CLASS COPY - Evidence for Change across Time – CLASS COPY
1. Genetics Suggest How Populations May Change
While initially ignored for many years, Mendel’s work with pea plants and genetics eventually sparked a conversation
about how species are defined and how they could change over time. Knowing that different traits are passed from
parent to offspring, and that these traits are passed randomly, shifted how we viewed genetics.
Originally, scientists thought that only the “strongest” genes would be passed from parent to offspring. Modern
genetics has taught us that each individual has two alleles for each gene. The chance for each allele to be passed to
offspring is exactly the same: 50/50. If a mother (or father) has brown hair but has a recessive blonde allele, we know
that the chance of her passing on the brown allele is exactly the same as passing on the blonde allele (50%). In addition,
this 50% chance does not change with each child! Child 1 will have a 50% chance of inheriting either allele and Child 2
will also have a 50% chance of inheriting either allele.
What does genetics research say about how species relate to each other? This knowledge of genetics shows how alleles
could change frequency in different populations. Furthermore, there could be a random change number of different
alleles (and their frequency) in a population that these changes could lead to a completely different population from the
original population. This means that we have to look beyond the physical appearance of organisms (phenotype) and also
consider their combination of alleles (genotype).
This can also be imagined with coin flips. If you flip a coin 10 times, you won’t always get 5 heads and 5 tails. The reason
for this is because it is a 50% chance for either heads or tails every time you flip the coin. This is the same for passing on
alleles in genetics. This means that alleles could be lost to a population simply by chance.
Example: Take two populations of orange bugs. The orange (T) allele is dominant to the yellow (t) allele.
Both populations start with 10 bugs that are heterozygous orange (Tt). This means that there are 20 total alleles in each
population. Each population has 10 orange alleles (T) and 10 yellow alleles (t). The chance of passing on the yellow allele
(t) is exactly the same as passing on the orange allele (T). Imagine that each population has a generation of exactly the
same size (2 bugs mate and create 2 offspring).
By random chance after 1 generation, Population 1 now has 8 yellow alleles and 12 orange alleles and Population 2 has
13 orange alleles and 7 yellow alleles. Below is a table of the first 6 generations.
Start
1 Generation
2 Generations
3 Generations
4 Generations
5 Generations
6 Generations
Population 1
T allele (orange)
t allele (yellow)
10
10
12
8
10
10
8
12
9
11
6
14
8
12
Population 2
T allele (orange) t allele (yellow)
10
10
13
7
15
5
15
5
14
6
16
4
14
6
CLASS COPY - Evidence for Change across Time – CLASS COPY
2. The Geographic Distribution of Species
Mammals are split into two major groups: Marsupial and Placental mammals. Marsupial mammals have a pouch that
the embryo develops within. Placental mammals hold the embryo within the uterus until birth.
Australia’s climate is well suited for mammals, and similar climates to North America. When analyzing the types of
mammals present, North America has a very small percent of marsupials whereas Australia has a very high percentage
of marsupials.
The marsupials in Australia are all very similar in structure and development. These marsupials are unlike the marsupials
elsewhere on Earth. There are approximately 140 marsupial species in Australia and nearby island of New Guinea has
about 53 species. There are only a total of 334 species of Marsupials on Earth (meaning about 130 that are not on New
Guinea or Australia). Not only this, but there are very few placental mammals in Australia, while most of the mammals in
North America are placental mammals.
Researchers have shown that when Marsupial and Placental Mammals are in the same environment, placental mammals
often outcompete the marsupials. The marsupial mammals then die out. Some event must have caused marsupial
mammals to become isolated on Australia and New Guinea and thrive without the presence of placental mammals.
For researchers, the question about mammals became: Why are there so many different marsupial species on the
isolated islands of Australia and New Guinea compared to the rest of the world?
