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Biology 30
Module 4
Organic Variations
Lesson 14
Evidences of Change
Copyright: Ministry of Education, Saskatchewan
May be reproduced for educational purposes
Biology 30
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Lesson 14
Biology 30
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Lesson 14
Lesson 14 Evidences of Change
Directions for completing the lesson:
Text References for Suggested Readings:

Read BSCS: An Ecological Approach
Sections 9.1-9.5, pages 119-208; 9.8-9.9, pages 211-216
OR
Nelson Biology
Chapter 29, pages 678-694

Study the instructional portion of the lesson.

Review the vocabulary list.

Do Assignment 14.
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Lesson 14
Vocabulary
analogous structures
Charles Darwin
convergent evolution
divergent evolution
divine concept
evolution
extinct
fossil
genesis
half-life
Jean Lamarck
Biology 30
macroevolution
microevolution
natural selection
paleontology
Pangaea
phylogenetic trees
sedimentary rock
tectonic plates
uniformitarianism
vestigial
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Lesson 14
Lesson 14 – Evidences of Change
Introduction
One noticeable aspect of the development of our planet centers
on the variety of physical and biological processes. Variations
can be seen in the continual physical forces and their effects
shaping our continental surfaces, climatic changes and the
aging processes within the earth’s crust and interior. Biological
changes are in evidence in changing plant and animal
distributions, changing cell and body actions and changes in
individual organisms over time.
This lesson will focus on changes or variations as they occur in gene pools or
populations over time. The term evolution could be used to describe these gene pool
changes or population variations. At this point, it should be stressed that the term
evolution is not the same as the Theory of Evolution. The latter states that present
species evolved from ancestral forms, which could have been quite different in
appearances. It also implies that the first living forms somehow arose through some
natural causes present at the time. It then attempts to explain the reasons for
variations occurring. This lesson will present some thoughts and information on
species’ variations. It will be noticed that there has been an “evolution” in thinking
over the years. At the present time, much of the human population agrees that
change or evolution is taking place. Differences arise in trying to explain why or how
evolution does take place. Such differences have been, and still are, largely
responsible for the disagreements that arise among people or between groups on the
topic.
Different sciences all have a common characteristic. Each science builds a body of
knowledge based on the scientific method. Collecting data, experimenting, observing
and then interpreting are some of the major steps in establishing facts or theories.
The ability to repeat observations or experiments by different people with the same
results is a main feature in the development of a fact or theory. This does not mean
that a theory is necessarily correct. Newer or more precise scientific techniques can
show some theories to be inaccurate or unable to answer all the questions about a
particular subject. Modified or new theories can take their place, based on the
information available at a particular time. This method of gathering scientific
knowledge is not the same as that of knowledge gained through religion. Religious
knowledge is based on beliefs developed and held by specific groups. The differences
in methods of gathering knowledge have probably been the main source of
disagreements between groups on the topic of evolution. With biology being a
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Lesson 14
science, the material presented in this module will concentrate on data gathered
through the scientific method. This does not mean that the course is trying to
convince individuals to absolutely accept or reject certain kinds of information or
beliefs. How individuals resolve or treat certain information or issues ultimately
depends upon themselves.
This lesson will look at the evidences or indications which confirm that changes do
occur in both the physical and the biological aspects of planet earth. On the
biological side, fossils have been, and still are, a major confirmation of evolution. In
addition, they are valuable sources of information about once living organisms and
the external conditions they experienced. Other sources of information about
changes occurring and perhaps the relatedness of species are based on such things
as: comparative anatomy, embryology, vestigial organs, biochemical similarities and
others.
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Lesson 14
After completing this lesson you should be able to:
•
define the term “evolution”, as it relates to living things.
•
trace some of the thoughts and manners of thinking with
respect to evolution from early times to the present.
•
discuss some of the contributions made by geologists to the
ideas of evolution.
•
discuss the various evidences of evolution.
•
discuss some of the major forces affecting the earth’s crust and
their effects on both physical and biotic conditions.
•
present the main points of the various theories on origins of
organisms and evolution.
•
explain the relationship between genetic variation, natural
selection and evolution.
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Lesson 14
Understanding Evolution
If one were to try and find a definition of evolution from different sources, it would be
quite probable that a number of different ones could be stated. Evolution (according
to different references) is:
•
•
•
a process by which populations change and develop adaptations to their
environments;
a progressive change in gene frequencies and pools;
the development of new species from pre-existing ones.
Examining such definitions more closely, a comparison could be made to finding the
meaning of a word like “help” from a dictionary. This one word could have a large
variety of slightly different definitions for particular situations. All have essentially
the same meaning or could be a part of the same general process. In the same way,
the three definitions (and others) given for evolution could fit together. A population
may evolve or change over time, through genetic changes and natural selection.
Evolution, or evolving, can occur in a number of ways. Some species may have few
changes taking place, others become extinct, and new species develop.
Attitudes and Thoughts on Evolution
Generalizations are often made by people based on what is observed. Early in
human history, observations of many actions lacked details or understandings of
what was actually taking place. People would frequently supply explanations based
on what they thought was occurring. With respect to life or living things, records of
human thoughts probably first began in the sixth to the fourth centuries, B.C. From
this time until about the l700’s, the theory of spontaneous generation, renamed
abiogenesis by Thomas Huxley, was commonly accepted.
