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UNIT 2 – GEOLOGIC TIME AND EVOLUTION
Earth History 1404 -Study and Lecture Guide
(Revised 7/11)
UNIT 2 HOMEWORK
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What Is Time?
- Time is not always easy to define. Time can mean different things to different people. Time can be
defined by the methods or comparisons by which we measure it. For us, the time of the Egyptian
pyramids at 2550 BC is very old.
- The concept of long periods of time (billions of years) is almost incomprehensible for most of us.
- Later, we will learn that space and time are unalterably linked to form a space-time continuum and
that without space – there can be no time.
- “Deep Time” is a concept that Earth has had a multi-billion year history.
- Earth is about 4.6 billion years old and our universe is about 13.7 billion years old.
- If we compare the age of Earth with a 1,000 sheet roll toilet paper, each sheet would represent 4.6
million years! (How many millions of years did you use today?)
- Applying vast amounts of time (billions of years) to scientific principles sets geology apart from all
other sciences except astronomy.
Types of Time
- Geology uses two different frameworks for the references of time.
- Relative Age Dating: placing events in sequential order (which came first, second, etc.)
- Absolute or Actual Age Dating: specific dates or specific units of time
Relative Age Dating Examples
- This method does not tell us how long ago a particular event took place; only that one event preceded
another, like comparing granddaughter to grandmother.
Absolute Age Dating Examples
- This would include your age, dates of historical events, etc.
Using Geologic Relative Age Dating
- We can decipher the geologic history of an area by the application of stratigraphic principles to the
various rock layers.
- Although geologists can match up rocks on similar rock type and superposition, correlation of this
type can be done only in a limited area. To correlate age-equivalent rock units over a larger area, we
need to incorporate fossils (using the principle of fossil succession).
That which a person does not understand, he tends to reject.
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Using Fossils to Map Rock Layers of Earth
- Fossils (previous life forms) are useful because most lived for a certain length of relative time in the
geologic past.
- The Principle of Fossil Succession (life becomes more complex over geologic time) would become
important for mapping large areas across Earth. But not just any fossil – we need special fossils
called guide or index fossils.
- Guide (index) Fossils: Fossils that are easy to identify, were geographically widespread but existed
only for a relatively short time period (quick appearance and quick extinction).
Mapping Relative Geologic Time (Determining the geologic ages of rock layers or strata)
- The Principle of Superposition and the Principle of Fossil Succession and logic were used in
constructing the first geologic time scales. In turn these principles were also used to create a
composite geologic map of the Earth’s history.
- The early maps and time scales were constructed haphazardly during the 19th century by researchers
in Europe. It was not the result of an organized effort.
- Over the century, when all of the strata studies and maps were placed together, the geologists created
a relative time scale based on the strata of rock being arranged in the correct sequential order
(according to fossil data of that time).
Advanced Stratigraphy: The study of the composition, origin, areal distribution and age relationship of
layered rocks
Relative Geologic Time Scales – Two Subdivisions
- Relative time - Intervals of the Earth's geologic time, in a specific order based upon relative age
relationships (most commonly - stratigraphic position and fossils).
- (Relative here means that we can determine if something is younger than or older than something
else, but not how long it existed.)
1. Time Unit (Geochronologic units): An interval of relative time based on specific fossil
assemblages
2. Time Stratigraphic Units: Units of rocks (layers of rocks) that were deposited during specific
relative time intervals (according to the fossil record)
Relative Time Units (Geochronologic units)
- Time units are simply units defining various intervals of geologic time (quasi life based).
- There are no specific time intervals assigned to these individual units – BUT we can generally assign
a beginning time and an ending time for each individual unit (through radioactive age dating)
- The major relative time units, in order of longest to shortest time intervals: Eon, Era, Period, Epoch,
and Age. These units were finally established by 1900.
