Download ModBio11-2 Evolution

OBJECTIVE SHEET
EVOLUTION
1. Describe the contributions of
Hutton, Lyell, Malthus, and
Lamarck toward the evolutionary
thought of Charles Darwin.
2. Explain Natural Selection as a
mechanism to describe how evolution occurs.
3. Discuss the various clues that nature provides as
evidence for evolution.
4. Define the following:
gene, allele, segregation, independent assortment,
mutation, chromosome, phenotype, genotype, dominant allele, recessive allele,
incomplete/codominance, polygenic traits, diploid, haploid, homozygous,
heterozygous
5. Solve single (hybrid) and double (dihybrid) factor crosses using Punnett squares.
6. Discuss how genes and gene variation can provide
us with a genetic definition of how populations evolve.
7. Discuss how natural selection can affect the distribution of phenotypes in any of
three ways: directional, stabilizing or disruptive selection.
8. Identify how Genetic Drift and Gene Flow can also provide a source of
evolutionary change. Provide an example of the “founder effect” as an example
of genetic drift.
9. Explain how the following Isolating Mechanisms help bring about Speciation:
Allopatric and Sympatric Isolation. Include a discussion on geographical,
temporal, ecological, behavioural, mechanical, and gametic isolation.
10. Explain the role of extinction in macroevolution. Differentiate among and give
examples of Adaptive radiation, convergent evolution and co-evolution.
11. Differentiate the pace of evolution as either gradualism or punctuated equilibrium.
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“There is grandeur in this view of life… from so simple a beginning endless forms most
beautiful and most wonderful have been, and are being evolved.”
– Charles Darwin 1859.
It has been called by some the greatest idea that any human has ever had. Charles
Darwin was able to unite the two most disparate features of our universe, the world of
purposeless, meaningless, matter in motion and the world of meaning, purpose, and life.
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A new era of biology began on November 24, 1859, the day Charles Darwin published
On the Origin of Species by Means of Natural Selection. Darwin’s book drew a
cohesive picture of life by connecting the dots among what had once seemed a
bewildering array of unrelated observations. The Origin of Species focused biologists’
attention on the great diversity of organisms—their origins and relationships, their
similarities and differences, their geographic distribution, and their adaptations to
surrounding environments.
Darwin made two major points in The Origin of Species. First, he presented evidence
that the many species of organisms presently inhabiting Earth are descendants of
ancestral species that were different from the modern species. Second, he proposed a
mechanism for this evolutionary process, which he termed natural selection.
Evolution is such a fundamental concept that its study illuminates biology at
every level from molecules to ecosystems, and it continues to transform
medicine, agriculture, biotechnology , conservation biology and many others.
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The H.M.S. Beagle Sets Sail
Ref. 369-377
There is an astounding diversity of life on our planet. Evolutionary theory accounts for
this diversity. It is a collection of scientific facts, observations, and hypotheses tested
over the last 150 years. It has explained the differences and similarities observed with all
life forms both extinct and living today.
A scientific theory is a well-supported testable explanation of naturally occurring
phenomena in the world.
Darwin set sail aboard the H.M.S. Beagle on a trip around the world in 1831. No one
knew that this voyage was one of the most important voyages in the history of science.
Starting from the British Isles, follow the route taken by Darwin to South America, the
Galapagos Islands and around the southern tips of Australia and Africa, only to return to
South America again before heading home to England.
If there was one thing that Darwin excelled at, it was his powers of observation. His
curiosity and analytical nature were ultimately the keys to his success as a scientist.
During his travels, Darwin came to view every new finding as a piece in an extraordinary
puzzle: a scientific explanation for the diversity of life on this planet.
Darwin made some keen observations on his travels. He was puzzled by the distribution
of life forms in the variety of habitats that he encountered along the way. The patterns
of diversity posed a challenge to Darwin.
Besides the living organisms encountered on his voyage, Darwin collected
a multitude of fossils aboard the ship and brought them back to England for further study.
It did not escape him that the fossils below ground resembled the living organisms above
them with some modification.
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Of all the Beagle’s ports of call, the one that influenced Darwin the most was a group of
small islands located 1000 km west of Ecuador. These islands are called the Galapagos
Islands. Although they were close together, the islands had unique climates and geology.
Darwin was most intrigued with the three different species of land tortoises.
Darwin also noticed the strange variety of finches on the island. The different islands
produced birds with a multitude of beak shapes. What interested Darwin was that a
particular beak shape allowed a bird to forage for the type of seed found only on the
island inhabited by that species of bird.