What observations lead to abiogenesis?
One common observation was that people observed that maggots seemed to suddenly
appear in rotting meat after several days. Today we know that this is a result of eggs
that hatched, not because the meat spontaneously changed into maggots. This was
proved experimentally by Francesco Redi. There were further experiments which
disproved abiogenesis; these were experiments by Spallanzani and Louis Pasteur.
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Lesson 14
Religious beliefs were a common basis for evolution; these beliefs originate from the
Old Testament. The Divine Concept put forward that a divine being made the world
and all its living inhabitants in six days, with man coming last. Special Creation, in
various other forms, has been part of the cultures of many different human societies.
Some cultures have attributed the different physical and biotic conditions on this
planet as being under the charge of different divine beings.
Other theories of evolution or creation were based on the ideas of the influential
philosophers such as Plato and Aristotle. A common belief, dating back prior to the
1800’s and traced to some of Aristotle’s work, centred on the “Great Chain of Being”.
This belief put forward that original creation, when it occurred, was complete and
perfect. Species remained as they were formed. Each species also had a permanent
place in a “divine order” of things. This not only applied to species and life, but it
governed other aspects. Some of these included the places or roles of specific human
individuals in societies. It favoured such ideas as the divine right of kings and, that
once established, social and economic positions of individuals in societies would
remain unchanged. For instance, an individual born into some low-ranking position
or order would remain there.
These early theories were not to remain unchallenged. The early l800’s began to see
more and more resistance to the belief of divine order and the world remaining static
in so many of its conditions. Much of the resistance could have come from the
dissatisfaction and feeling of helplessness many people were feeling in being locked
into rigid social and economic frameworks. Such rigid systems offered little hope of
advancement or betterment for those in low-ranking positions. At the same time,
almost all sciences were also challenging some of the “old” ideas, such as the earth
being the centre of the universe and the world remaining static and “divinely
ordered”. Previous generalizations and conclusions, such as abiogenesis, were being
disproved by experimentations and new observations. Development of a rational
scientific method and new testing techniques (especially with the use of better
microscopy) were important factors in this.
A major force in challenging the idea of a static earth was the developing science
of geology and the works of a number of prominent geologists. In l788, James
Hutton published his Theory of the Earth. Among his conclusions in the
publication was that the earth was continually changing. Also, that the same
geological forces in action at the present time (volcanism, erosion, deposition...)
had been shaping the earth over a long period of time. The emphasis on time was
shifted from the immediate and relatively short time intervals, to “deep” time,
where there is “no vestige of a beginning – no prospect of an end”. In l830,
another geologist by the name of Charles Lyell published Principles of Geology,
which emphasized Hutton’s conclusions of a changing earth. In addition, this
publication expressed a theory of uniformitarianism. This related not only to
geology, but made generalizations which could apply to other sciences.
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The principles of uniformitarianism include the following:
l.
Natural laws are constant in time and space.
2.
Scientists should attempt to explain past events through the same
processes that can be seen today.
3.
Most geological change occurs slowly and gradually, not through
catastrophic events.
The first two principles could almost be regarded as scientific attitudes or methods
which could be utilized by all sciences. These could be used to explain many
events or changes which have occurred or are occurring on earth. The third
principle was considered not only by geologists, but was incorporated into the
studies and findings of other scientists, including Charles Darwin. Darwin’s
conclusions on evolution included the idea of changes to living forms taking place
at gradual or uniform rates.
By the mid-l800’s, individuals could see changes in many things around them and
the idea of change began to include living forms more and more. Even though the
idea of change or evolution was being shared by increasing numbers of people, the
still unanswered questions of how changes took place and why they did,
presented major difficulties. Attempts to answer these questions created
divisions, which persist in varying degrees to the present time, among and within
groups. Applying the concept of evolution or change to humans made the study
that much more difficult.
The next portion of the lesson will examine the various
concepts and theories on changes in life forms. The
remainder of the lesson will concentrate on the observations
or evidences used by people in the past and present to
confirm that life forms or species do change.
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Lesson 14
Origins of Life
The time at which the first forms of life were created (or genesis) and just how this
took place could follow one of several views. One of these was previously mentioned.
Special Creation
Different cultures view genesis or the creation of the first life as having been under
the influence of a divine being or beings. The Old Testament and the Divine Concept
puts forward that the world and all its inhabitants were created in six days, with man
coming last. Other cultures stated that the many conditions, both living and nonliving, were created by different divine beings.