CLASS COPY - Evidence for Change across Time – CLASS COPY
3. Chromosomal DNA Compared Across Species
DNA sequence analysis and gene mapping is the most common way to compare species today. We can sequence the
DNA and see how similar the sequence is between species. In most cases, the DNA sequence does not change very
rapidly. Sometimes even very small changes can take thousands of generations. Once scientists started mapping genes,
they saw commonalities between many species. This was especially interesting with chimpanzee and human DNA.
At first observation, it does not seem as if humans and chimpanzees have very similar DNA. Humans have 23 pairs of
chromosomes while chimpanzees have 24 pairs. Closer examination investigated the specific arrangement of genes on
chromosomes. Below is the 2nd chromosome from humans and the 23rd and 24th (labeled 2p and 2q) chromosomes from
chimpanzees. The chromosomes are numbered from largest to smallest, so we are looking at the second largest
chromosome in humans the two smallest chromosomes in chimpanzees. The dark spots represent different pieces of
DNA; many of them representing groups of genes. Scientists noticed that the arrangement of these dark spots line up
almost exactly with that of the human when the chimpanzee chromosomes are lined up end to end. Once sequenced,
scientists determined that the same gene sequences are present on both human and chimpanzee chromosomes in the
same order!
Not only do human and chimpanzee chromosomes have
almost identical DNA, but the human chromosome has
some vestigial pieces (pieces that are not used).
Chromosomes usually have a single centromere
location. The 2nd chromosome in humans has a second
vestigial centromere location. Chromosomes also have
long strands of repeating nucleotides at the end of
chromosomes called “telomeres.” Most chromosomes
have one on either end. The 2nd human chromosome
has a third vestigial telomere in the middle. It appears
as if the common ancestor of humans and chimpanzees
had two chromosomes that fused into a single
chromosome in the human lineage.
CLASS COPY - Evidence for Change across Time – CLASS COPY
4. Molecular Biology: DNA Comparison
DNA codes for proteins that your body needs to live. One of those proteins is the β hemoglobin, proteins that occur in
your blood. This protein carries oxygen and carbon dioxide as they circulate in your bloodstream. Other animals’ blood
also contains β hemoglobin protein. However, not all of these proteins are identical. The differences between β
hemoglobin protein caused by differences in the DNA that codes for them.
One reason developmental biologists are interested in comparing DNA and proteins of organisms is to learn about
relatedness. The more similar two organisms’ DNA are, the more likely it is that the organisms are closely related.
Review the comparisons of between genetic codes for the following organisms:
Orangutan
Gorilla
Chimpanzee
Human
Write answers or explanations to all of the BOLD questions or statements below in your lab journal.
1. Study the DNA handout that shows a comparison of the DNA that codes for the α and β hemoglobin proteins
that are found in primate blood.
a. Compare the DNA sequences from the beta hemoglobin of four different primates.
b. Circle any bases in each of the three primates that are different from each base in human DNA.
c. Count the total number of different bases for each primate.
d. Calculate the percent difference for each beta hemoglobin DNA sequence from Homo sapiens. There
are 180 total bases. Divide the number of differences by 180 for each primate to calculate the percent
difference.
e. Based on the percentages above, which primate may humans be most closely related to
evolutionarily? Explain your answer.
2. Why is comparing DNA evidence for evolution and how it can be used to “look into the past?” Be specific and
refer to how changes in organisms begin in the first place.
CLASS COPY - Evidence for Change across Time – CLASS COPY
5. The Fossil Record
These fossils were discovered at numerous locations around the Earth. Through radiometric dating the fossils were
dated to the times recorded. Scientists also observed that only one type of fossil was found during each time period.
For example, the Pliohippus fossils always dated back to one million years ago (mya) while the Merychippus was always
found at 8 mya and Mesohippus at 15 mya. None of these fossils would be found during the same time period. This
demonstrated that these organisms lived during different periods of time.
A comparison of the skeleton and bones shows an extremely similar bone structure between these organisms while the
shapes and sizes vary slightly.
