Performance Benchmark L.12.D.5
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
http://evolution.berkeley.edu/evosite/evo101/IIIDGeneticdrift.shtml
To observe a simulation of genetic drift go to
http://www.biology.arizona.edu/evolution/act/drift/drift.html
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
http://anthro.palomar.edu/vary/vary_3.htm
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:
http://www.pbs.org/wgbh/evolution/library/01/6/l_016_01.html
To learn more about the three types of natural selection go to:
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Evolution.html
To learn more about natural selection go to
http://evolution.berkeley.edu/evosite/evo101/IIIENaturalSelection.shtml
Performance Benchmark L.12.D.5
Students know that biological evolution explains the diversity of life. E/S
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
http://www.pbs.org/wgbh/evolution/change/grand/index.html
Also view the following webpage to see why it might be to a cheetah advantage to be a faster
runner
http://evolution.berkeley.edu/evosite/evo101/IIIE6aFitenough.shtml
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
http://evolution.berkeley.edu/evosite/evo101/IIIE6bBadgenes.shtml
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.
http://evolution.berkeley.edu/evosite/misconceps/IEneeds.shtml
To learn more about why Lamarck ideas are not valid explanations for evolutionary change go to
http://necsi.org/projects/evolution/lamarck/webelieve/lamarck_webelieve.html
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
http://www.indiana.edu/~ensiweb/lessons/lam.darw.html
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 offspring 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:
http://evolution.berkeley.edu/evolibrary/article/_0/history_09
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
http://www.talkorigins.org/faqs/chance/chance.html
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
http://evolution.berkeley.edu/evosite/nature/index.shtml
Performance Benchmark L.12.D.5
Students know that biological evolution explains the diversity of life. E/S
Sample Questions
1. Natural selection can best be defined as the
a. survival of the biggest and strongest organisms in a population
b. elimination of the smallest organisms by the biggest organisms
c. survival and reproduction of the organisms that occupy the largest area
d. survival and reproduction of the organisms that are genetically best adapted
2. Natural selection acts
a. on all genes in population.
b. on phenotypes that are expressed.
c. only on recessive alleles.
d. on all somatic mutations.
3. Gene flow describes the
a. movement of genes from one generation to the next.
b. exchange of genes during recombination.
c. movement of genes from one population to another.
d. sexual recombination of genes in a population
4. The major unifying concept in biology that provides for an explanation for the vast diversity
of life on our planet is
a. the theory of evolution.
b. the cell theory.
c. the classification theorem.
d. gene-chromosome theorem.
5. In the illustration below which answer best explains the difference in the length of the
animals’ necks?
http://necsi.org/projects/evolution/lamarck/lamarck/giraffes.jpg
a. The theory of use and disuse.
b. The change in gene pool due to mutation.
c. The theory of natural selection.
d. The desire for organisms to change and adapt.
6. Genetic variations are the raw material for evolution. These variations cannot be acted upon
by natural selection unless
a. they are expressed in the phenotype of the organism.
b. they produce only unfavorable characteristics.
c. they produce only favorable characteristics.
d. they are expressed in non-sex cells.
Performance Benchmark L.12.D.5
Students know that biological evolution explains the diversity of life. E/S
Answers to Sample Questions
1. (d)
2. (b)
3. (c)
4. (a)
5. (c)
6. (a)
Performance Benchmark L.12.D.5
Students know that biological evolution explains the diversity of life. E/S
Intervention Strategies and Resources
The following list of intervention strategies and resources 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
http://www.ucmp.berkeley.edu/
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 http://www.ucmp.berkeley.edu/education/explorations/tours/geotime/guide/index.html
Or use the animation entitled “Understanding Geologic Time” at
http://www.ucmp.berkeley.edu/education/explorations/tours/geotime/index.html
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
http://www.pbs.org/wgbh/evolution/
To utilize the activity “Sex and the Single Guppy” go to
http://www.pbs.org/wgbh/evolution/sex/guppy/habitat.html
To review the list of activities on evolution and diversity the PBS site, go to
http://www.pbs.org/wgbh/evolution/library/05/index.html
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
http://www.biologycorner.com/worksheets/pepperedmoth.html
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
http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/life4e_15-6-OSU.swf
To learn more about the Hardy-Weinberg formula and its use go to
http://anthro.palomar.edu/synthetic/synth_2.htm
Another useful site for the Hardy-Weinberg formula is
http://www.phschool.com/science/biology_place/labbench/lab8/intro.html
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.
http://www.nap.edu/readingroom/books/evolution98/
The National Center for Science Education at
http://www.ncseweb.org/
The TalkOrigins Archive at
http://www.talkorigins.org/