The Scientific View
Following the immediate formation of planet earth, scientists indicate that conditions
would not have allowed for any living forms. Neither the atmosphere nor the surface
conditions were favourable to the appearance or development of organic forms of life,
however simple. Ironically, this put science in the position of trying to explain how
living organisms could develop from the non-living, when they had been trying so
hard to disprove spontaneous generation. The possible answer existed in the
conditions prevalent at the time. Conditions then were not as they are now. Oparin,
a Russian scientist, stated in 1936 that certain surface and atmospheric conditions
of primitive Earth could have resulted in the formation of organic molecules. In an
experiment in 1953, carried out by an American scientist (Miller), an electrical
discharge through a “soup” of inorganic matter (methane, ammonia, hydrogen and
water) did actually form organic molecules. These were short chains of amino acids,
which are the actual building blocks of proteins. Other scientists, trying to simulate
the possible conditions on earth as they may have been over 4 billion years ago, have
also been able to produce organic molecules. Even though amino acids and other
organic molecules have been created, no one has so far been able to produce organic
matter which somehow has a genetic component and is able to carry out some of the
life-sustaining actions of living cells as well as reproduction. However, some
conditions or actions about 3.5 billion years ago could have combined organic matter
into complete living cells. The oldest fossil cells were discovered in rock dated at
approximately 3.4 billion years old. Other cells may then have developed from these.
A theory, sometimes called the Heterotroph Hypothesis, centers on the idea of the
first living cells being anaerobic and heterotrophic; anaerobic, because of the initial
absence of oxygen and heterotrophic, because photosynthetic or chemosynthetic
structures are quite complex and would probably need more time to develop. It
would not seem likely that cells could have been formed having both life-sustaining
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and photosynthetic capabilities. Heterotrophic cells could have lived on the short
chain organic compounds that were being formed. The more complex autotrophs,
with photosynthetic structures, could then have evolved from these. With
photosynthesis, oxygen began to be produced and the Earth’s ozone layer began to be
formed. With these processes going on, it is likely that the numbers of anaerobic
cells or organisms would have begun declining, since oxygen would really have been
“poisonous” to them. This seems to be borne out today, with anaerobes being
present in relatively limited numbers and in places where there is no oxygen.
In addition to Oparin’s and Miller’s theories and work on creation of life forms, there
are other ideas. One indicates the possibility that the first organic molecules fell to
the earth as passengers on meteorites or comets. This idea is reinforced by
discoveries of organic molecules in space matter. Going a step further, there is a
chance that perhaps some of the space matter actually carried living cells. Another
theory contends that life could have begun around deep-sea hydrothermal vents.
This theory puts forward the idea that such vents provide a steady flow of energy and
inorganic nutrients. The energy and the nutrients could have, at some point,
resulted in some matter becoming animate.
Evolution After Creation
Special Creation
Subscribers to Special Creation differ in ideas on evolution. One view is that once
created, organisms or species are immutable or unchanging. Species in existence
appear in forms as they were created. This view presents a problem in that fossils
have shown different life forms in different periods of time during earth’s history. To
account for this, some believe that there could have been a number of creations,
perhaps five or six, followed by catastrophes. Each of the catastrophes and special
creations would have been by “divine decree” – rare events. Some references refer to
this as Catastrophism. Interestingly, some scientists subscribe to a Theory of
Catastrophism as well. However, rather than ascribing the catastrophes to divine
influence(s), the scientists lay their causes to sudden geologic, climatic or other
natural events. Another view embodies the idea of Special Creation and then sees
evolution as occurring, but under a “guiding influence”.
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Lesson 14
The Scientific View
Biological theories on evolution, after the first appearance of life, have been
experiencing a progression of thoughts. Some early Greek scientists-philosophers felt
that species experienced “successions” in forms. They could not explain why changes
in forms took place, but did feel that the changes were in the direction of “better or
more perfect” forms. Since that time, various other theories have been expressed.
Some of the more prominent theories on reasons for evolution which were put
forward at different times are described in the following paragraphs.
Jean Lamarck Theory of Evolution
In the early 1800’s, French biologist Jean Lamarck expressed his ideas on organisms
or species changing from generation to generation. His descriptions and accounts of
actual evolutionary processes and evidences of others were quite accurate. He was
also the first to suggest that interactions between organisms and their environments
were the basis of evolution. It is unfortunate that Lamarck is perhaps best known for
his reasons in attempting to explain why evolution took place, which have proved to
be inaccurate. His reasons for evolution were based on two main thoughts:
1. The fist thought is that the use or disuse of different parts would result in
different developments of those parts over time. A common example of
short-necked animals continually stretching their necks for a number of
generations, resulting in giraffe-like characteristics, would illustrate this.
Sometimes associated with this, is a related idea about an inner need to change.
That is, a change could be initiated or brought on by some conscious desire on
the part of an individual organism itself.
2. The second major thought in Lamarck’s works was the theory of acquired traits
being inheritable. This expressed the idea that traits which organisms developed
or acquired in their lifetimes could be passed on to their offspring. For instance,
if an organism lost a limb, its future offspring could be born without that limb.
This particular idea became a major failing of Lamarck’s entire work related to
trying to explain how evolution took place. Later genetic studies showed that
acquired body traits could not be inherited. Only changes affecting genes of
gametes could be passed on.
Although his proposed theory of evolution is known to be incorrect, it provoked much
thought and discussion on the subject, influencing the thoughts of others, including
Charles Darwin.