Darwin began to hypothesize that these separate species of bird would have evolved from
a single original South American ancestor species after becoming isolated from one
another. This was truly a revolutionary idea that would shock the world.
Ideas That Shaped Darwin’s Thinking
James Hutton
Known as the father of modern geology, Charles Darwin
enthusiastically read all of Hutton’s books explaining for the first
time how the Earth’s features were formed by natural processes
over deep time. He estimated the Earth to be many millions of
years old and not just a few thousand years old. He established
geology as a legitimate science through observation and carefully
reasoned geological arguments.
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Charles Lyell
A geologist, paleontologist, and friend of Darwin, he strongly
influenced Darwin’s thinking about evolution as a slow process
in which small changes gradually accumulate over immense
spans of time. He emphasized natural law as an explanation for
geological formations contrary to biblical flood stories. Darwin
saw natural law as a way to explain the change and diversity of
organisms on Earth. Lyell assisted Darwin with fossil
interpretations.
Thomas Malthus
An economist who wrote an essay about human population
growth strongly influenced Darwin’s thinking about life. Malthus
predicted that the human population will grow faster than the
space and food supplies needed to sustain it. Darwin made
comparisons of this idea to the struggle for existence among
organisms.
Jean-Baptiste Lamarck
Lamarck’s ideas were found to be incorrect however he was one
of the first to propose that life forms could change and evolve
over time. His ideas involved the concept of “Use and Disuse.”
He believed that through selective use or disuse of organs,
organisms acquired or lost certain traits during a lifetime. He
incorrectly thought that these traits could then be passed on to
their offspring.
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Lamarck’s ideas about species change contrasts with Darwin’s ideas in interesting ways:
Lamarck’s view
The original short
necked ancestor
Keeps stretching
neck to reach
leaves higher up
on the tree.
and continues
stretching until
neck becomes
progressively
longer
finally, a longneck descendant
after very many
generations
Darwin’s view
Original
group has
variation in
neck length.
Natural selection favors
longer necks
The favored characteristic gets
passed on to next generation in
greater proportion than the
shorter neck.
After many, many generations, the group is
still variable but showing a general increase
in neck length.
For many years, biologists hypothesized the origins of the giraffe’s long neck. Initially
the explanation by early biologists rested on the assumption that the giraffe had the
advantage of eating leaves higher on the trees where other animals could not reach.
Careful observation through the years indicates that most of their grazing occurs at lower
levels.
The latest evidence indicates that the giraffe’s neck evolved due to sexual selection as
male giraffes battle each other for reproductive rights by swinging their necks and hitting
each other with their ossicones (antler-like protuberances) with tremendous leverage.
Eventually one of the giraffes would back-down and the victor would become the
dominant male with reproductive rights. What’s interesting to note is that the giraffe has
the same number of vertebrae as you and I.
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Evolution By Natural Selection
The simplicity of Darwin’s theory of evolution can be beautifully summarized as follows:
•
Individual organisms differ, and some of this variation is heritable.
•
Organisms produce more offspring than can survive, and many that do survive do
not reproduce.
•
Because more organisms are produced than can survive, they compete for limited
resources.
•
Each unique organism has different advantages and disadvantages in the struggle
for existence. Individuals best suited to their environment survive and reproduce
most successfully. These organisms pass their heritable traits to their offspring.
Other organisms die or leave fewer organisms. This process of natural selection
causes species to change over time.
•
Species alive today are descended with modification from ancestral species that
lived in the distant past. This process by which diverse species evolved from
common ancestors unites all organisms on Earth into a single tree of life.
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The Mountain of Evidence That Is Evolution
The RNA World
All life on Earth shares the self-replicating DNA molecule. DNA is a large, complex
molecule whose origin may be linked to RNA. RNA is a smaller, simpler molecule than
DNA. Scientists were surprised to discover that RNA can actually act like an enzyme to
help it make copies of itself and forge some very simple proteins. It has been
hypothesized that RNA might have existed before DNA and that this simple molecule
evolved into the more complex, stable, and efficient molecule of DNA that all life forms
share today.
Homologous Structures
Ref. 384-5
Homologous structures suggest a common ancestor. The limbs above appear to be
different and they even have different functions, but they all have similar bone structure
and are developed from the same embryonic genes.
“Evolution lies exposed in the imperfections that record a history of common descent.