Geologic time scale
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Time Unit Eon: This represents the longest intervals of geologic time. Each has been assigned a
different time length based today on radioactive age dating. There are four named eons (with the
oldest at the bottom)
4. Phanerozoic Eon: (542 Million Years to Today) time of visible life
3. Proterozoic Eon: (2.5-0.542 Billion Years) time of unicellular AND multi-cellular life
2. Archean Eon: (4.0-2.5 Billion Years) ancient, represents most of the oldest know rocks; life
on Earth at this time probably contained only unicellular life
1. Hadean Eon: (4.56-4.0 Billion Years) time before most of the oldest known rocks on Earth;
(most often found with meteorites)
Oldest: HADEAN EON____ARCHEAN EON___PROTEROZOIC EON______PHANEROZOIC EON: Youngest
- These four eons represent the major “time” subdivisions of Earth’s history and have different time
intervals.
Time Unit Era: Time unit of ancient life that is a subdivision of an Eon.
- Eras are generally separated by catastrophic life extinctions boundaries
- Eras can extend for up to hundreds of millions of years.
- Eras was established in 1840 on the basis of the then known occurrences of “mass extinctions”.
- Mass extinction: greatly accelerated extinction rate for plant and animal life that results in a major
decrease in biodiversity (varieties of life)
- In this class, we will principally study only the eras for the Phanerozoic Eon, although all eons have
assigned eras.
- Phanerozoic Eon has a subdivision of three Eras. (Starting with the oldest at the bottom)
3. Cenozoic Era: (Ceno = younger) (65 Million Years to Today) age of the mammals,
flowering plants and birds
2. Mesozoic Era: (Meso = middle) (251-65 Million Years) age of the dinosaurs
1. Paleozoic Era: (Paleo = older) (542-251 Million Years) early complex life; evolution of fish,
insects, vascular plant life, life’s conquest of land
Phanerozoic Eon subdivided: PALEOZOIC ERA_____MESOZOIC ERA___CENOZOIC ERA
Time Unit Period: Eras are subdivided into multiple time intervals of ancient life called Periods.
- Examples would be:
Cretaceous Period (145-65 Million Years)
Jurassic Period (200-145 Million years).
Triassic Period (251-200 Million Years)
- These units of time must be defined by specific fossil types that are found world wide
-Periods were first established in Europe. One by one
- The Ordovician Period was established (1830’s) to settle an argument between two respected
geologists (Sedgwick and Murchison) in Great Britain.
Cenozoic Era subdivided: Tertiary Period ______ Quaternary Period
Mesozoic Era subdivided: Triassic Period ___Jurassic Period ____ Cretaceous Period
Paleozoic Era subdivided: Cambrian Period ___Ordovician Period ___Silurian Period____
___Devonian Period ____Mississippian Period _____Pennsylvanian Period _____Permian Period
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Time Unit Epoch: shorter time interval of ancient life, a “Period” that can last millions of years
- Commonly used in the US to subdivide the Tertiary Period of the Cenozoic Era
- Examples of the Tertiary Period would be:
Pliocene Epoch (5.3-1.8 Million Years)
Miocene Epoch (23.0-5.3 Million Years)
Quaternary Period subdivided: Pleistocene Epoch ____ Holocene Epoch
Tertiary Period subdivided: Paleozoic Epoch ___ Oligocene Epoch______ Eocene Epoch
___Miocene Epoch ______Pliocene Epoch ___
Time Unit Age: nebulous term for a division of time dominated by a major life form or a major
geologic event (Ice Age, Age of Fishes, etc.)
Informal Time Modifiers
- There is also an informal terminology of modifier adjectives that is used to divide any time unit:
- Late (last)
, Middle,
Early (first)
- Various examples would include Early Phanerozoic Eon or Middle Cenozoic Era or Late Jurassic
Period or any other combination.
- The preceding was a review of time intervals or time units that were based of life and extinctions and
did not involve the actual layers of rocks.
Time Stratigraphic Units
- To include the rock layers, we need a new classification system:
- Time Stratigraphic Units: Units of rocks (layers of rocks) that were deposited during a specific time
interval (Jurassic, etc.)
