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Content Benchmark L.12.D.5
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Life Science
Structure of Life
Organisms and Their Environment
Diversity of Life
Content Areas
Nature of Science (NOS)
Life Science
Earth Science
Physical Science

Students know that biological evolution explains the diversity of life. E/S

Biodiversity refers to the variety of organisms and their genetic diversity on our planet. Today over 1.5 million living and 300,000 extinct species have been named and described. The actual number of species on our planet is not known, but some believe it may be at least 15 to as many as 30 million species. With this tremendous amount of diversity the question becomes “Why or how are there so many forms of life on our planet?” The answer to this question is evolution. In 1973, Theodosius Dobzhansky wrote that “Nothing in biology make sense except in the light of evolution.” Today biologists use the process of evolution to explain the diversity of life on our planet.

Evolution or descent with modification explains how the gene pools of species or populations change over time leading to the development of new species and therefore diversity of life. Biologists have described several mechanisms that lead to changes in gene pools over time. They include:

a) Mutations
b) Gene recombination
c) Gene flow
d) Genetic drift
e) Natural Selection

Mutations refer to the genetic errors that accumulate in the genes within species and/or populations. Mutations that prove useful or allow organism to “better adapt” to their environment are rare and their accumulative effect is minor compared to the other factors, nevertheless mutations do allow for entirely new genes or DNA instructions to occur.

Recombination of genes in a sexually reproducing population is more important than mutations in producing variations that make adaptation possible. In sexual reproduction, individuals with a different array of genes are brought together. In this way some individuals in the population may be better adapted to the environment than others.

Gene flow results in the movement of genes between populations. Genes maybe introduced or removed from populations by immigration or emigration. The effect is to change the genetic composition by the addition or removal of genes. As compared to other factors, gene flow has a minor effect on gene pool composition.

Genetic drift results in the chance change in the gene pool. Genetic drift involves the random transfer of alleles from one generation to the next. This change can occur by what is known as a bottleneck or founder’s effect. In both cases alleles are passed on to the succeeding generations as a matter of chance rather than “fitness”.

A bottleneck occurs where a large population of organisms is reduced to a very few, which in turn grows back into a much larger population. The northern elephant seal is often used as an example of a bottleneck. Prior to 1890 there were tens of thousands of these seal along the Pacific Ocean coastline from Baja California to Alaska. However these animals were hunted to near extinction with less than 100 (some estimates being lower than 20) individuals remaining in 1890. Today upwards of 60,000 or more survive. Thus all northern seals today can trace their inheritance to those few surviving individuals of 1890. Examination by researchers of 24 gene loci in a representative sample of seals has shown no variations. Each of these genes has only one allele. When compared to the closely related southern seal no such similarity occurs. Other cited examples of bottleneck include the North American bison and the South African cheetah.

To learn more about genetic drift go to

To observe a simulation of genetic drift go to

Founder’s principle occurs where a statistically small population from an established population migrates and settles in a new area. The founder effect is likely responsible for near absence of blood group B in American Indians, whose ancestors arrived from Asia about 10,000 years ago. Northern Asian populations today have frequencies of blood type B ranging from 15 to 20%. More recent examples of founder’s principle are seen in religious isolates like the Dunkers and Old Order Amish of North America where their blood group gene frequencies are quite different from those in the surrounding populations, both in Europe and in North America.

To learn more about blood type frequencies go to

Next to genetic drift, natural selection has the greatest effect on the gene pool of a population or species. Nature produces variations within members of the same species or population which can result in uneven advantages in their ability to survive. This is sometimes referred to as the “survival of the fittest”. Those individuals that have the greatest ability to survive also have a greater chance of reproductive success (will reproduce and leave offspring) and therefore will pass their “better adapted” traits (or genes) to their offspring. Over time this can result in the change in the gene pool of a species. This process can lead to enough change that over time a new species appears. New species means increased biodiversity.

While natural selection accumulates and maintains favorable genotypes in a population it initially acts upon the phenotypes of the individuals rather than their genotypes. This can be illustrated in the three types of natural selection called directional, stabilizing and disruptive. In the 1970 Peter and Rosemary Grant were able to show a directional change in the beak size of the medium ground finch Geospiza fortis. Over a several year period the Grants were able to document an increase in beak size in response to drought and reduction of seeds these bird feed upon. While the size of the beak has been shown to be controlled by a gene, initially birds with larger beaks were favored by natural selection over birds with smaller beaks.

To learn more about the Grant’s work go to:

To learn more about the three types of natural selection go to:

To learn more about natural selection go to

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Performance Benchmark L.12.D.5

Students know that biological evolution explains the diversity of life. E/S

Common misconceptions associated with this benchmark:

1. Students incorrectly assume that evolution produces increasing complexity or more “perfect” organisms.

While natural selection tends to reduce the number of less fit organisms in a population, for evolution “good enough” is good enough. In nature, perfection is not necessary for survival. Some groups of organisms like sharks, fungi and mosses have changed little over the past 100 million years. On the other hand dinosaurs resided on earth for over 150 millions, but are no longer here. What is “better” at one time does not mean “better” at another time.