Why should a rat run, a bat fly, a porpoise swim, and I type this essay with structures
built of the same bones unless we all inherited them from a common ancestor?”
- Stephen Jay Gould, 1982.
It has not escaped biologists that even the genes and indeed the DNA that produces these
structures are also homologous.
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Vestigial Structures
Ever wonder why whales and snakes have hip
bones? Do chickens have teeth? Of course not,
but why do they have the gene for the production of
tooth dentin? Why do we have wisdom teeth (third
molars) when their only purpose seems to be to
crowd out our other teeth? Why do males have
nipples? Why do we all have muscles attached to
our ears? These and many more are vestigial
structures—which are remnants of our evolutionary
heritage from the past. Evolution doesn’t re-invent
structures and organs from scratch, it takes those
same inherited structures and “tinkers” with them to
see how successful any changes would be in a new
untested environment.
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The Fossil Record
Fossils and the order in which they appear in layers of rock – provides some of the
strongest evidence of evolution. Fossils found deeper are older than the fossil layers
found on top. The fossil record shows that animals and plants have appeared in a
historical sequence. Moreover, the sequence of fossils is consistent with what is known
from other lines of evidence. For example, evidence from biochemistry, geology,
molecular biology and cell biology places prokaryotes as the ancestor of all life. And,
indeed, the oldest known fossils dating from about 3.5 bya, are prokaryotes. Another
example is the chronological appearance of the different classes of vertebrates (animals
with backbones) in the fossil record. Fishlike fossils are the oldest fossils in the fossil
record; amphibians are next, followed by reptiles, then mammals, and finally extending
from the reptiles are the birds. This sequence is consistent with the history of vertebrate
descent as revealed by many other types of evidence.
Hundreds of years of fossil collection have produced a vast library, or catalogue, of the
ages of the Earth and the life on it. We can now identify general time periods when major
changes occurred. Interested in the origin of mammals? Go to rocks from the period
called the Early Mesozoic; geochemistry tells us that these rocks are likely about 210
million years old. Interested in the origin of primates? Go higher in the rock column, to
the cretaceous period, where rocks are about 80 million years old.
A trilobite fossil minus the rock around it.
This extinct species of Arthropod existed
around 300 mya. Many fossils of trilobite
species have been found in the Kootenay
region within the last few decades.
The fossil record even provides us with amazing predictive power about where to look
for particular transitional forms. Paleontologists look for rocks of the right age, rocks of
the right type to preserve fossils, and rocks that are exposed at the surface. A classic
example of this allowed Paleontologist Neil Shubin to find the most complete transitional
fossil from sea to land in 375 million year old rock in northern Canada.
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Neil Shubin knew that in rocks dated around 360-365 mya that he would find diverse
kinds of fossilized animals that we would all recognize as amphibians and reptiles. With
their necks, their ears, and their four legs, they do not look like fish. This rock layer
throughout the world consistently produced the same variety of extinct species. But in
rocks that are 385 million years old, we find whole fish that look like, well, fish. They
have fins, conical heads with eyes on top, and scales; and they have no necks so the head
moves with the body. It was no surprise that Shubin focused on rocks in the 375 mya
range in Canada to search for fossils showing the transition from sea to land. In the fall of
2004, his intuition was rewarded with the discovery of Tiktaalik roseae.
This extinct creature broke down the distinction between two different kinds of animal.
Like a fish, it has scales on its back and fins with fin webbing. But, like early land-living
animals, it has a flat head and a neck. And when we look inside the fin, we see bones that
correspond to the upper arm, the forearm, even parts of the wrist. The joints are there too;
this is a fish with shoulder, elbow, and wrist joints. All inside a fin with webbing…
Tiktaalik roseae
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Embryology
Embryologists study the development of embryos from
different species. There is striking similarity in the development of similar species. We understand that this similarity
is due to these organisms having similar genes. Their differences result from the timing that these genes turn on and
off, as well as even how long these genes remain turned on.
This similarity in genes is due to the fact that we share a
Common ancestor.
This provides further evidence to explain how evolution
occurs.
This is a human embryo at a stage of its
development called a blastocyst. There is no
differentiation or specialization of cells at this point
of development. Similar genes in similar organisms
cause certain cells to develop in predictable ways.
The timing of these genes can determine the shape
and make-up of these structures.
This is a tubal (ectopic) pregnancy of
a human embryo at about 6 weeks of
development. You can see limb-buds,
eyes, and the beginnings of a backbone.