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Charles Darwin’s Theory of Evolution
In the mid-1800’s, biologists Alfred Wallace and Charles Darwin independently
formulated similar ideas on reasons for evolution. It was acknowledged that Darwin
had spent more time and work and had formulated his ideas just before Wallace
actually presented his publicly. In 1859, Darwin published his ideas as the book On
the Origin of Species by Means of Natural Selection. His book was primarily based on
observations made of differences in organisms on scattered island chains in the
Galapagos. His observations are summarized as follows:



Darwin found fossils of extinct animals that appeared very similar to present day
animals living in the same area (the extinct glyptodont and the modern armadillo)
Plants and animals in the temperate regions of South America were more similar
to those in other tropical regions of the same continent as compared to those of
temperate regions elsewhere.
While on the Galapagos Islands, he noticed that animals, such as the tortoise and
the finch, varied slightly from island to island. This was perhaps the most critical
observation that helped develop his theory.
These observations formed questions in Darwin’s mind. In conjunction with his
reading of Charles Lyell’s work, the Wallace-Darwin Theory of Evolution eventually
centred on the idea of Natural Selection.
Natural Selection: Variations or changes in populations which gave organisms
better chances of survival were more likely to be passed on to future
generations.
Included in this idea was that change was random (or without purpose) and that
natural selection was based on the selective forces of a particular environment. A
random variation could be quite favourable in one particular environment and of no
value or even harmful in another environment. Over a period of time and a
succession of generations, groups of organisms could acquire enough different
(favourable) variations that they will have formed species different from their
ancestors. Darwin and Wallace were only different in their emphasis on the forces of
natural selection. Wallace emphasized food supply as the major selective force. In
Darwin’s view, selective forces could be quite varied, with food supply being just one
of many.
Darwin identified four factors that govern natural selection:
1. Organisms produce more offspring than can survive, and as a result, there is
competition for limited resources.
2. Individuals within a population vary, and this variation in heritable.
3. Individuals better suited to the local conditions survive to produce offspring
4. Processes for change are slow and gradual
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Similar to Lamarck, Darwin and Wallace were not able to explain why or how
variations took place. This was one contributing factor to the resistance from some
against this theory. Another objection probably arose from the viewpoint which
developed about the role of natural selection. Phrases adopted by some people such
as “survival of the fittest”, “elimination of the unfit” and “struggle for existence”
created negative feelings in some. Natural selection came to be looked upon as a
disruptive force, leading to continual struggle and suffering and death for those not
quite fit enough.
De Vries’ Theory of Evolution
In 1901, De Vries, presented his theory based on mutation. His theory stated that
mutation, or suddenly appearing and well-defined inheritable variation, is a force in
the evolution of species and origin of new species. This is in opposition to the slight,
cumulative changes present in Darwin’s theory. This theory is accepted by most
biologists as a fundamental concept in evolution. The gene is responsible for the
characteristics of a species and their offspring; changes in the genes would result in
changes in the characteristics of the offspring.
Current Theories of Evolution
The Wallace-Darwin Theory of Evolution, based on the idea of gradual species
changes as a result of natural selection over long periods of time, forms a basis for
many of the current thoughts. These current thoughts are not the products of any
single individual, but reflect the combined ideas of many.
Organisms have descended from species which have lived before them. The idea of
common descent implies that certain plants, animals and other organisms could be
placed into certain groups according to ancestors which they may have shared. The
other idea of gradual, but steady, changes is often referred to as Gradualism. If you
recall, this actually conforms to one of the principles of uniformitarianism.
Environmental conditions and natural selection are still regarded as powerful forces
in evolutionary changes. However, natural selection is not necessarily looked upon
as the only force resulting in change. Darwin’s Theory promotes the idea of diversity
and new strains of cells and organisms developing by means of natural selection.
Present day scientists see natural selection as a guiding force which often actually
tends to minimize or stabilize the number of changes, rather than maximize them.
Selective forces may not permit that many new variations to reproduce themselves.
In this way, natural selection really acts in a conservative manner. Present day
scientists also try to dispel the idea of natural selection as destructive or as a force
which somehow actively destroys the “unfit”. Most often, the success of a variation
depends upon an organism’s ability to reproduce or pass genes on to more offspring.
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Lesson 14
It is mainly this difference in individual reproductive abilities, rather than being
killed or destroyed at an early age, which determines whether a variation continues
into future generations. Unsuccessful variations are those which do not have good
reproductive abilities, but they may still live out their lives.
Another line of thought among some individuals and groups relates to rate of change.
Darwin’s Theory implies evolution is occurring gradually and over long periods of
time. There are now some who relate “catastrophism”, or sudden, catastrophic
actions, as having major effects in evolution. This idea was first mentioned under the
preceding heading of Special Creation. Some groups see catastrophic events
occurring periodically under divine influence. Some scientific groups also have a
theory of catastrophism. The scientific view maintains that populations tend to
remain fairly stable, until some major, natural events upset the normal balance in
large areas or even on the entire planet. Major transitions in populations can take
place at these times. Large numbers of individuals and even entire species can die
out. Survivors could be quite different in traits as they continue new lines of
development. To distinguish between the scientific and the divine concepts, the
scientific one is sometimes referred to as Punctuated Equilibrium.