Vestigial structure can point to the imperfect nature of life on our planet. Defined as organs that have reduced or no apparent function, these structures can present problems for some organisms in which they are found. Take the human appendix as an example. In plant eating vertebrates, the much larger appendix serves to help in the digestion of plant material. While in humans seems to have no apparent function. And as a testament to it apparent loss of functions doctors in the United States performed approximately 300,000 appendectomies in 2000.

To learn more about this topic review the essay about the “less than perfect” eye at

Also view the following webpage to see why it might be to a cheetah advantage to be a faster runner

In some cases “imperfection” is a better adaptation as in the case of malaria and sickle-cell anemia. For a discussion of this, go to

2. Students mistakenly assume that organisms can “will” themselves to change or they change because of an inner “need” to change and survive.

In the early 1800’s Jean-Baptiste Lamarck developed one of the earliest biological explanations for biological evolution. In his explanation, organisms could change because of the “need” to survive. Giraffes developed longer neck to satisfy the need to get leaves from trees. Among populations variations occur in gene pools. Either organisms have gene adaptations that are good enough to allow them to survive and reproduce or they do not. Whether genes are good enough for survival are determined by the requirement of the environment.

To learn more about why Lamarck ideas are not valid explanations for evolutionary change go to

Although Larmark and Darwin both believed that the environment plays a role in shaping the evolution of organisms, their ideas on what other factors produced change over time differed.

To view a lesson that illustrates the difference between Lamarck and Darwin go to

3. Students incorrectly assume organism can change by the use or under use of bodily structures or abilities.

At the time Lamarck developed his ideas, biologist lacked a basic understanding of how traits/genes were passed on. Changes that occur in somatic cells, such as those in the arms, legs or neck are not passed on. Despite all of his body building, Arnold Schwarzenegger will not pass his large muscles to his children. Only changes to genes in the gametes will be passed to his off-spring and such is the case with all living organisms.

Offspring inherit traits from parents, but the genes for these traits are found in the gametes. Any change in somatic or non-sex cells is not passed on to the offspring. A review of meiosis may be helpful at this point.

To learn more about Lamarck’s ideas go to:

4. Students mistakenly assume that life changes by chance alone.

Chance does play a role in altering gene pool composition, random mutation and genetic drift can add or remove genes from a population. Yet, natural selection will ultimately determine which traits are “best adapted” for survival. Organisms that use camouflage as a protection from predators survive depending how “good” their camouflage is or is not. Those with the better camouflage survive in greater number and pass on their traits. Over time these “better adapted” individuals may increase in numbers.

In everyday language “chance” is often associated with fortune, luck or an usual event. In evolutionary biology a “chance” event is simply an event which is not caused by the organism itself. Nor is it an event which we could have predicted.

To learn more about evolution and chance go to

5. Students incorrectly assume that evolution is “just” a theory and therefore is not a valid explanation for diversity of organisms on our planet.

This occurs because students have failed to understand the difference between the scientific meaning of theory and the everyday pedestrian use of the term. Scientific theories are explanations which are based on lines of observable evidence, enable valid predictions, and have been tested in many ways.

To review the nature and process of science go here

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Content Benchmark L.12.D.5

Students know biological evolution explains the diversity of life. E/S

Sample Test Questions

1st Item Specification: Explain that organisms change over time as a result of biological evolution.

Depth of Knowledge Level 1

  1. Natural selection can best be defined as the
    1. survival of the biggest and strongest organisms in a population.
    2. elimination of the smallest organisms by the biggest organisms.
    3. survival and reproduction of the organisms that occupy the largest area.
    4. survival and reproduction of the organisms that are genetically best adapted.
  1. Natural selection acts on
    1. all genes in population.
    2. phenotypes that are expressed.
    3. recessive alleles.
    4. all somatic mutations.
  1. Gene flow describes the
    1. movement of genes from one generation to the next.
    2. exchange of genes during recombination.
    3. movement of genes from one population to another.
    4. sexual recombination of genes in a population.
  1. The major unifying concept in biology that provides an explanation for the vast diversity of life on our planet is the
    1. theory of evolution.
    2. cell theory.
    3. classification theorem.
    4. gene-chromosome theorem.
  1. Which idea below best explains how organisms adapt and change over time?
    1. The theory of use and disuse.
    2. The change in gene pool due to mutation.
    3. The theory of natural selection.
    4. The desire for organisms to change and adapt.

Depth of Knowledge Level 2

  1. In an African population 9% of individuals are homozygous recessive for sickle-cell anemia. What percentage of the population would be expected to be heterozygous for this trait?
    1. 30%
    2. 42%
    3. 49%
    4. 70%
  1. In watermelons striped green is dominant to solid green. In a garden of 100 plants 25 are solid green. What percentage would be expected to be heterozygous striped?
    1. 0%
    2. 25%
    3. 50%
    4. 75%
2nd Item Specification: Recognize that all populations have some genetic variations.