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Life’s Origins
The question about how life on Earth started is not within the scope of evolutionary
theory. Life from non-life remains one of the greatest mysteries in life science research.
Again, biologists see very compelling clues and evidence in nature today that provide us
with insight into how life may have started.
Under certain conditions, large organic molecules can form tiny bubbles similar to cell
membranes. They contain higher than normal concentrations of the molecules of life.
Theses tiny microspheres can
contain extremely high
concentrations of organic material
(which is the building blocks of
life). The spheres even contain
lipid membranes similar to cell
membranes
The Miller-Urey Experiment
Stanley Miller and Harold Urey were
able to re-create the conditions of the
early Earth and produced amino acids,
some of the building blocks of protein
by passing sparks through a mixture of
H2, CH4, NH3, and water. Although they
did NOT create life they did show that
experimentally under the right abiotic
conditions, the building blocks of all
living things could be made.
This became known as the Miller-Urey
experiment in the early 1950’s.
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The Endosymbiont Hypothesis
Given the evidence that life presents us we can visualize how life may have started. An
even greater challenge occurs when we began to wonder how simple prokaryotic cells
could become the complex eukaryotic cells complete with organelles.
Convincing evidence today shows us that about 2 billion years ago, prokaryotic cells
began to evolve internal cell membranes. Other prokaryotic organisms entered into these
cells and began living as they were surrounded by membrane. Over time, a mutualistic
relationship developed. This became known as the Endosymbiont Hypothesis.
Eukaryotic cells formed from a symbiosis among several different prokaryotic organisms
living together. One prokaryotic organism could generate ATP and evolved into a
mitochondrion while another prokaryote cell that carried out limited photosynthesis
evolved into the chloroplasts found in plant cells today.
Label the following diagram (pg.427) to explain the ENDOSYMBIONT HYPOTHESIS
It is interesting to note that the internal structure of chloroplasts and mitochondria look
very similar to these types of prokaryotic bacteria. Chloroplasts and mitochondria
continue to have their own separate and unique DNA from the rest of the DNA in your
cell’s nucleus.
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Molecular Genetics
It’s fascinating that all life forms share the same
molecule, the DNA molecule, to transfer traits to
future generations. The sequence (or order) of base
pairs (the building blocks of DNA) can be read by
biologists to determine genetic relatedness among
species. Of the 3 billion base pairs found in humans
and chimps, over 98% of the sequence is exactly the
same – base for base, producing over a 96% match
in genes. We did not evolve from chimpanzees,
rather they are our evolutionary cousins as we both
separated from a common ancestor about 3 mya.
We even share sequences and genes with yeast,
mice, corn, and long ago extinct species.
Of the five great apes, Homo sapiens are the only primate with 23 pairs of chromosomes
in our cells. The other four great apes, including Gorillas, Chimps, Bonobos, and
Orangutans all have 24 pairs of chromosomes.
How could this happen? The loss of an
entire chromosome would be lethal to
any developing embryo. The evidence
is found on human chromosome number 2.
A distinct fusion of two ancestral chromosomes
12 and 13 from our shared common ancestor
occurred. We still contain all the same genes.
Molecular genetics studies have confirmed in fantastic detail the evolutionary
relationships among all living things on Earth in ways that would have astounded Charles
Darwin.
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Developmental Genes and Body Plans
Reference text pg. 440
Biologists have long suspected that changes in genes during embryological development
could produce drastic transformations in body plans of organisms. By turning on and off
the “master control genes” called hox genes we have seen various changes occur. If one
gene called “wingless” is turned on in an insect body segment, that segment grows no
wings. This is interesting because some ancient insects had wing-like structures on all
body segments. Modern insects today have wings on only one or two segments.
Small changes in the timing of cell differentiation and gene expression can make the
difference between chimpanzee brains and human brains!
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Natural Selection and the Beak of the Finch
Introduction
There are 13 different species of finch on the Galapagos Islands off the coast of Ecuador.
On one of the islands, Daphne Major, biologists Peter and Rosemary Grant have devoted
many years to studying four of these bird species. The Grants have studied the effects of
drought and periods of plenty on the finches, and the results of their experiments have
had an enormous impact on evolutionary science.
For this challenge, you will first analyze the characteristics of the 13 species of finch
found on the Galapagos Islands. Then you will watch a short film about the research
conducted by the Grants. Based on the information presented in the film (The Origin of
Species: The Beak of the Finch, at http://www.hhmi.org/biointeractive/origin-speciesbeak-finch) and your own observations, you will conduct an argument and make
predictions about the role of natural selection on the evolution of finch populations.
