10/2/2015
Name:______________________________
Objective:
Describe the evidence of evolution including: fossils, homologous structures, vestigial structures,
molecular similarities, and biogeography.
Homology vs. Analogy (Station 1)
Read the station notes.
2. How does each concept provide evidence to support evolution?
1.
3. Using the image below describe which wings are homologous and which are analogous?
Moth
Pterodactyl
Bird
Bat
3.
The “wings” of sugar gliders and flying squirrels are analogous. What does that tell you
about their evolutionary history?
4.
The limbs above are homologous structures. What does that tell you about the evolutionary
history of these organisms?
Embryos (Station 2)
1. Read the station notes.
2. How is embryology important to the study of evolutionary relationships? What does it tell us
about the evolutionary history of these organisms? (fish, salamander, tortoise, chicken, pig,
rabbit, and human)
3. A naturalist cataloging species in a remote forest discovers two different varieties of beetles.
Both have a large horn-like structure on their heads that they use to dig into the ground to
find food, but they are otherwise not very similar. What conclusions can the naturalist draw
from this comparison of traits?
Fossils (Station 3)
1.
2.
Read the station notes
What are intermediate forms/transitional fossils? How do they provide evidence for
evolution?
3.
Discuss how different organisms living in different niches might have very different
probabilities of being fossilized.
4.
How does the layering of the Earth provide clues to the evolutionary history of organisms?
Vestigial Structures (Station 4)
Below are three different diagrams illustrating vestigial structures. Identify the vestigial structures
and describe why it is considered a vestigial structure. Also state its evolutionary significance.
4. Vestigial Pelvis and Legs in Snakes
5.
Use the tenets of natural selection to explain why fish species that live in completely dark
caves have vestigial, non-functional eyes.
Biogeography (Station 5)
1. Read the station notes.
2. Use the figures below to describe the concept of biogeography. How is this evidence
supporting evolution?
Molecular Biology (Station 6)
1. Why do humans and a banana share 50% of their DNA sequences?
Listed below are the amino acid sequences of the Cytochrome-c protein for several organisms.
2. Why are we going to compare amino acid sequences?
Dog
D V E K K K I V Q K A Q T V E K G G K H T N H L F G K T K N K G I T G E E T L M E
D I E K K K I V Q K S Q T V E K G G K H T N H L F G K T K N K G I T G E D T L M E
Penguin
Dog
D V E K K K I V Q K A Q T V E K G G K H T N H L F G K T K N K G I T G E E T L M E
D V E K K K I V Q K A Q T C E K G G K H V N Y L I G K T Q A A F S T D K N K G I T
Frog
Dog
D V E K K K I V Q K A Q T V E K G G K H T N H L F G K T K N K G I T G E E T L M E
N A E N K K I V Q R A Q T V E A G G K H V N H F Y G K T Q A P F S S N K A K G I T
Silkworm
3. Using the sequences above, write the number of amino acid differences between the two
species.
Dog vs. Penguin ___________ Dog vs. Frog ________ Dog vs. Silkworm _________
4. Based on the DNA data, which organism is most closely related to a dog? Explain your
answer.
______________________________________________________________________________
______________________________________________________________________________
5. Based on the DNA data, which organism is most distantly related to a dog? Explain your
answer.
______________________________________________________________________________
______________________________________________________________________________
Homology vs. Analogy
Homology
Analog
y
Embryos
Background:
In the 1700s scientists were fascinated by the observation that different animals looked
very similar in their early stages of development. They noticed that as different organisms
develop they become less and less alike. Scientists today still compare the developmental stages
of animals of different species to help identify evolutionary relationships.
The study of the early developmental stages of living things is called embryology.
An embryo is an early stage of a living thing. In most vertebrate animals, embryos develop inside
the mother. Comparison of early embryos, particularly those of vertebrates, illustrates
remarkable similarities. For example, early embryos of fish, reptiles, birds, and mammals all
have tails and gill slits. Fish keep the gills as they develop, while the other vertebrates lose them.
In humans, the tail disappears in later stages, but in other vertebrates such as fish, birds, and
reptiles, the tail remains.