- These units are easier to visualize by using comparisons
Based on Life
Based on Life and Rocks
TIME UNITS
TIME STRATIGRAPHIC UNITS
Age
Stage - ignore
Epoch ------------------------------------- Series (mostly for Cenozoic Era rocks)
Period ----------------------------------- System (Mostly for Paleozoic & Mesozoic rocks)
Era
Eon
Erathem – rarely used, ignore
Enonothem – rarely used, ignore
- System – a time-stratigraphic unit in which layers of rocks were deposited during a specific Period
(which we generally use for the Periods of the Paleozoic Era and Mesozoic Era rocks
-A Jurassic System represents layers of rocks that were deposited during the Jurassic Period of the
Mesozoic Era
- A system does not describe the physical attributes of the rocks
- Example:
TIME UNITS
TIME STRATIGRAPHIC UNITS (Strata)
Cretaceous Period
Cretaceous System (of rocks)
Jurassic Period
Jurassic System (of rocks)
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- Series – a time stratigraphic unit of rocks that were deposited during a specific Epoch (which we
generally use for the Tertiary Period of the Cenozoic Era rocks)
- A series does not describe the physical attributes of the rocks
- Example:
TIME UNITS
TIME STRATIGRAPHIC UNITS (Strata)
Pliocene Epoch
Pliocene Series (of rocks)
Miocene Epoch
Miocene Series (of rocks)
example of time stratigraphic section
Informal modifiers of the above System and Series units:
TIME UNITS
TIME STRATIGRAPHIC UNITS
Late
Upper
Middle
Middle
Early
Lower
Rock Stratigraphic Unit: Formation
- We need an alternative to Time Stratigraphic Units because there are too many limits to pure
stratigraphic units.
- Thus a “rock-friendly” alternative to the time-stratigraphic units of systems and series is now in order
where time is ignored.
- Rock Stratigraphic Unit: rocks defined by the physical attributes and characteristics of the described
rocks (time is ignored)
- The basic Rock Stratigraphic Unit is the formation.
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- Formation: a mappable rock unit with distinctive upper and lower boundaries
- Formations are characterized by:
a. being defined only on the physical aspects of the rocks (not fossils) and there may be a suite
of related rocks rather than just one rock type
b. being formed under generally uniform conditions or with a specific geologic environment
such as marine or desert or shoreline, etc.
c. being tabular in shape and extent
d. being time-transgressive, and may not be the same age everywhere throughout its thickness
Map showing various Texas rock formations
- Formation Naming Conventions
- Formations have a two-part name
1. First part = geographic location (type section locality)
2. Second Part = rock type of the formation
(Example: Oakville Sandstone found near Oakville TX)
- If a formation consists of more than one rock type, the word “Formation” is used in place of a
specific rock type (example: Frio Formation of South TX consists of both sandstone and shale).
- Lumping of Formations (FYI)
- Lumping
Group: more than one formation
Supergroup: more than one group
- Splitting of Formations
- Member: laterally formations can be split into members if the rock type changes
- Members reflect lateral environmental changes in the rock type (like from the sandy beach to
the mud-shale bay)
Showing “members” f formation
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College is not a contest. You don't have to compete with anyone else for your grade. Learn at
your own pace and don't feel inferior if you don't understand something the first time around.
Biostratigraphic Units
- Petroleum geologists often collect cuttings from drill holes which they examine with a microscope,
looking for fossils (mud loggers are often paid to do the same)
- They will correlate lengths of drill cuttings that have the same set of fossils (associated with oil)
- Biostratigraphic Unit: Bodies of strata that contain recognizably distinct fossils
- Fossil content is the only criterion used to define this unit.
- Can vary from a thin stratum to strata thousands of feet thick
- Biostratigraphic unit is sometimes referred to as a biozone
Biostratigraphic unit
Stratigraphic Section
- Stratigraphic Section: Any sequence of rock units found in a given region
Also called Columnar Section, Geologic Section, Stratigraphic Column, Geologic Column
Example of stratigraphic section
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Type Section
- Type Section: a rock unit (stratigraphic section) at a location that is most representative for
comparison purposes
Type sections for Abo Formation
AGE DATING AND TIME SCALES
Age Dating And Time Scales
- Up to now, we have been discussing the methods used to correlate strata and fossils based on relative
age dates.
- We will now begin addressing detailed age dating. They were many early failed attempts to reach
this goal.
- We should not use the term “absolute” dating because this implies a high degree of accuracy (our
book does).
- We should use the term “actual” age date which implies an accuracy range: 2 BY ± 2MY
An hour of study is defined as studying for 45 minutes and a break of 15 minutes. Ten hours of
continuous study without a break is defined as one hour of study.