Depth of Knowledge Level 1

  1. Genetic variations are the raw material for evolution. These variations cannot be acted upon by natural selection unless they
    1. are expressed in the phenotype of the organism.
    2. produce only unfavorable characteristics.
    3. produce only favorable characteristics.
    4. are expressed in non-sex cells.
  1. Which statement below is TRUE about natural selection?
    1. It produces changes in a species’ environment.
    2. It can lead to changes in variations among a species.
    3. It will lead to the overpopulation of a species.
    4. It generally leads to less competition in nature.

Depth of Knowledge Level 2

  1. Use the passage and the illustration below to answer the next question. The illustration below shows the variation in beak depth of three finch species found among these different islands in the Galapagos Archipelago.

How would a biologist explain the variation in the beak sized of these three species of finches?

  1. The beak sizes slowly changed over numerous generations with some growing larger and some growing smaller.
  2. The various habitats of the islands induced desirable genetic changes producing the variation seen today.
  3. They developed differences in beak size because of their need to be able to match the size of their beaks with the different types of food on these islands.
  4. Variation in beak sizes occurred by chance and when a match between beak size and available food occurred, those birds produce more offspring.
  1. The picture below illustrates the location and types of tortoises that Charles Darwin observed on the Galapagos Islands. The tortoises have necks of different lengths and shells that differ in overall shape and in the size of the opening for the neck.
(Adapted from ExamView)

The vegetation on Hood Island is sparse and sometimes hard to reach. Which of the following options BEST EXPLAINS how vegetation may have affected the evolution of the Hood Island tortoise?

  1. The tortoises stretched their necks to reach the vegetation and each generation the tortoises’ offspring grew longer necks.
  2. The tortoises with long necks and a shell that provided greater neck movement obtained food more easily, survived longer, and produced more offspring than other tortoises.
  3. The tortoises learned to climb to reach the vegetation and this trait was inherited by each successive generation.
  4. The tortoises inherited flexible shells that enabled them to reach the higher vegetation and over time, the shape of their shell changed to allow more movement.

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Content Benchmark L.12.D.5

Students know the concepts of natural and artificial selection. E/S

Answers to Sample Test Questions

  1. D, DOK Level 1
  2. B, DOK Level 1
  3. C, DOK Level 1
  4. A, DOK Level 1
  5. C, DOK Level 1
  6. B, DOK Level 2
  7. C, DOK Level 2
  8. A, DOK Level 1
  9. B, DOK Level 1
  10. D, DOK Level 2
  11. B, DOK Level 2

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Content Benchmark L.12.D.5

Students know that biological evolution explains the diversity of life. E/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

The University of California at Berkeley maintains a website called Understanding Evolution that provides teachers and students with resources for understanding the nature and process of evolution. Of particular interest is a section called “How does evolution impact my life? Within this section are examples of how evolution can be seen today in agriculture, conservation and in medicine.

1. Guidelines for teaching Evolution: This site also maintains lesson plans to teach various aspects of evolution and some guidelines in teaching evolution to students.

To review this site go to the URL below and select Understanding Evolution

2. Understanding Geologic Time: Students often cannot comprehend the vast amount of time that life has been on our planet and therefore sometimes find it difficult to see how life on our planet has been able to change.

To better understand this vast concept of time use the activity “How Big is a Billion” found at

Or use the animation entitled “Understanding Geologic Time” at

3. The PBS site “Evolution” also provides a number of activities for teachers and students alike. This website provides support materials on the series “Evolution” that appeared on PBS several years ago. The site provides activities, videos and teacher’s guides. An activity that can help your students understand natural selection is Sex and the Single Guppy.

To utilize this site and its resources go to

To utilize the activity “Sex and the Single Guppy” go to

To review the list of activities on evolution and diversity the PBS site, go to

4. The Understanding Evolution website at UC Berkeley also has a simulation of natural selection using the “classic” example of the peppered moth. The two simulations last 5 minutes as the student plays the role of bluejays trying to eat the light and dark forms of the peppered moth in different forest environment. An analysis page is also provided.

To use this activity go to

5. Hardy-Weinberg Formula: Teachers of honors or AP biology classes should have their students use the Hardy-Weinberg Formula to predict the allele and genotypes frequencies in a population.

To view an animation of the Hardy-Weinberg condition in action go to

To learn more about the Hardy-Weinberg formula and its use go to

Another useful site for the Hardy-Weinberg formula is

6. Additional sites: The following sites contain useful information and activities that are useful to the teacher.

Teaching About Evolution and the Nature of Science supported by the National Academy of Sciences.

The National Center for Science Education at

The TalkOrigins Archive at

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Support Pages

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Sample Questions:
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Intervention Strategies & Resources:
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