Materials
Finch cards (13 cards per student team)
Computer access
Camera (optional)
Sticky notes Graph paper Notebook paper for writing
Different colored pens (option)
Procedure
Break into your team as instructed and find your team’s location marked in the room.
Each team will receive 13 cards of different finch species. Follow the instructions below,
recording observations and answers to questions in your Biology notebook or on a sheet
of paper.
PART 1: What Do You Already Know?
1. Working with others on your team, examine the cards of the Galapagos finches
and arrange the species into groups on your counter space based on their
characteristics. Grouping species according to shared characteristics can provide
clues to how they have evolved.
2. Give each group an informative name and write that name down next to each
group. On sticky notes, list the justification or reason why you created each group.
3. Pause for a gallery walk. Walk around the class and examine the displays by the
other teams, paying attention to the following:
• How were other teams’ groupings similar to your team’s? How were they
different?
• What evidence did your classmates use to justify their groupings?
• How does the evidence they provided support their groupings?
On your own, write two questions about each team’s presentation on a sticky
notes. Put your initials on your question.
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4. Based on what you observed during your gallery walk, does your team want to
make any changes to its own groupings? What additional evidence would you
need to better justify your team’s groupings?
Make your changes and write down in your notebook or on a sheet of paper your
rationale for revising (or not revising) your groupings. Write additional
justification on your sticky notes or replace them all together.
PART 2: Sorting Finch Groups
5. Watch the first segment of the film The Origin of Species: Beak of the Finch at
http://www.hhmi.org/biointeractive/origin-species-beak-finch), from the
beginning to 5:36 minutes. As you watch listen for answers to the following
questions:
• What do the different beaks tell us about the different finch species?
• What evidence did scientists use to determine that the 13 species of
finches on the Galapagos arose from a single common ancestor?
• What was an alternative explanation, and how did the scientists discount
it?
• Why was the scientific conclusion of common ancestry important for
understanding the effects of natural selection on these bird species?
6. Your teacher will pause for a short discussion on these answers.
7. Return to your finch groupings at your counter from Part 1. Work with others on
your team and, if necessary, rearrange the bird groupings based on what you
heard in the film. Then use the information from the film to revise the names and
your justifications on your sticky notes. Answer the following questions in your
notebook or on a sheet of paper.
a. What did you change?
b. What evidence from the film convinced you to make the change?
c. What do the different groups of finches that you created represent?
PART 3: Examining Finch Beaks
8. Watch the second segment of the video, from time stamp 5:36 minutes to 9:00. As
you watch, listen carefully for evidence to help you answer the following
questions.
a. Describe beak sizes of the medium ground finch population (species 12 finch
cards)
b. How did the population of medium ground finches on the island of Daphne
Major change as a result of environmental changes?
9. Make a prediction. After watching the segment, create a bar graph on your own
that shows the beak sizes of the population of medium ground finches before and
after the drought. Your graph should indicated the number of medium ground
finches with each of four different beak sizes (from smallest to largest) before and
after the drought. (Hint: You will create two bars for each category of beak size,
one representing the populations before the drought, and one representing the
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populations after the drought.) Include the following categories of beak sizes in
your graph:
• Medium ground finches with much smaller beaks
• Medium ground finches with smaller beaks
• Medium ground finches with larger beaks
• Medium ground finches with much larger beaks
10. Share your graph with others on your team and provide feedback by asking your
team members two or more questions about their graphs. Be ready to explain your
own graph.
11. Watch the third segment of the video, from time stamp 9:00 minutes to 11:12
minutes. After watching the film, answer the following questions.
a. How did your graph compare to the graph in the film? Did anyone on your
team have a graph that was similar?
b. If no one on your team had a graph that was similar, what evidence did you
not consider?
c. If your graph was close to the one in the film, what part of your thinking was
the same as that of the scientists in the film?
d. Why did the drought have such an impact on the medium ground finch
population?
12. Class prediction: What was the response of Peter and Rosemary Grant to the
dramatic change in the distribution of beak sizes in just one generation of birds? If
the drought had continued longer, what would you expect your beak graph to look
like?
PART 4: Understanding Speciation (the making of new species)
13. Watch the final segment of the video from time stamp 11:12 minutes to 15:45
minutes. As you watch the film, listen carefully for an answer to the following:
• How did one ancestral finch population give rise to 13 species, each with
different characteristics?