Early Attempts to Date Earth
- Early Christian scholars, lead by Bishop James Ussher (1654) tried to establish the date of creation by
analyzing historical records and genealogies found in the Scriptures. This study found that the Earth
was created 6,000 years ago on October 26, 4004 B.C. at nightfall.
- John Lightfoot in 1642 derived that date to be September 17, 3938 BC.
- Bishop Lloyd’ Bible was the first to publish the date 4004 BC in the margins of the Great Edition of
the English Bible in 1701.
- This 6,000 years age has pervaded many Western chronologies of Earth history since the 1700s.
- Other studies would indicate that Earth was much older than 6,000 years.
Earth created: 10/26/4004 BC at night or 9/17/3938 BC
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6,000 years ago ????
_____1642 AD____1654 AD_____1778 AD______1864 AD_____1868 AD______1889 AD______
Time Based on Earth Cooling Rates
- French zoologist Georges Louis de Buffon (1778) assumed Earth cooled slowing to its present state
from a molten beginning. By melting various balls and cooling them, he extrapolated that Earth was
75,000 years old.
- English physicist Lord Kevin (1864) re-evaluated the molten rock theory and estimated the Earth was
a maximum of 400 million years old.
- However, neither knew about the radioactive decay that is heating the Earth.
Time Based on Seawater Salinities
- John Joly (1889) tried using ocean salinity to date the age of the Earth
- Salt content for ocean water: 2.2 pounds per cubic foot seawater
- Salt content for fresh water: less than an ounce per cubic foot fresh water
- Joly assumed: the oceans are as old as Earth, were originally all fresh water, and that the oceans are
getting saltier with NaCl all throughout time.
- Joly measured the ocean salinity, estimated the volume of the oceans, and estimated the rate of river
water salt delivery.
- This would give an age to Earth as 90,000,000 (90 million) years.
- Joly did not take into account the huge deposits of salt from evaporated seawater; these deposits
occur throughout geologic time.
- Ocean salts other than NaCl give a totally different time.
Time Based on Average Rate of Sediment Deposition
- Based on assumptions that the average rate of sediment deposition is constant through time
- Derived by estimating average rate of world-wide sediment deposition, measure the total thickness of
all known sedimentary rock, then calculated the time required for all this to happen
- By 1900, most estimates were 100,000,000 (100 million) years for the age of the Earth
- False assumptions included: gaps in sedimentary rocks are common (remember unconformities);
rates of deposition are not constant and sediments are compacted after deposition
Time Based on Rates of Evolution
- Darwin and many others tried to calculate Earth’s age based on the evolution of all types of animals.
- Darwin (1868) estimated that 300 million years had passed since the age of the dinosaurs (only 65
million in reality).
- Everyone came up with totally different ages using evolution.
- But evolution did show that Earth had to be very old.
Time Based on Seasonal Changes
- Dendrochronology: tree ring dating
- Growth rings are added annually; each ring consists of a light (wet season) and adjacent dark (dry
season) layer of woody growth.
- Count the rings and compare to nearby dead trees to get ages
- Oldest known tree is the bristlecone pine of Ca and NV: 4,500 years old.
- Year-by-year chronology can extend time back to 12,000 years (more than the 6,000 year estimate).
- It is not a very useful method in geology.
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Time Based on Seasonal Changes (continued)
- Varves: thin layers of sediments that are added during the various seasons
- Glacial Lake Varves: sediments derived from melting glaciers that form two distinct layers
- These varves consist of pairs of light-colored summer silt and dark-colored winter clays. They
average only 1-2 cm in thickness each year.
- Some glacial lakes have been found to be more than 12,000 years old.
- Evaporate Lake Varves: layer of sediment added during wet season (dark-colored layers) and dry
seasons (light colored seasons)
- Deposition of the 1,800 feet thick Permian-aged Casteel gypsum Formation of West Texas and
Southern New Mexico required about 300,000 years to form!
Casteel Formation gypsum varves
Glacial Ice Cores
- Ice cores: long cylinders of ice pulled out of deep holes drilled into glaciers.
- Samples of ice cores from Greenland can stretch back 250,000 years; in Antarctica – back to 740,000
years.
- Layers in glacial ice are added annually and can represent each year’s snow.
- Trapped within the ice are tiny bubbles of air that contain CO2 and oxygen isotopes (unchanged?).