Punctuated Equilibrium was put forth by American scientists Gould and Eldredge in
l972, based on their observations that many species appeared suddenly and
disappeared suddenly. Between appearing and disappearing, the species tended to
remain relatively unchanged. Looked at another way, one could say that intervals of
stability or “equilibrium” were periodically interrupted, or “punctuated”, by brief
periods of change. During a change, new species would evolve from pre-existing
ones. Fossil, geologic and other evidences which indicate significant changes
occurring periodically in the earth’s conditions add credibility to the theory of
Punctuated Equilibrium. These significant changes are often marked by mass
extinctions. Two of the more notable ones were about 225 million years ago, when an
estimated one-half of all marine organisms died, and the other change was between
60 and 70 million years ago. During this last interval, most of the large reptiles or
dinosaurs disappeared and mammals made their appearance. The following graphic
depicts the earth’s history and some highlights occurring during various time
periods.
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Lesson 14
Era
Cenozoic
Period
Quaternary
Tertiary
Mesozoic
Paleozoic
Precambrian
Millions
of
Years
Ago
1
Main Events
Humans
64
“Modern” mammals, angiosperms ...
Cretaceous
136
Dinosaurs declining; first flowering
plants; small mammals increasing.
Jurassic
195
Dinosaurs’ numbers peaking; first
mammals, birds.
Triassic
225
Reptiles and gymnosperms become
prominent. Many marine species
disappear.
Permian
280
First gymnosperms; many insect orders.
Carboniferous
345
Great numbers of ferns, amphibians;
first reptiles, winged insects...
Devonian
410
Many fish, insects; first trees,
amphibians.
Silurian
440
First land plants.
Ordovician
530
First fish (vertebrates).
Cambrian
570
Appearance of many invertebrates:
snails, clams, sponges, corals...
4,500
Few fossils. Some monerans, algae,
fungi, worms...
Questions are continually raised about the reasons for the two major extinctions
mentioned and about four or five others which apparently took place. The most recent
speculation about the last mass extinction (especially involving dinosaurs), included the
possibility of an impact between earth and a large meteor or meteors. If this occurred, a
large amount of dust and gases could have entered the atmosphere, beginning
significant climatic and other changes. Some scientists suggest that the extinctions
were gradual rather than sudden, which would tend to downplay the possibility of an
impact with a meteor. Longer term climatic changes could have been tied in with
certain geologic events, such as continents slowly rising or being uplifted and inland
seas retreating. Climate could have become less “tempered”. That is, the ranges could
have become greater, with hot days hotter and cold days colder.
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Lesson 14
Increase in volcanic activities could also have played a part, with the addition of ash and
dust into the atmosphere. There is even a possibility that all of the gradual changes
could have been occurring when a meteor strike did take place, simply altering the
speed of change at some point.
Trying to establish the accuracy of various theories regarding origin and evolution is
a difficult task. It is likely that there will never be completely acceptable answers to
everyone. Perhaps, elements of all the various beliefs and theories have some
validity. As time goes on, new methods of research and new findings will probably
change some ways of thinking and perhaps even introduce completely new theories.
So even as there is physical and biological evolution, there is also an “evolution” in
the thinking and theories on the concept.
Evidences of Evolution
Many techniques have to been utilized to study the changing nature of organisms
over periods of time. In some instances, evolution can be seen in laboratory
situations – particularly in organisms with short generation times. Strains of
viruses and bacteria subjected to certain physical conditions, chemicals or drugs
(mutagens) have been found to change in the nature of their genes in relatively
short periods of time.
Scientists currently regard our planet as being approximately 4.6 billion years old.
The oldest fossils of once living cells have been dated as being about 3.4 billion
years. Many evolutionary changes among organisms take place over many years.
As a result, other techniques must be used as direct human observation, even if
carried out over a number of generations, is not sufficient
Fossils
Paleontology, or the study of plant and animal fossils, has provided a wealth of
information, supporting and strengthening the idea of evolution. Fossil evidence
not only provides information about previous life, but can indirectly reveal some
interesting conditions as they may have existed with respect to the physical forces
shaping our planet and the changing climates. Examples are:


Finding fossils of tropical plants in Antarctica and a fossilized deciduous forest
in an area of the Canadian Arctic has supported the theory of continental plate
movements or continental drift.
Significant climatic and earth surface changes could also be speculated on
when fossils of obviously sea-living creatures are found in deserts or
mountains.
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Lesson 14
Fossils are considered to be any signs of previously existing life. The following are
some common forms of fossils.






Preserved and frozen fossils. Actual cells or bodies of organisms themselves.
There can be considered the best fossils. Quicksand, tar pits (a source of
saber-toothed tiger fossils), peat bogs, tree sap and permafrost (a source of
wooly mammoth fossils) are some conditions or places where remarkably
well-preserved remains have been found. These are formed when an organism
was quickly trapped in ice or tree sap which hardened into amber.
Petrified fossils. Minerals penetrate and replace the hard parts of an
organism. The result is exact replications of cell shapes or tissue formations in
these petrified fossils which can provide scientists with many details.
Trace fossils. The fossils show signs of organisms, although the actual
remains are not present. Common trace fossils include body or body part
imprints, usually in sedimentary rock layers. Outlines of entire bodies, cell
features or even animal (or human) tracks leave information about organisms,
even though actual remains have long disappeared.
Molds. A mold of an organism forms when it is buried in sediment and then
decays, leaving an empty space.