14. With your team, create a graphic representation of the process that led to 13
different finch species. You may use the cards with the finches to construct your
graphic. Prepare your presentation like a museum exhibit, so it will stand alone
without your needing to explain it. However, you can include a written caption, as
museum exhibits do.
You will get a chance to look around at other exhibits and then you may possibly
want to revise yours. Offer suggestions or feedback on other exhibits as you see
fit. Make sure your representation can stand up to peer review after you finalize it.
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Genetics, The Study of Heredity
It is very interesting to note that as Charles Darwin was deriving his monumental ideas
on Evolution and Natural Selection, he did not know of a reason for inherited traits.
Gregor Mendel (the father of modern genetics) was devising his principles of genetics at
the same time.
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Characteristics appear to be repeated from generation to generation. The passing of traits
from parents to offspring is called heredity. Your biological traits are controlled by
genes located on chromosomes that are found in every cell of your body. You inherited
half of your 46 chromosomes from your mother and the other 23 from your father.
Essential Terminology
1. Chromosomes, Chromatids, Centromere and Homologous chromosomes
A chromosome is a long DNA molecule containing many genes. In eukaryotes, it is
associated with protein. During prophase, the chromosomal material has duplicated itself
and the two new halves are called “sister chromatids”. They are joined at the centromere
and the centromere is attached to the spindle. A homologous chromosome is the
matching chromosome inherited from the second parent.
It is important to note that if the top
paternal and maternal strands are
both called individual chromosomes,
then when they duplicate below to
form sister chromatids, each of the
new duplicated paternal and maternal
double strands are now considered to
be only a single chromosome.
Homologous chromosomes contain the same genes. For example, the gene for eye colour
would be found on both the paternal and the maternal chromosome. That’s what
homologous means—there are two chromosomes that have the same genes, one from
mother and the other from father. An important point to remember is that even though
there are two copies of a single gene, they may be different versions of that gene. We
call these different versions of the same gene alleles. There might be the allele for green
eyes, blue eyes or brown eyes present on a chromosome but it’s still the gene for eye
colour. Which allele an offspring inherits is completely random.
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2. Genes
Genes are units of nucleotide base pairs located on the chromosome that provide
instructions to a cell to produce a specific trait. You have about 20,00 genes (i.e. the
gene for hair colour). Many genes can interact with other genes to produce various
effects.
3. Alleles
Alleles are two or more alternate forms of a gene. (i.e. an allele for black hair or blonde
hair are forms of the gene for hair colour).
4. Dominant genes (alleles
These genes (or alleles) determine the expression (the way you look) of the genetic trait.
We represent dominant alleles with capital letters.
5. Recessive genes (alleles)
These alleles are masked by the dominant alleles and are not expressed (do not show up).
We represent recessive alleles with lower case letters.
6. Genotype
Genotype refers to the genes (represented by letter combinations) of a trait for an
organism. eg. BB or Bb
7. Phenotype
Phenotype refers to the physical appearance of the traits in an organism. It tells us how
the genotype actually looks like. i.e. Blue eyes, green eyes, curly hair.
8. Homozygous
Homozygous refers to a genotype in which both genes of a pair are identical (i.e. RR, aa)
9. Heterozygous
Heterozygous refers to a genotype in which the gene pairs are different (i.e. Rr, Aa)
10. Incomplete dominance
When two alleles interact and are equally dominant, they produce a new phenotype which
is a blending of the two traits. (i.e. a red flower makes a pink flower when crossed with a
white flower.)
11. Codominance
Both genes are expressed at the same time producing a mottled effect. (eg. Red and
white flowers make flowers with red and white specks)
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All of your body cells contain a full complement of 46 chromosomes. (23 pairs) These
cells are referred to as somatic cells. The male sperm cell or the female ova (egg) are
called gamete cells and are the only cells to contain only ½ of this number.
A gene for a particular trait can be found on one of your chromosomes. The alleles come
in pairs on your chromosomes (one from mom and one from dad). During the formation
of gametes (sperm or egg) the alleles on a chromosome will segregate (separate). This
ensures that each gamete cell has only one copy of the alleles for that trait. Because of
this independent assortment, a male’s sperm cell may contain the allele for blue eyes
while another of his sperm may carry the allele for brown eyes. Both gametes carry the
gene for eye colour and both gametes have only one allele (form) of the gene.