CLASS COPY - Evidence for Change across Time – CLASS COPY
6. Anatomy Comparisons between Different Species
The figure below shows a comparison of the humerus, ulna, radius, carpals, metacarpals, and phalanges of different
species with internal skeletons. The name (definition) of each bone is given based on the number of joints, the
connections the bones form, the size of the bone, and the purpose. Scientists noticed that even species that seemed to
have nothing in common, had similar bone structures. For colored photos, please see page 108 in BSCS: AHA textbook.
CLASS COPY - Evidence for Change across Time – CLASS COPY
7. Vestigial Organs
Sometimes scientists discover organs or parts of organisms that seem to not be used by that organism. These organs are
known as vestigial organs and are defined as having no known purpose in that organism. When observing organisms that
are similar, yet different species, what is vestigial to one species may be critical to the other species. For example, the
human appendix has no known purpose while in other mammals with appendixes, it is critical for digesting plants.
Other human vestigial organs include: earlobes, coccyx (tail bone), body hair, ear muscles, tonsils, wisdom teeth. Also,
goose bumps (the little bumps that appear on the skin when cold) are vestigial. They are similar to the reaction of birds
and mammals to fluff their feathers or fur, but we have neither.
Vestigial organs do not impact a species ability to thrive or have offspring. They do not appear to serve any function.
Scientists believe that vestigial organs are the remains of structures in an ancestor that would have used them. Maybe
humans were herbivores once? Did whales walk on land?
Whales have
bones for hind
legs, but no
hind legs!
CLASS COPY - Evidence for Change across Time – CLASS COPY
8. Embryology
(the study of embryos)
Once scientists developed the technology to observe fetal development in organisms, it was apparent that this would be
a way to compare species. This began the science of embryology and allowed scientists to explore the similarities
between the embryos of different species. Development of the embryo requires the activation of genes within the DNA.
Therefore scientists can compare the function of genes between organisms by observing how the embryo develops.
Below is an image of embryo development of six different species. Notice that human embryos have gill slits as well as a
tail that both go away throughout development.
Visit class webpage to watch the following videos of embryonic development: Human, Chick and Zebrafish.
CLASS COPY - Evidence for Change across Time – CLASS COPY
9. Phylogeny (a.k.a. evolutionary trees)
Visit Class Website for quick link to: http://www.ucmp.berkeley.edu/education/explorations/tours/Trex/index.html
Procedure:
1. Open the above website and click on “Student Start”
2. Use the drop-down menu and pick folder 2. Read through each slide and when finished, click on the purple
word (usually “more”)
3. Continue onto folder 3. Stop when you get to the slide that says “let’s explore vertebrates.”
4. Now answer each of the following questions in your journal.
1. What is a cladogram?
2. Draw a cladogram with a frog, cow and horse correctly placed.
3. In your journal, write the statement that best completes this statement: A "common ancestor" is:
a. one that is very commonly seen in the fossil record.
b. one that is shared by two or more organisms.
c. one that has no distinguishing features and is therefore very "common."
4. Read the following statements then answer the question below.
A. All organisms inherit their features from their ancestors.
B. Organisms resemble their most recent ancestors more closely than distant ancestors.
C. Over time evolution occurs and new features appear.
Which of these statements are accurate?
a.
b.
c.
d.
A and C.
B and C.
All of the above.
None of the above.
5. Examine the following diagram and the statements that follow.
Statements:
A.
B.
C.
D.
Crocodiles are more closely related to squirrels than to frippities.
Frippities share a more recent ancestor with Triceratops than with Pigeons.
Frippities probably laid eggs.
All of these animals share the same ancestor.
Which of the statements are accurate?
CLASS COPY - Evidence for Change across Time – CLASS COPY
a.
b.
c.
d.
A and C.
B and D.
C and D.
A, B, C, and D.
5. Below is a cladogram of the major groups of life. You will notice it is upside down compared to those you have
seen.
a. To which group do humans belong?
b. What must have been true about the common ancestor of amphibians, turtles, snakes and lizards,
crocodiles and birds, and mammals (hint: what does tetrapod mean?)
c. Based on your answer to b, what must have happened to snakes?
d. Do humans share a more common ancestor with insects or with flatworms?