Casts. When minerals fill a space left by a decayed organism, a cast of the
organism is formed.
Imprints. Thin objects, such as leaves, can leave imprints when the sediment
that they fall into hardens.
Scientists have been able to date many fossils in various ways. Knowing the ages
of particular geologic strata or earth layers has been one way. Another method of
testing is based on the rate of radioactive decay of certain molecules. Living
bodies ingest and incorporate certain radioactive molecules into their tissues.
After death, these molecules decay or break down into other forms at very specific
rates. A radioactive form of carbon, Carbon 14, has a half-life of approximately
5700 years. That is, it takes about 5700 years for one-half of a sample of Carbon
14 to change to more stable molecules (of nitrogen l4), another 5700 years for onehalf of the remaining half to decay, and so on. By comparing the proportions of
radioactive to non-radioactive substances in fossils to those in still-living
organisms, scientists can quite accurately date many fossil finds. Carbon dating
is fairly accurate for fossils not much older than 50 000 years. Another
radioactive substance, Potassium 40 (Calcium-Argon) has a half-life of about 1.3
billion years and is often used for dating fossils much older than 50 000 years.
Rather then testing the radioactive matter in a fossil, potassium dating is carried
out on the rock matter in which the fossil is found. Proportions of radioactive to
non-radioactive matter are then compared with those in relatively recent rock or
earth strata.
Comparisons of fossils in different earth layers and in different locations have
shown changes taking place in species over time. One species which has been
commonly used as an example is the horse, which evolved from a much
smaller-sized, many-toed form to what it is today. Other examples of organisms
that have been studied to observe transitions within a species are clams,
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rhinoceroses and apes. Some fossils hint at
divergence between major organism groups. Such
an example could be that of bird-reptiles, with
some fossil remains showing birdlike forms with
some reptilian traits. The following illustration
represents one such bird-reptile – an
Archaeopteryx.
Despite being found in diverse locations and kinds of places, fossils are really not
that common for several reasons.
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

Bodies of organisms, especially soft-bodied ones, tend to be broken down
relatively quickly. Unless they are preserved almost immediately, as in amber
or resin, low oxygen or low temperature conditions, or in some other manners,
they will not remain intact very long.
Places where such preservations can happen are not extensive
Places were fossils may be found, such as in sedimentary rock, may not be
very easy to get at or to search through.
For some of these reasons, fossil records can show considerable numbers of gaps
(like having pages of books missing). Gaps in fossil records especially bothered
Darwin in his studies and observations. Since he felt that changes proceeded
slowly and not in leaps, there should have been many long lines of fossil records.
These were not readily evident in his time. Since then, many more fossils have
been found and many gaps have been filled in. However, there are still many
“blank spaces” in trying to trace the histories of various groups of organisms.
Comparative Anatomy
The study and comparison of body forms of plants and animals has probably been
the most common method of trying to establish the degrees of relationships
between organisms. Such comparisons also show that changes are taking place
within and between groups. Observations of general conformations or external
body features have been (and still are) frequently used to place individuals into
specific groups. The more alike that organisms looked, the more closely related
they were assumed to be. The resemblance of structure and anatomy can be
classified as either homologous structures or analogous structures.
Homologous structures are body parts of different (species of) organisms which
appear to be derived from the same embryonic tissues. That is, they seem to have
common origins. However, different environments have resulted in modifications
to produce slight differences in structures and often different uses.
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Homologous organs: these vertebrates have the same basic
arrangements of bones, but they have been put to different uses.
Homologous structures are interpreted as results of divergent evolution. This is
an evolutionary trend where a certain group splits and the resulting sub-groups
begin to show greater diversities over time. The many breeds of dogs evolving from
a common wolf-like ancestor would be an example of divergence taking place.
Opposite to this divergent type of change is one where some unrelated groups
begin to show convergent evolution over time. This is shown by analogous
structures. Analogous structure are where embryonically unrelated parts have
come to function similarly. (Different origins, similar functions.) In the following
diagram, the whale paddle and fish fin have similar functions, but have different
origins, likewise with the bird wing and the fly wing.
Examples of convergent evolution.
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In addition to the preceding examples of convergence, other examples involve
many marsupial species in Australia and placentals in other areas. A wombat in
Australia is somewhat similar to the groundhog (or woodchuck) in Saskatchewan,
and the Tasmanian devil has shown convergence in numerous features with
badgers and wolverines. People using comparative anatomy as a means of trying
to determine relationships between organisms must be careful to distinguish
structures as either homologous or analogous.
Studies of homologous and analogous parts, along with fossil, biochemical and
other studies, are used by scientists to assist in setting up phylogenetic trees.
These diagrammatic representations indicate how some scientists think certain
plant or animal groups developed and their relationships with others.
Vestigial Organs
Vestigial organs are structures that were functional in the organisms’ ancestors
but over time have become much reduced in size and often non-functional. In a
human, such vestigial features could include the sometimes troublesome
appendix, the remains of another eyelid and reduced facial-head muscles, such as
those controlling ear movements.
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Embryology
Studies of embryos of different species can sometimes show the degree of
relatedness between them. In many species, early embryonic development is very
similar. As developments progress, changes become more noticeable and the less
related that species are, the greater the differences.
