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The Process of Evolution
The Theory of Evolution
Discoveries in modern science have shown that, over many thousands of years, populations of living
things change. In 1859, the English naturalist Charles Darwin proposed a theory to explain how
organisms change over time. It is called the theory of evolution.
The theory of evolution explains how the immense variety of living things on Earth has developed
from ancestral forms during the past three billion years. This theory is considered to be the most
important unifying idea in biology. It offers an explanation, based on fossil and other scientific
evidence, of how Earth came to be populated by the millions of species (different kinds of organisms)
now alive and of how these species are related to one another. As a result of the changes that occur in
living things and in the environment over time, nearly all species that once lived on Earth are no longer
living today.
A Struggle for Existence
During its lifetime, a female elephant may produce six offspring. Darwin calculated that, over
hundreds of years, millions of elephants could descend from one original pair of elephants. Similarly,
if a plant produced only two seeds a year, in 20 years there could be a million new plants descended
from the original parent plants. However, this does not happen. In fact, only a relatively small number
of offspring of any species survive to produce their own offspring. Darwin realized that Earth cannot
support huge increases in populations; thus, they do not increase that dramatically. He concluded that
there is a “struggle for existence” in which only a few offspring of any type survive to maturity and
reproduce.
In this struggle for existence, there is competition among organisms for various resources. Lack of
food and space are two factors that can limit an organism’s chances to survive and reproduce. Plants
must have minerals, sunlight, and space in order to make their own food and grow. Animals also need
resources such as food, water, space, and shelter in order to survive. Competition for available
resources exists among individuals of the same species and between members of different species
living in the same area.
Genetic Variation
Through the process of reproduction, characteristics are passed on from parents to offspring. The
resulting offspring resemble their parents. However, all the offspring produced by one pair of parents
are not identical. The various characteristics that are passed on from one generation to the next are
called hereditary traits. The differences among the offspring that inherit these characteristics are
called genetic variations.
It is very important to recognize the difference between hereditary, or inherited, characteristics and
changes that occur to an individual during its lifetime. For example, eye color is inherited. It is a
hereditary trait. By contrast, becoming extremely muscular due to weight lifting is an acquired trait. It
is not inherited. A characteristic that is hereditary can be passed along to offspring. An acquired
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characteristic cannot be passed on to offspring. Thus, only changes that are in the sex cells, or
gametes, of the parents can become the basis for evolutionary change.
There are two types of reproduction. In asexually
reproducing organisms, a single parent organism
splits in two to produce two new organisms. (See
Figure 1-2.) In sexually reproducing organisms, a
male and a female mate to produce offspring.
Sexual reproduction involves the combining of
genetic material from two individuals. The traits in
the offspring are the result of a new assortment, or
recombination, of traits inherited from both
parents. Thus, sexual reproduction produces greater
genetic variability among offspring than does
asexual reproduction.
In addition to the recombination of genetic information that occurs during sexual reproduction, genetic
variation can arise from mutations. A mutation is a sudden change that occurs in the genetic material
of an organism. These changes occur randomly and spontaneously, and may be caused by radiation or
chemicals. A mutation in the gametes may produce a small change in the resulting offspring, a major
effect in the offspring, or no noticeable effect in the offspring at all.
Darwin recognized that there is variation among individuals produced by the same parents. He
wondered how the differences within a group of organisms could lead to differences between groups of
organisms. Darwin asked this question about the species of small birds, called finches, that he had
observed on the Galápagos Islands. He realized that there probably were small differences among the
first finches that arrived on the islands from the South American mainland. (See Figure 1-3.)
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How did these minor variations within the ancestral finch species lead to
the significant differences that now exist between the groups of finches?
In other words, how did they develop into separate species? (See Figure
1-4.) Darwin recognized two important facts that play a role in the
development of new species:
 There is a struggle for existence, which limits the number of
offspring that survive.
 There are differences among offspring due to individual,
inherited variations.
Perhaps more important, Darwin posed the question: What determines
which individuals survive to reproduce and thus become the parents of
the next generation of offspring? His answer to this question formed the
basis for his theory of evolution and revolutionized our understanding of
how various forms of life have come to be.
Natural Selection
The special characteristics that make an organism well suited to a
particular environment are called adaptations. How do organisms
evolve the adaptations that enable them to survive so well in a particular
environment?
Darwin attempted to answer this question. He developed an answer by combining what he knew about
the inheritance of traits with what he observed about an organism’s struggle for existence. He
concluded that whatever slight variations an organism had that gave it an advantage over other
individuals in that environment would make it more likely to survive. That is what is meant by
“survival of the fittest.” An organism that was more likely to survive also would be more likely to
reproduce and pass on its genetic variations to future offspring. Those individuals that did not have
such characteristics would be less likely to reproduce and pass on their characteristics. Darwin used the
term natural selection to describe the way that environmental conditions determine which organisms
survive to reproduce. Over time, the proportion of those more fit individuals would increase in a
population. This is because of the increase in frequency of the genes responsible for those traits that
give the surviving individuals the advantage.