Similarities in embryology of different living things suggest strong evolutionary
relationships. Scientists often study the embryology of different organisms to help understand
their evolutionary relationships. Embryological evidence suggests many animals share a
common ancestry- the more closely associated the embryology of different organism, the closer
their evolutionary relationship.
Fossils
Although it was Darwin, above all others, who first marshaled convincing evidence for
biological evolution, earlier scholars had recognized that organisms on Earth had changed
systematically over long periods of time. For example, in 1799 an engineer named William
Smith reported that, in undisrupted layers of rock, fossils occurred in a definite sequential order,
with more modern-appearing ones closer to the top. Because bottom layers of rock logically
were laid down earlier and thus are older than top layers, the sequence of fossils also could be
given a chronology from oldest to youngest. His findings were confirmed and extended in the
1830s by the paleontologist William Lonsdale, who recognized that fossil remains of organisms
from lower strata were more primitive than the ones above. Today, many thousands of ancient
rock deposits have been identified that show corresponding successions of fossil organisms.
Thus, the general sequence of fossils had already been recognized before Darwin conceived of
descent with modification. But the paleontologists and geologists before Darwin used the
sequence of fossils in rocks not as proof of biological evolution, but as a basis for working out
the original sequence of rock strata that had been structurally disturbed by earthquakes and other
forces.
In Darwin's time, paleontology was still a rudimentary science. Large parts of the geological
succession of stratified rocks were unknown or inadequately studied. Darwin, therefore, worried
about the rarity of intermediate forms between some major groups of organisms.
Today, many of the gaps in the paleontological record have been filled by the research of
paleontologists. Hundreds of thousands of fossil organisms, found in well-dated rock sequences,
represent successions of forms through time and manifest many evolutionary transitions. As
mentioned earlier, microbial life of the simplest type was already in existence 3.5 billion years
ago. The oldest evidence of more complex organisms (that is, eukaryotic cells, which are more
complex than bacteria) has been discovered in fossils sealed in rocks approximately 2 billion
years old. Multicellular organisms, which are the familiar fungi, plants, and animals, have been
found only in younger geological strata. The following list presents the order in which
increasingly complex forms of life appeared:
So many intermediate forms have been discovered between fish and amphibians, between
amphibians and reptiles, between reptiles and mammals, and along the primate lines of descent
that it often is difficult to identify categorically when the transition occurs from one to another
particular species. Actually, nearly all fossils can be regarded as intermediates in some sense;
they are life forms that come between the forms that preceded them and those that followed.
The fossil record thus provides consistent evidence of systematic change through time—of
descent with modification. From this huge body of evidence, it can be predicted that no reversals
will be found in future paleontological studies. That is, amphibians will not appear before fishes,
nor mammals before reptiles, and no complex life will occur in the geological record before the
oldest eukaryotic cells. This prediction has been upheld by the evidence that has accumulated
until now: no reversals have been found.
Vestigial Structures
Background: Some living things, including humans, have organs or structures with no apparent
function. For example, some snakes possess the remnants of hind legs and a pelvis embedded in their
body. Manatees are animals which live their lives in the water, but retain the bones of a pelvis that
are found in land dwelling vertebrates. These are just a couple examples of what scientists call
vestigial structures. A vestigial structure is a body part of an organism that tends to be reduced in size
and does not seem to have a function. It is thought that vestigial structures are parts or organs that
once functioned in the ancestors of organisms. While the part had an important function in the
ancestor, it lost its usefulness in the new organism. For this reason, vestigial structures are considered
a valuable piece of evolutionary evidence.
Vestigial Pelvis and Legs in Snakes
Biogeography
The field of biogeography is concerned with the distribution of species in relation both to
geography and to other species. Biogeography comprises two disciplines: historical
biogeography, which is concerned with the origins and evolutionary histories of species on a
long time scale, and ecological biogeography, which deals with the current interactions of
species with their environments and each other on a much shorter time scale.
Historical Biogeography
Historical biogeographers also make use of a tool called an area cladogram. This diagram is
made by taking a taxonomic tree, which shows various species and their relatedness, and
replacing the species names with the geographic location in which those species are found. This
new tree allows scientists to determine how the differences in environments have affected the
evolutionary history of different species of common origin. A sample area cladogram is shown
in.