Comparative Biochemistry
Scientists are able to study and compare the structures of chlorophylls,
hemoglobins, enzymes, hormones and other compounds of different organisms.
Proteins and, in particular, their amino acid sequencing can be analyzed in quite a
bit of detail. Researchers now have the ability to determine the actual amino acid
sequences of most individual proteins. As well, they have the ability to test
proteins from different organisms and to determine just how many amino acids
may be different. For example, one particular cell protein in humans and
chimpanzees has l04 amino acids and all are the same. This similar protein in a
certain type of fungus has 44 amino acids which are different. In constructing
phylogenetic trees, scientists will sometimes use the number of differences in the
amino acids to determine when groups may have split apart. The fewer the
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differences, the more recent the split and the more closely related groups are.
Some individuals claim to be able to use a molecular clock to date particular
groups according to how many amino acids are different between them. That is,
an assumption is made that amino acid changes occur at fairly regular time
intervals in most species. The number of changes or differences in selected
proteins between two groups can be multiplied by the estimated time for each
change. This would give the estimated time when the two groups split or
branched.
Chromosome or Genetic Similarity
When organisms reproduce, they transmit certain characteristics to their
offspring. Transmission of characteristics is accomplished by a substance known
as DNA, which makes up chromosomes and genes. The actual structure of DNA
and the way in which it passes along an organism’s characteristics have already
being examined in other parts of this course. In the preceding section on
biochemistry, amino acids and amino acid sequencing were mentioned as
important factors in comparing species. It is the DNA which determines the
identity of the amino acids used and their sequencing in protein synthesis.
Therefore, similarities or differences between organisms are really based on the
genetic component. As with previous techniques, the greater the similarity between
organisms in their chromosomes, the greater the chance of being more closely
related. General chromosome numbers and their structures could be observed
and used as a basis for making relatively fast, general comparisons of organisms.
If more precise analysis is needed, scientists can study nucleotide sequencing in
DNA to determine just how similar two chromosomes from different organisms
are.
On a general level, determining whether or not certain individuals belong to the
same or related species can be done by noting the reproductive success between
them. The numbers and structures of chromosomes play an important part in
deciding if two individuals can mate or cross to produce fertile offspring. If an egg
and a sperm differ in the numbers or structures of their chromosomes, it is unlikely
that successful fertilization or reproduction will take place. Since different species
do have different chromosome numbers and characteristics, this is one factor
keeping them separate.
In some situations, crossing between two related species can
occur, with offspring being produced. However, in most
instances, the offspring of such crosses are sterile or infertile.
The most common cross is that between horses and donkeys.
Breeding a male donkey and a female horse results in an
infertile offspring – the mule. Mules have greater endurance
and are stronger than horses, in addition that have the
potential to grow taller than either parent. These
characteristics are due to hybrid vigour.
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Actual Evolution in Action
Evidences of particular actions or situations are most reliable when actual
observations are made. This applies to organic variations or evolution as readily
as for other situations. As was mentioned earlier, changes in organisms have
been noted – especially in those with relatively short generation times. Changes
are commonly noticed in viral or bacterial strains, either in laboratories or outside
of them.
Other changes, recorded for study purposes, have included a population of
peppered moths in England, which changed in colour as environmental conditions
changed. In the 1700’s, trees in the countryside were covered by gray-green
lichens. Two types of moths could be found, light coloured ones and dark
coloured ones. The dark coloured ones were rare because they could be seen on
the tree trunks and they were eaten. But as the Industrial Revolution occurred,
trees became darkened by soot. The light moths were eaten and the population of
moths became predominately dark.
Another change is the squirrels in the Grand Canyon in the United States.
Originally a single population, it was apparently split into two groups by a change
in the course of the Colorado River. As time progressed, changes in the two
groups resulted in the formation of two separate species: the Abert squirrel and
the Kaibab squirrel.
Microevolutions, or small changes occurring in populations, are possible to see
in many other groups of animals or plants. Humans often increase the
frequencies of these, especially in domesticated organisms, by selective breeding
or other practices. At certain points in time, smaller changes eventually
accumulate to where a species becomes quite different than it was previously.
Macroevolution, or major changes occurring over a short period of time may also
occur. This would result in the organism no longer being the same species it was
previously. That is, if it were possible to bring them together, crossing and
reproduction would not take place between individuals of the “old” and “new”
groups.
In studying evolution in general it is common to note many pathways coming to
an end. Extinctions are common features of phylogenetic trees. Scientists
estimate that over 99% of all species that lived at one time are now extinct. It is
not difficult to think of examples of organisms that are presently endangered on
the prairies. Some of these could include prairie dogs, burrowing owls, swift
foxes, whooping cranes and bison.
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Major Forces or Conditions Affecting the Earth’s
Crust
Tectonic Plate Movements
In l9l5, German meteorologist and geophysicist Alfred Wegener published some of his
conclusions on the concept of continental drift. The major idea was that continents
at one time were more or less one large land mass. Fragmentation took place and the
various land masses drifted apart. Wegener stated that the fragmentation took place
about 200 million years ago. Initially greeted with some skepticism, continental drift
or tectonic plate movement is now a well-established fact. Extensive studies and
mappings of the crusts beneath ocean surfaces (especially with the use of satellite
technologies), along with other findings, has added a great deal of information as to
what has happened and even as to what is likely to happen.