- These trapped gases can give us a record of past greenhouse gases on Earth (controversial).
- Scientific study of ice cores can reveal ancient climates going back 250,000 years (controversial).
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CHEMISTRY AND ACTUAL AGE DATING
______1896 AD_____1902 AD_______ 1903 AD_______1906-1907 AD____1912-1913 AD____
Brief History of Radiometric Age Dating
- 1896: Becquerel discovered radioactivity in uranium.
- 1902: Rutherford and Soddy discovered that radioactive elements break down into other elements in
a predictable sequence.
- Radioactive Decay: elements that break down into new elements at a known rate of time (a
breakthrough for age dating)
- 1903: The Curies discovered that radioactive radium gives off heat.
- 1906: Radioactivity was also discovered in thorium, rubidium, and an isotope of potassium. It was
determined that the Earth’s core remains hot because of radioactive decay.
- 1907: Boltwood published a rock date of 2.2 billion years, using radioactive dating of rocks and
minerals.
- 1912: The nucleus of atoms was discovered.
- 1913: First crude separation of isotopes took place.
Atoms and Elements Review
- Element: a substance that cannot be separated into a simpler substance (it is made up of one type of
atom)
- Atom: smallest particle of an element that retains the properties of that element
- Atom is composed of electrons (negative charge), neutrons (no charge) and protons (positive charge)
- Protons and neutrons for the “nucleus” of an atom; electrons form “clouds” or “shells” that orbit the
nucleus
- -The neutron and proton are nearly equal in mass and are 1850 times heavier than an electron
- Change the number of protons (atomic number) and you get a different element.
- There are 92 naturally occurring elements plus many man-made elements
Helium atom
- The elements are listed in a table referred to as a Periodic Table of elements (Pages 64) textbook.
- Periodic Tables present an arrangement of elements according to their repeating physical and
chemical similarities
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Plato- Knowledge is the food of the soul
Isotopes
- Isotopes: are created by adding a neutron(s) to an atom
-Isotopes are variations of the same type of parent element
-Most elements can have isotopes
- Some isotopes are radioactive (unstable nuclei)
- Other isotopes are NOT radioactive (stable nuclei)
Radioactive Decay
- Isotopes that have unstable nuclei can change spontaneously to a lower energy state.
- During radioactive decay, one type of element (a parent) will change into a different type of element
(a daughter product)
- This will be addressed more in lab.
- Thus one element can change into another element by radioactive decay.
Cloud Chambers and Geiger Counters
- We can visually see some radioactive decay within “cloud chambers”
- We can also hear some radioactive decay with Geiger counters, especially with uranium-rich rocks
Radioactive Decay Processes (FYI)
- Radioactive decay processes are very important for age dating
1. Alpha Decay
2. Beta minus decay
3. Beta plus decay
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- Gamma radiation is the high energy photon emitted from the nucleus of the atom during radioactive
decay
Modes of Decay:
1. One Step: for example rubidium to strontium, one atom changing into another
2. Decay Series: many steps, for example from uranium, through many elements, to lead
Radon 222
- Radon is an invisible, odorless, tasteless cancer-causing radioactive gas.