Antibiotic Resistance in Bacteria – A Case Study in Natural Selection
Penicillin can kill some of the bacteria that cause diseases in humans, leaving human cells unaffected.
Penicillin was the first antibiotic discovered. Later ,many more antibiotics with the ability to kill
different kinds of bacteria were discovered.
Over time, scientists noticed that some strains of bacteria once killed by antibiotics were no longer
affected. They had developed a resistance to penicillin and some of the other antibiotics. How did the
bacteria develop this resistance? Were there genetic variations that made some bacteria naturally
resistant to the antibiotic, without having prior exposure to
it? (See Figure 1-5.)
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If some bacteria were resistant to antibiotics from the
start, they would have a survival advantage when such
chemicals were added to their environment. In fact,
this is what happened. By killing off nonresistant
bacteria, the antibiotics had decreased the competition
for food that existed in the original population. Many
more resistant bacterial cells could survive, grow, and
reproduce. The result would be a strain of bacteria
that has resistance to antibiotics. Similar results have
been noted in some insects exposed to pesticides;
those that are resistant can survive to reproduce.
These are examples of natural selection at work—
where external factors affect the survival of
individuals within a population.
Artificial Selection
People who raise dogs to perform certain tasks intentionally select, train, and breed those pups that
have the characteristics best suited to their intended function. This selection by people, of organisms
with specific characteristics, is known as selective breeding, or artificial selection. It is a process
similar to natural selection. However, humans—not the natural environment—select the organisms that
have certain desirable traits and decide which ones will survive and pass on those traits to their
offspring. Plant and animal breeders have practiced selective breeding for centuries, resulting in a
variety of domestic livestock breeds and crops that are different from their wild ancestors.
The Scientific Method
Experiments conducted by Darwin, as well as by other researchers, show how knowledge has been
gained in science, namely by the use of the scientific method—an organized approach to problem
solving. The main steps in this process are as follows:
 State the problem (in question form)
 Collect information
 Form a hypothesis
 Perform an experiment: Use experimental group with a variable; use control group without the
variable
 Record observations and data
 Check results; redo experiment
 Draw your conclusions
 Communicate your results
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Review Questions:
1. How did Darwin explain the fact that only a small number of offspring of any species survive to
reproduce?
A. Each species acts to limit the size of its own population.
B. Every species is limited to a certain number of offspring.
C. The members of a species allow only specific offspring to reproduce.
D. Offspring must compete for available resources in order to survive.
2. Which statement best describes competition? It exists
A. only among individuals within the same species
B. only between different species in the same area
C. only between different species living in different areas
D. among individuals of the same species and between different species living in the same
area
3. Suppose two animals live in the same location and eat the same kind of food. What adaptation
would decrease the competition between them?
A. Both animals eat at the same time.
B. Both animals breed at the same time.
C. One animal has hair and the other has feathers.
D. One eats during the day and the other eats at night.
4. Heredity is best described as
A. behavioral differences among offspring
B. the struggle for existence among living things
C. traits that are passed from one generation to the next
D. the slow and gradual change in organisms over millions of years
5. A couple had two children. One child has blue eyes and the other child has brown eyes. This
difference is an example of
A. natural selection
B. artificial selection
C. genetic variation
D. acquired characteristics
6. Which description relates to an acquired characteristic?
A. Jamal is tall and thin.
B. Olivia has curly, blond hair.
C. Brittney has a widow’s peak like her father.
D. Jose has large muscles from doing exercises.
7. What happens during asexual reproduction?
A. Two organisms join together to become one new organism.
B. A single parent organism splits to produce two organisms.
C. Two organisms mate to produce a new single offspring.
D. A single organism forms from the halves of two organisms.
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8. In sexually reproducing organisms, the offspring inherit a combination of genetic traits from the
A. mother only
B. father only
C. mother and father
D. grandparents only
9. When compared to asexual reproduction, sexual reproduction produces
A. less genetic variation among offspring
B. greater genetic variation among offspring
C. offspring that are identical to the parents
D. offspring that are identical to one another
10. A mutation results from
A. artificial selection by humans
B. the fact that only the fittest organisms survive
C. a sudden change in the genetic material of an organism
D. competition for resources such as food and water
11. When mutations occur in body cells, they can be passed along to
A. sex cells only
B. other body cells only
C. offspring only
D. gametes only
12. Which statement best describes the current understanding of natural selection?
A. Natural selection influences the frequency of adaptive traits in a population.
B. Changes in gene frequencies due to natural selection have little effect on evolution.
C. Natural selection has been discarded as an important concept in evolution.
D. New mutations of genetic material are due to natural selection.
13. The Florida panther, a member of the cat family, has a population of fewer than 100 individuals
and has limited genetic variation. Based on this information, which inference would be most valid?