Movements beneath the earth’s surface, owing to its molten interior, have translated
into exterior crustal movements. The lighter outer crust of the earth is made up of a
series of plates, some of which are part of continents and some are part of ocean
floors. These plates “float” on top of more dense, molten matter. The plates move in
various ways according to the convectional movements beneath them. They move
apart in places, which appears to be the main action in areas of the Atlantic Ocean
presently. In other areas, they have collided or come together to add to the process of
continent formation or mountain building, as one plate slides beneath the other. In
still other places, plates slide past each other, scraping and deforming as they do so.
The edges of plates are frequently marked by volcanic activity or earthquakes or both.
Scientists now say that the land masses have fragmented, moved, come together and
fragmented a number of times. The last time that they were all close together was
about 200 million years ago and the name Pangaea was given to this “super”
continent. It consisted of a northern continent called Laurasia and a southern land
mass called Gondwanaland. The northern mass eventually fragmented into North
America and Eurasia and smaller masses. The southern mass eventually produced
the continents of South America, Africa, Antarctica and Australia. India, which was
part of Gondwanaland, eventually pushed into the Eurasian land mass. The
following graphics illustrate how scientists think movements of continental masses
occurred after Pangaea fragmented.
You may be interested in reading about, or seeing, some of the future projections that
some scientists are making about plate movements. If you are, some of the more
recent geology texts will likely have these.
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Pangaea: 200 million years ago
135 million years ago
65 million years ago
Present
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Lesson 14
Significance of Tectonic Plate Movements
In the short time intervals that humans are familiar with, most movements of the
earth’s crust are insignificant in individuals’ life times. The only major exceptions are
when individuals are directly affected by some of the consequences related to these
movements – such as volcanic actions, earthquakes and related actions. Less
obvious are some of the indirect effects which could involve very large land areas and
possibly the entire planet. Some of these could include climatic changes due to
volcanic ash or dust from major eruptions. These effects are generally not as
dramatic and not as obvious as the conditions which actually caused them. Over
thousands and millions of years, however, plate movements do have major effects on
many physical conditions.
Climatic Effects of Plate Movements
Plate movements would obviously affect climate. For instance, being close to the
equator at one time permitted the present continent of Antarctica to experience much
warmer and more moist conditions. This is reflected in some of the fossils found
there being that of tropical organisms. Climate is not just determined or affected by
latitude. Different positions of continents establish and affect ocean currents. These
have major influences on climatic conditions and climatic patterns over adjacent land
masses. Climates are also affected by the mountain ranges created by volcanism or
usually by plates converging or moving together.
Temperature and precipitation or moisture levels are important influences on living
organisms. As these conditions changed over millions of years, they affected the
developments and changes of species. Changes in climate are related to other
conditions which also had effects on life forms.
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Some of these other conditions could have been the effects or results of climatic
changes. The melting of ice caps or the onset of glaciations are included in this,
along with accompanying ocean level changes. Other conditions could have
themselves caused climatic changes and subsequent results. The ash or dust
from volcanoes was already mentioned.

Currently, the greenhouse effect and global warming is receiving considerable
attention. The burning or combustion of fossil fuels is releasing carbon and other
gases into the atmosphere. These have the effect of permitting short wave
radiation through to the earth and keeping long wave radiation from escaping.
This results in temperatures going up.
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Lesson 14

Another item of consideration is the depletion of some atmospheric ozone over
various regions of the planet. This is allowing higher levels of ultraviolet light to
reach the earth’s surface. The full significance of this on plant and animal life is
not yet known but, for the most part, it does not seem to be favourable.
Other Physical Influences
While the preceding actions or conditions have been mentioned as playing significant
roles in changes occurring on this planet, they are not the only ones. Additional ones
could include atmospheric conditions (such as oxygen levels), soil conditions, water
salinities and topographies or relief.
Summary
Signs or evidences of change among living things can take a variety of forms.
Where possible, a number of these evidences are used together in attempting to
establish patterns of development, changes and relationships among groups. In
the process, the attentions of different groups such as paleontologists, geologists,
comparative anatomists, geneticists and physiologists, may all be utilized.
However, accuracy of interpretation is only as good as the data or collected
information. For this reason, while some evolutionary pathways and relationships
seem clear, others are not. Many of these doubtful ones will likely remain so,
considering the long periods of time involved and the scarcity of information or
clues at various points.
Evolving or changing is a noticeable action in all that is around us. It is apparent
in the non-living and living and from very small areas of our planet out into the
greater expanses of our galaxies and universe. Most people accept the concept of
these changes or evolutions happening. Differences of opinions do exist on actual
mechanisms causing the changes and especially on the reasons for the changes
actually taking place. These reasons range from Divine guidance to genetic and
environmental influences. To different people, the various reasons bring different
degrees of acceptance or non-acceptance. It may very well be that no one reason
is totally correct and none are entirely wrong either. Perhaps if there is one
reason in the “why” of evolution, it may involve some aspect of all the reasons
presented thus far and some that we may not yet be fully aware of.
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