- It is created by the natural breakdown of uranium in rocks and soil
- It is the second leading cause of lung cancer in the US
- A basement in an area of granite sands and soils can be a problem; proper venting is critical for safety
Decay Constant
- Decay Constant: actual rate of decay (this information is needed to age date an isotope)
-Can be determined by graphing and is unaffected by temperature or pressure
Half life
- Half life: the amount of time it takes for one half of the atoms in a sample to decay into something
else
-Each half-life takes as long as the previous half life
-Each isotope has its own half life; some as short as a millisecond and others as long as tens of
billions of years
-Half life is the most common and convenient way to specify rates of decay, especially with
isotopes (MEMORIZE THIS)
-Rate of decay for a half-life is geometric and not linear
half life
Radiometric Age Dating of Rocks and Minerals
- To date a mineral: the age of incorporation of the atoms of a radioactive isotope into a mineral, which
occurred when the mineral was formed (and NOT the age of the creation of the isotope, for example
within the sun)
- Method: Compare the ratio of the relative amounts of a radioactive parent isotope and radiogenic
daughter product in a mineral; this ratio is a function only of the time elapsed since the radioactive
isotope was initially incorporated into the mineral
- Since the decay rate is known and constant, the age of a mineral can be calculated from the
measurements of the relative amounts of radioactive parent isotope and radiogenic daughter product
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Assumptions of Radiometric Dating
1. Decay rate is constant during geologic time
2. Measurements of amounts of parent isotope and daughter product are accurate
3. No new parent or daughter product is added to the mineral
4. No daughter product was initially present when the mineral was formed
#3 & #4 can be statistically accounted for
Isotope Pairs used:
- Uranium to Lead – half life = 4.5 BY; minerals = zircon, monazite, sphene, apatite
- Potassium to Argon – half life = 1.3 BY; minerals = micas, hornblende, orthoclase, glauconite
- Rubidium to Strontium – half life = 49 BY; minerals = micas, hornblende, orthoclase, glauconite
- Samarium to Neodymium - – half life = 106 BY; minerals = garnets, meteorites, old basalts
Fission Track Dating
- Radioactive decay of uranium can leave tiny pits or “tracks” that remain in mineral crystals; the
number of tracks – compared to the remaining uranium- gives a date for that mineral; limited use for
40,000 years to 1.5 million years
Carbon 14 Isotope Age Dating
- Carbon 14 is a rare radioactive isotope of carbon that forms naturally in the atmosphere
- Carbon 14 is created from Nitrogen gas in the upper atmosphere by cosmic rays from outer space
- After about 75,000 years, most of the Carbon 14 will decay back into Nitrogen and is too old to use
- All living plants and animals continually acquire carbon 14 into their cells by photosynthesis and
ingestion. When the plant or animal dies, no new carbon 14 is incorporated into their cells.
- Thus only organic material can be used for carbon 14 dating.
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Incorporating Radiometric Age Dating with Relative Age Dating
- Igneous and metamorphic rocks are best for radiometric age dating
- Sedimentary rocks can rarely be age-dated (except for volcanic ash). A sedimentary rock that
contains the mineral glauconite can be dated. If a sedimentary rock is cut with a volcanic dike
(igneous rock), that sedimentary rock can be relative age-dated.
- Radiometric dating showed that the Phanerozoic Eon represents a very short relative time frame.
- Radiometric dating allows us to assign actual age dates to all of our geologic columns
- The oldest know rocks that have been dated have come from outer space (meteorites)
Most citizens are not equipped to personally assess the facts, nor often even to separate the facts
from opinion or political spin; science from non-science. They therefore are likely to be
predominantly influenced on these issues by the prevailing perception in their communities.
REVIEW OF CHAPTER 18 Evolution – The Theory and Its Supporting Evidence
- Chapter 18 begins with the history of the concept of evolution; then the chapter discusses fossil
evidence of how certain life forms (fossils) were different over geologic time.
Popular Misconceptions
- 1. Evolution proceeds strictly by chance and not genetics (false)
- 2. Nothing less than fully developed structures such as eyes are of any use (false – sometimes only
light sensitivity is needed by an organism)
- 3. There are no transition fossils that connect ancestors with descendents (false – we do find missing
links)
- 4. Humans evolved from monkeys, so monkeys should no long exist. (false – monkeys are their own
species)
Definitions
- Theory of Evolution: Organisms descended with modifications of ancestors that lived in the past
- Paleontology: The geological study of life history as revealed by fossils
The Theory that Acquired Traits (or Characteristics) Are Inherited
- Jean-Baptiste de Lamarck had the first widely accepted proposal for evolution in1802. It was based
on the false assumption that acquired traits are inherited (genetics not yet discovered).
- The classic example he used were giraffes. He felt that giraffes’ necks got longer because they
stretched to reach higher leaves, and this neck stretching was passed on to their babies who acquired
longer necks.
- Another example of his theory more apparent was that men who developed large arm muscles
because they were blacksmiths, tennis players, or weight-lifters would have babies born with larger
than normal arm muscles.
- However, one experiment involved amputation of mouse tails for successive generations, showing
that even after twenty generations, there was no effect: baby mice were still born with tails.
Genes and DNA – The Key To All Life Forms
- The units of heredity know as genes, cannot be altered by any physical effort by an organism during
its lifetime.
