananas, cheerfully laying
their eggs in the mushy fruit...

Disaster strikes: A hurricane washes the bananas and the
immature fruit flies they
contain out to sea. The
banana bunch eventually
washes up on an island off the coast of the mainland. The fruit flies
mature and emerge from their slimy nursery onto the lonely island.
The two portions of the population, mainland and island, are now
too far apart for gene flow to unite them. At this point, speciation
has not occurred—any fruit flies that got back to the mainland could
mate and produce healthy offspring with the mainland flies.
The populations diverge: Ecological conditions are slightly
different on the island, and the island population evolves under
different selective pressures and experiences different random
events than the mainland population does. Morphology, food
preferences, and courtship displays change over the course of
many generations of natural selection.

So we meet again: When another storm reintroduces the
island flies to the mainland, they will not readily mate with the
mainland flies since they’ve evolved different courtship
behaviors. The few that do mate with the mainland flies, produce unviable eggs because of other genetic
differences between the two populations. The lineage has split
now that genes cannot flow between the populations.
This is a simplified model of speciation by geographic
isolation, but it gives an idea of some of the processes that
might be at work in speciation. In most real-life cases, we can
only put together part of the story from the available
evidence. However, the evidence that this sort of process
does happen is strong.
1. What is speciation and how can it occur?
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2. Describe how speciation has occurred in this scenario.
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3. When the island flies meet again with the mainland flies are they able to mate? Explain. In your
response include the words courtship, unviable, and geographic isolation.
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4. Label each graph with the type of natural selection it represents: directional, disruptive, or stabilizing.
5. Which graph could result in speciation? Explain. ____________________________________________
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