Ecological Biogeography
Unlike historical biogeographers, ecological biogeographers make extensive use of current
population information. They study the ways in which species develop and interact in the
presence of other species and different environments. Many ecological biogeographers mimic
Darwin: they study island communities as a type of experimental system to test hypotheses about
species development.
Much of ecological biogeography is concerned with species richness, the number of different
species an area supports. In specific, ecological biographers have developed the species richness
equilibrium model. The model begins with an uninhabited "island" that can be either a literal
island or an area of like habitats completely surrounded by unlike habitats. All species available
to colonize the new area are called the "species pool." As more and more new species enter the
new area, the species pool becomes smaller and smaller, and the immigration rate (the
probability that any given species moving into the area will be a new species) decreases. At the
same time, the island becomes more and more crowded and supplies become scarce, causing the
extinction rate to increase. The point at which the extinction rate and the immigration rate
balance is called the equilibrium point. The model predicts that changes in extinction and
immigration rates will tend toward the equilibrium point, which is different for every island,
depending on resources and degree of separation from other areas. This is shown graphically in
the figure below.
Molecular Biology
Since Darwin's day, science has made astounding advances in the ways in which it can study
organisms. One of the most useful advances has been the development of molecular biology. In
this field, scientists look at the proteins and other molecules that control life processes. While
these molecules can evolve just as an entire organism can, some important molecules are highly
conserved among species. The slight changes that occur over time in these conserved molecules,
which are often called molecular clocks, can help shed light on past evolutionary events.
Molecular Clocks
The key to using biological molecules as molecular clocks is the hypothesis of neutral evolution.
This hypothesis states that most of the variability in molecular structure does not affect the
molecule's functionality. This is because most of the variability occurs outside of the functional
regions of the molecule. Changes that do not affect functionality are called "neutral
substitutions" and their accumulation is not affected by natural selection. As a result, neutral
substitutions occur at a fairly regular rate, though that rate is different for different molecules.
Not every molecule makes a good molecular clock, however. To serve as a molecular clock, a
molecule must meet two requirements: 1) it must be present in all of the organisms being
studied; 2) it must be under strong functional constraint so that the functional regions are highly
conserved. Examples of molecules that have been used to study evolution are cytochrome c,
which is vital to the respiratory pathway, and ribosomal RNA, which performs protein synthesis.
Once a good molecular clock is identified, using it to compare species is fairly simple. The most
complicated step is the comparison of molecular sequences. The sequences of the molecule in
the different species must be compared so that the number of amino acid or nucleic acid bases
that differ can be counted. This number is then plotted against the rate at which the molecule is
known to undergo neutral base pair substitutions to determine the point at which two species last
shared a common ancestor. Depending on the rate of substitution, molecules may be used to
determine ancient relationships or relatively recent ones. Ribosomal RNA has a very slow rate of
substitution, so it is most commonly used in conjunction with fossil information to determine
relationships between extremely ancient species.
Molecular Biology
Cytochrome c is part of the electron transport chain down which electrons are passed to oxygen
during cellular respiration. Cytochrome c is found in the mitochondria of every aerobic
eukaryote — animal, plant, and protist. The amino acid sequences of many of these have been
determined, and comparing them shows that they are related.
Human cytochrome c contains 104 amino acids, and 37 of these have been found at equivalent
positions in every cytochrome c that has been sequenced. We assume that each of these
molecules has descended from a precursor cytochrome in a primitive microbe that existed over 2
billion years ago. In other words, these molecules are homologous.
The first step in comparing cytochrome c sequences is to align them to find the maximum
number of positions that have the same amino acid. Sometimes gaps are introduced to maximize
the number of identities in the alignment. Gaps correct for insertions and deletions that occurred
during the evolution of the molecule.
We assume that the more identities there are between two molecules, the more recently they have
evolved from a common ancestral molecule and thus the closer the kinship of their owners. Thus
the cytochrome c of the rhesus monkey is identical to that of humans except for one amino acid,
whereas yeast cytochrome c differs from that of humans at 44 positions. (There are no
differences between the cytochrome c of humans and that of chimpanzees.)