When fertilization is complete, the first somatic cell is created. This cell is called a
zygote. The zygote has a full 46 chromosomes in 23 pairs.
Monohybrid Crosses
Go and observe the Chart called “GENETICS II” at the front of the class. The Law of
Segregation is crucial to understanding how organisms can inherit alleles. The Law of
Segregation states that
This law ensures that alleles received from the haploid sperm and egg to the offspring
occur at random, but they form the first diploid cell in the offspring.
Genetic crosses that involve the study of only one trait, such as seed colour, are called
one-factor crosses because we are working out the possible genetics for only one factor.
Use the letter from the chart below for the expression of the dominant and recessive
alleles for the genes controlling these traits.
TRAIT
Pod shape
Pod Colour
Flower Position
Plant height
DOMINANT ALLELE
Smooth (N)
Green (G)
Axial (A)
Tall (T)
RECESSIVE ALLELE
Constricted (n)
Yellow (g)
Terminal (a)
Short (t)
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For each of the following problems, use the Punnett square and the chart on page 29 to
answer the following
Example: A plant that is heterozygous for green pods is crossed with a plant that has
yellow pods. What are the probably genotypic and phenotypic ratios in the offspring?
Answer: draw a Punnett square and use the letter combinations in the above chart:
g
G
Gg
g
Gg
g
gg
gg
With the Punnett square complete, you can now determine the phenotypes of the
offspring:
Since green (G) is dominant over yellow (g), plants that have G in their genotypes have
green pods. Only plants with genotype gg have yellow pods. In this example, 1 out of 2
of the offspring have green pods and 1 out of 2 have yellow pods.
In this example, the genotypic ratio is 2 Gg: 2 gg or simply reduced to 1:1. The
phenotypic ratio is 2 green: 2 yellow or simply again 1:1. Always reduce the ratios to
their simplest form.
For each of the following problems draw a Punnett square and check your work up at the
front desk:
1.
Nn x NN
Genotypic ratio:
Phenotypic ratio:
2.
Aa x aa
Genotypic ratio:
Phenotypic ratio:
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3.
Tt x Tt
Genotypic ratio:
Phenotypic ratio:
4. Show a cross for two plants that are heterozygous for green pods.
Genotypic ratio:
Phenotypic ratio:
5. Cross a plant that is heterozygous for axial flowers with a plant that has terminal
flowers.
Genotypic ratio:
Phenotypic ratio:
6. Cross a homozygous tall plant with a short plant.
Genotypic ratio:
Phenotypic ratio:
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7. Cross a plant that is heterozygous for smooth pods with a plant that has
constricted pods.
Genotypic ratio:
Phenotypic ratio:
8. When a tall plant is crossed with a short plant, some of the offspring are short. What
are the genotypes of the parents and the F1 generation? What is the phenotypic ratio
of the F1?
9. ¾ of the plants produced by a cross between two unknown pea plants have axial
flowers and ¼ have terminal flowers. What are the parent plant genotypes?
10. What cross would result in ½ of the offspring having green pods and ½ of the
offspring having yellow pods?
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Dihybrid Crosses
Crosses that involve two traits, such as pod colour and pod shape, are called dihybrid
crosses or sometimes (two-factor crosses). Predicting the outcome of these crosses
requires basically the same procedure as that for single factor crosses. Keep in mind that
in dihybrid crosses, the genes controlling the two different traits are located on
nonhomologous chromosomes. During meiosis, nonhomologous chromosomes assort
independently. This means that each of the chromosomes of any pair of homologous
chromosomes has an equal probability of ending up in a gamete with either chromosome
from any other pair of homologous chromosomes. The genes that are located on
nonhomologous chromosomes also assort independently as you can see below:
GgNn x GgNn
Because of independent assortment, a plant that is heterozygous for two traits (genotype
AaBb) will produce equal numbers of four types of gametes – AB, Ab,aB, and ab. In the
example that follows, we will predict the results of a cross between two plants that are
heterozygous for both pod colour and pod shape.
Example: What are the genotypic and phenotypic ratios in the F1 resulting from a cross
between two pea plants that are heterozygous for pod colour and pod shape? What is the
phenotype of the parents in this cross?
Answer: choose letters to represent the alleles in the cross and write the letters of the
parent genotypes.
GgNn x GgNn
Determine the possible gametes that the parents can produce. Independently assorted,
there would be four possible types… GN, Gn, gN, and gn. Enter the possible gametes at
the top and side of a Punnett square and complete the F1 generation.