A. The panthers will begin to evolve very rapidly.
B. The panthers can easily adapt to their environment.
C. Over time, the panthers will be less likely to survive in a changing environment.
D. Over time, the panthers will become more resistant to diseases
14. Which statement represents the major concept of the biological theory of evolution?
A. A new species moves into a habitat when another species becomes extinct.
B. Present-day organisms on Earth developed from earlier, different organisms.
C. Every period of time in Earth’s history has its own group of organisms.
D. Every location on Earth’s surface has its own unique group of organisms.
15. Which concept is not a part of the theory of evolution?
A. Present-day species developed from earlier species.
B. Complex organisms develop from simple organisms over time.
C. Some species die out when environmental changes occur.
D. Change occurs according to the needs of an individual organism to survive.
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16. Which statement best illustrates a rapid biological adaptation that has actually occurred?
A. Pesticide-resistant insects have developed in certain environments.
B. Paving large areas of land has decreased habitats for certain organisms.
C. Scientific evidence indicates that dinosaurs once lived on land.
D. The characteristics of sharks have remained unchanged over a long period of time.
17. When a breeder allows only the strongest and fastest horses to reproduce, she is practicing
A. artificial selection
B. natural selection
C. artificial mutation
D. asexual reproduction
18. Unlike in natural selection, in artificial selection
A. genetic information is passed from one generation to the next
B. humans, not the natural environment, decide which organisms will reproduce
C. the natural environment, not humans, decides which organisms will reproduce
D. mating is random and all organisms may pass their traits on to their offspring
19. People can develop new varieties of cultivated plants by carrying out
A. selective breeding for all traits
B. random breeding for all traits
C. selective breeding for specific traits
D. random breeding for specific traits
20. Selective breeding for particular traits can be used to
A. develop cultivated plants only
B. develop domesticated animals only
C. develop cultivated plants and domesticated animals
D. breed rare, wild animal species only
21. Which situation would most likely result in the highest rate of natural selection in a population?
A. reproduction of organisms by an asexual method in an unchanging environment
B. reproduction of organisms in an unchanging environment that has few predators
C. reproduction of organisms that have a very low mutation rate in a changing
environment
D. reproduction of organisms that show genetic differences due to mutations in a changing
environment
22. Some behaviors, such as mating and caring for the young, are genetically determined in most
species of birds. The presence of these behaviors is most likely due to the fact that
A. birds do not have the ability to learn
B. these behaviors helped birds to survive in the past
C. individual birds need to learn to survive and reproduce
D. within their lifetimes, birds developed these behaviors
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23. According to the theory of natural selection, why are some individuals more likely than others to
survive and reproduce?
A. Some individuals pass on to their offspring new characteristics they have acquired
during their lifetimes.
B. Some individuals do not pass on to their offspring new characteristics they have
acquired during their lifetimes.
C. Some individuals are better adapted to exist in their environment than others are.
D. Some individuals tend to produce fewer offspring than others in the same environment.
24. According to modern evolutionary theory, genes responsible for new traits that help a species
survive in a particular environment will usually
A. not change in frequency over time
B. decrease rapidly in frequency
C. decrease gradually in frequency
D. increase in frequency over time
25. The analysis of data gathered during a particular experiment is necessary in order to
A. formulate a hypothesis for that experiment
B. develop a research plan for that experiment
C. design a control for that experiment
D. draw a valid conclusion for that experiment
Base your answer to question 26 on the information and data table below.
A student hypothesized that lettuce seeds would not sprout (germinate) unless they were exposed to
darkness. The student planted 10 lettuce seeds under a layer of soil and scattered 10 lettuce seeds on
top of the soil. The data collected are shown in the table below.
26. One way the student could improve the validity of these results would be to
A. conclude that darkness is necessary for lettuce seed germination
B. conclude that light is necessary for lettuce seed germination
C. revise the hypothesis
D. repeat the experiment
27. Suppose there are two types of fur color, brown and white, in a species of rabbit that lives in an
area with very little snow all year. Most of the rabbits have brown fur. Then the environment
changes so that there is snow much of the year. Based on your knowledge of natural selection, you
might predict that the proportion of white fur to brown fur in the new climate would change so
that
A. equal numbers of rabbits would have brown fur and white fur
B. more rabbits would have white fur than brown fur
C. more rabbits would have brown fur than white fur
D. more rabbits would have white fur with brown splotches
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28. Which concept is best illustrated by the above diagram, which shows changes in the body size and
form of horses over time?
A. acquired characteristics
B. artificial selection
C. partial inheritance
D. evolution by natural selection
29. How is the “struggle for existence” important to the study of evolution? Give an example.
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30. In terms of evolution, why are the variations among individuals within a population more
important than the similarities between them?
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31. Explain why sexual reproduction is this important for the process of evolution than asexual
reproduction.
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32. Briefly explain why mutations are important to evolutionary change.
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33. Give two possible causes of genetic mutations.
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(2) _______________________________________________________________
34. What cells would these mutations have to occur in to be passed along to offspring? Explain.
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