- Traits can only be acquired through genes
- Genes: units of heredity.
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- Repeat: genetics cannot be altered by any physical effort of an organism during its life time.
Other Scientists And Evolutionary Studies
- Linnaean Society is the world’s oldest biological society, founded in London in 1788.
- In 1859 - Charles Darwin and Alfred Wallace proposed Natural Selection as the mechanism for
evolutionary change to the Linnaean Society. But Darwin was the “first” to make the proposal (via a
read correspondence) and was given the credit for first presenting the theory of evolution.
- Gregor Mendel (1860) performed a series of garden pea experiments, in which the plants have
different colors of flowers. As Mendel raised the peas, he made specific crosses between certain
plants and did something very unusual for biology in those days: He counted the results. From this he
developed a theory of genetics.
- Thus, after Mendel’s work was rediscovered in 1900, researchers started seeing parallels between his
theory of genetics and about chromosomes. From this, people figured out that Mendel’s genes were
on the chromosomes. In the 1940s, people finally started fitting the two together. People began to
realize that the DNA in the chromosomes was the genetic material.
Genes and Heredity
- Genes: The hereditary determinants in all organisms; alleles are an alternate of genes.
- Genetic information is carried in the chromosomes of cells.
- Only the genes in the chromosomes of sex cells are inheritable (natural selection).
- Natural selection works on variations of genetic material within populations that can survive.
Mutations and Genes
- Any new variation arises by mutation of the genes:
- Mutation: a change in chromosomes or genes. This is a change in heredity information.
- Mutations account for most variations in populations,
- We can have chromosomal mutations (altered large segments of a chromosome) or a point
mutations (change with a particular gene).
- Mutations are random in regards to fitness: The mutation could be beneficial, neutral or harmful.
- If a species is well adapted to its environment, most mutations would not be beneficial and could be
harmful.
- If the environment changes, the harmful mutation may become useful and can lead to the survival of
the species.
Causes of Mutations
- Many are caused by mutagens: Agents that bring higher mutation rates
- Mutagens include: Chemicals, ultraviolet radiation, X-rays, radiation, and extreme temperatures
- Some mutations are spontaneous taking place in the absence of any known mutagen.
- Some small scale evolution is taking place with insects and rodents because they are becoming
resistant to chemical pesticides.
Species and Speciation
- Species: A biological term for a population of similar individuals that in nature interbreed and
produce fertile offspring
- Speciation: The phenomenon of a new species arising from an ancestral species
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- Allopathic speciation: Species that arise when a small part of its population is isolated from its parent
population. The isolated group is subjected to a different selection pressures. (important to know)
Allopathic (Darwin’s finches)
Speed of Evolutionary Changes
- Phyletic gradualism: The gradual accumulation of minor changes that can bring about the origin of a
new species
- Punctuated Equilibrium: Evolution that occurs rapidly through mutations, giving rise to a new species
within a few thousand years (important to know)
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Directions of Evolution
- Divergent evolution: Divergent descendants adapted to various aspects of the environment
(freshwater fish and saltwater fish examples)
- Convergent Evolution: The development of similar characteristics in distantly related organisms
(camel and llama example)
- Parallel Evolution: Similar characteristics arising in closely related organisms (kangaroo rat and
Jerboa)
Evolutionary Trends
- Cannot be predictive for the long-term future
- Size changes of species (larger or smaller) are common as their environment changes (genetic
survival of the fittest)
- Evolution by natural selection is a two set process
-variations must be produced and maintained in interbreeding populations
-favorable mutations that allowed for survival
Extinctions
- Extinctions take place continually because organisms cannot adapt to a changing environment
- Perhaps 99% of all species ever on Earth are now extinct.
- Extinction can be caused environmentally related changes.
- Some species of bacteria have survived for billions of years.
- Background Extinctions: Continual extinctions of species like we have today
- Mass extinctions: Marked decreases in Earth’s biodiversity have occurred several times.
Theory Support
- The theory of evolution is truly scientific because we can make observations to support or deny it.
- Much of the evidence supporting evolution comes from classifications, embryology, genetics,
biochemistry, molecular biology and present-day small-scale evolution.
- Whether you choose to accept any of this above as valid is through your own decision.
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