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Because of independent assortment, a plant that is heterozygous for two traits (genotype
AaBb) will produce equal numbers of four types of gametes – AB, Ab,aB, and ab. In the
example that follows, we will predict the results of a cross between two plants that are
heterozygous for both pod colour and pod shape.
Example: What are the genotypic and phenotypic ratios in the F1 resulting from a cross
between two pea plants that are heterozygous for pod colour and pod shape? What is the
phenotype of the parents in this cross?
Answer: Choose letters to represent the alleles in the cross and write the letters of the
parent genotypes.
GgNn x GgNn
Determine the possible gametes that the parents can produce. Independently assorted,
there would be four possible types… GN, Gn, gN, and gn. Enter the possible gametes at
the top and side of a Punnett square and complete the F1 generation.
GgNn x GgNn
GN
Gn
gN
gn
GN
GGNN
GGNn
GgNN
GgNn
Gn
GGNn
GGnn
GgNn
Ggnn
gN
GgNN
GgNn
ggNN
ggNn
Gn
GgNn
Ggnn
ggNn
ggnn
The phenotypic ratio is 9:3:3:1 There are 9 green smooth to 3 green constricted to 3
yellow smooth to 1 yellow constricted.
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In mice, the ability to run normally is a dominant trait. Mice with this trait are called
running mice (R). The recessive trait causes mice to run in circles only. Mice with this
trait are called waltzing mice (r). Hair colour is also inherited in mice. Black hair (B) is
dominant over brown hair (b). For each of the following problems, draw a Punnett
square to answer the following:
1. Cross a heterozygous running, heterozygous black mouse with a homozygous
running, homozygous black mouse. Identify the phenotypic ratio.
2. Cross a homozygous running, homozygous black mouse with a heterozygous
running, brown mouse. Identify the phenotypic ratio.
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3. Cross a waltzing brown mouse with a waltzing brown mouse.
phenotypic ratio.
Identify the
4. Cross a homozygous running, heterozygous black mouse with a waltzing brown
mouse. Identify the phenotypic ratio.
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5. Cross a heterozygous running, brown mouse with a heterozygous running,
homozygous black mouse. Identify the phenotypic ratio.
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The Evolution of Populations
Genetic variation is studied in populations. It is important to note that individuals do not
evolve —populations evolve. A population is a group of individuals of the same species
that interbreed to produce fertile offspring under natural conditions. Because members of
a population interbreed, they share a common group of all genes called a gene pool.
A gene pool consists of all the genes, including all the different alleles that are present in
a population.
The relative frequency of an allele is the number of times that the allele occurs in a gene
pool, compared with the number of times other alleles for the same gene occur.
Gene pools are important to evolutionary theory because evolution involves changes in
population over time. In genetic terms:
** Evolution is any change in the relative frequency of an allele in a population. **
Changes in the allele frequency of a population can occur in numerous ways. All of the
changes are still subject to sorting by natural selection. If the change in allele frequency
aids in the survival of the population in their environment, we would expect to see an
increase in the relative frequency of that favorable allele. The biological “fitness” of the
species would increase. “Fitness” refers to the ability to reproduce and produce offspring.
The opposite would be true if the allele change hinders the survival of a population in an
environment. The fitness of the population would decrease.
As environments change, populations constantly evolve, and those with the best-suited
adaptations are more likely to reproduce and leave offspring who have inherited these
characteristics.
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Five Different Ways To Cause Evolutionary Change In A Population
The Agents of Evolutionary Change
1. Natural Selection
When biologists study how a population of organisms have evolved, they choose a
particular trait, characteristic, or adaptation found in that population and make
comparisons of that trait to earlier times. It is very useful to plot this information on a
graph.
When studying population characteristics, a Normal Distribution Graph can show the
relative numbers of individuals that currently have a particular characteristic or trait.
The Normal Distribution Graph
This graph shows the various heights for a population of people. 5’8” is the average
height for this population and coincidently has the greatest number of individuals. We
would expect less numbers of really short and really tall individuals. This is also called a
typical “bell-curve” graph.
Normal Distribution graphs can show how the distribution of genes and therefore allele
frequencies are in a given population. By graphing gene frequencies as they occur over
time in a population being studied; and comparing these to previous graphs, a biologist
can determine if evolution is occurring. In other words, biologists can tell if there is a
change in the relative frequency of an allele in a population over a given time period.
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