Unit 8: Evolution
Content Outline: Geologic Time and Processes (8.3) – Part 1
I.
James Hutton (1726 – 1797)
A. He was a Geologist. This is someone who studies rocks and earth’s processes.
B. He proposed the Theory of Gradualism. This theory tries to explain that that the earth must be very, very
old because in order for some processes to occur, such as mountain formation or canyon formation, it
would require enormous amounts of time.
1. According to the theory, Earth must be very old. This is very important to Darwin’s theory of
Natural Selection because the theory of gradualism supports the time frame needed for Natural
Selection to transform species over generations.
II.
Charles Lyell (1797 – 1875)
A. He was also a Geologist.
1. He wrote a book titled Principles of Geology. (Darwin took this book on the Beagle voyage.)
2. In the book, Lyell proposed the Theory of Uniformitarianism. (“The key to the past is the
present”.) The theory tries to explain that the same geologic processes that are occurring today,
also occurred in the past. These processes helped to create, over millions of years, the geologic
formations we see today. For example, erosion, over millions of years and still today, led to the
formation of the Grand Canyon.
3. For this theory, Earth must be hundreds of millions of years old. (This also supports Darwin’s
theory… it provides enough time to pass so that we get the millions of different species to
evolve.)
B. Review Georges Cuvier and his Theory of Catastrophism at this point. Tie the three people together by
the work that they all did and relate it back to Darwin’s Theory of Natural Selection as time is important
to the theory.
III.
Dating Fossils to create a geologic “time line”:
A. Two ways this can be achieved:
1. Absolute “Radiometric” dating
a. Uses the half-life of radioactive elements that accumulate in an organism over time.
b. C-14 (Used for thousands.); U-238 (Used for millions  billions.)
2. Relative Dating
a. Uses the different strata of rock and index fossils to establish a time line. Index Fossils
are fossils that we know a specific time period they existed. They are compared to the
location or strata where we found an “unknown” dated fossil. If the “unknown” is found
above the index, the “unknown” is younger. If the “unknown” is found below the index,
the “unknown” is older. Positions are relative to time.
B. Plate tectonics
1. Refers to the moving of the continental plates.
2. The continents moving are a direct cause of the biogeography of species.
3. Collisions? (Mountains form.)
4. Separation? (Sea floor spreading… see the “world’s Zipper” in the Atlantic Ocean floor.)
5. Subduction and slipping? (Plates being pushed under the other, such as by Japan; Plates sliding
past one another, as in California.)
6. “Ring of Fire” (Refers to all the volcanoes around the Pacific Plate.)
Geologic Time and Processes (8.3) – Part 2 (Microevolution vs. Macroevolution)
I. Microevolution (Evolution/change on a small scale.)
1. This term usually refers to changes in allele frequency within a population of a species.
a. This is what we’ve studied so far. Basically, just that whoever reproduces more
will have more of their genes in the future gene pool.
b. Hardy-Weinberg allows us to study microevolution as we can monitor the
allele/gene frequencies in a population.
c. The types of natural selection (disruptive, stabilizing, and directional) show
changes phenotypically of a population because of microevolution.
2. With enough time and enough changes in DNA, microevolution can eventually lead to
macroevolution - The evolution of a new species or higher taxon in the classification
system from a pre-existing species.
II. Macroevolution (Evolution/Change on a large scale.)
A. This term refers to the evolution of a new taxon from a pre-existing taxon. (Basically, the evolution of a
new species or higher on the classification scale.)
B. Speciation – the creation of a completely new species from a past species.
a. Allopatric speciation – speciation that occurs because of GEOGRAPHIC ISOLATION of a
population
1. Steps that lead to allopatric speciation
1. Geographical separation of species (with its gene pool) from other populations
2. No gene flow. Mutations are inevitable so the separated gene pools (from
population A and population B) will continue to have separate mutations. If there
is no gene flow, then these mutations are not transferred to the other population
and the gene pools become increasingly different.
3. Greater genetic drift (because of smaller populations)
4. Eventually, the DNA of the 2 populations becomes so different that the
organisms cannot reproduce even if they are mixed back together.
b. Sympatric speciation – Speciation that occurs with no geographic isolation.
1. This type of speciation is more rare but does occur. It occurs because there is some type
of reproductive isolation amongst the species that prevents them from breeding. Since
there is no breeding, there is no gene flow. Without gene flow, the 2 groups can
accumulate differences and eventually become a new species.
C. Reproductive isolation – If 2 populations cannot reproduce for whatever reason then they can eventually
become new species because of differential environmental pressures and differential mutations between
the 2 species.
a. Types of barriers that create reproductive isolation
1. Prezygotic barriers– reproductive isolation that occurs before a zygote is formed
2. Postzygotic barriers – reproductive isolation that occurs after a zygote is formed
Visually depicting speciation through Cladograms
I.
Homologous structure vs. Analogous structures
a. Remember, Homologous structures indicate common ancestry.
b. Remember, Analogous do not indicate common ancestry (Just similar function.)
1. Fly wing vs. Bird wing
c. Convergent Evolution –Remember, only appear to be related due to similar environments and pressures.
II.
Cladogram or Phylogenic Tree
a. It is an “Either – or” evolutionary tree based on shared characteristics.
1. Based on a new trait developing and being passed down to descendants
b. Common ancestors are indicated at the “Y”
c. Each branch called a clade.
1. Must be common ancestor and all of its descendants.
d. Construction of one:
1. Character Table - Table used to generate phylogenetic trees by comparing characteristics of
species.
a. Each new character represents the divergence of two groups from a common ancestor
(one without the new characteristic and one with it) and thus shows the order in which
new traits evolved
b. Ingroup (group of taxa that is actually being analyzed) vs. Outgroup – species or group
of species that is known to have diverged before the lineage that contains the groups we
are studying
a. Ingroup organisms get a 1; outgroup organism gets a 0.
c. Rules for constructing character table:
a. Traits go on the left side
b. List traits from newest to oldest going from top to bottom
c. The traits that are shown by the LEAST organisms will be the YOUNGEST
because they appeared more recently; therefore, there would not have been as
much time for speciation to occur creating multiple species with the trait.
d. The traits that are shown by the MOST organisms will be the OLDEST because
they appeared early in the evolutionary lineage; therefore, all organisms that
diverged after that trait appeared would acquire that trait from their ancestor.
2. Shared Primitive Character
a. Trait that is common to many taxons or clades. (For example, a Backbone.)
3. Shared Derived Character
a. Trait that is common to one clade only. (For example. Hair on mammals.)
E. Tips for reading a cladogram
1. Extant (living species) will be up top
2. Extinct species will be on bottom
3. They can be turned either way and be read the same.
4. The youngest species will be the one with the shortest divergence line (and the oldest will be the one
with the longest)
5. Each node represents divergent evolution. Typically, cladograms are based off of the emergence of a
new morphological trait that all species from that point forward will show (because they come from
this ancestor that had the trait) but none of the organisms before will (because they evolved before the
trait appeared)
6. A species will be most related to (because they share the most recent common ancestor with) the
species that evolves after them.
7. The most recent common ancestor of 2 extant species can be found by tracing the line back to where
they diverged.
Geologic Time and Processes (8.3) – Part 3: Part A (Types of evolutionary change)
1. Gradualism
a. This theory tries to explain that that the earth must be very, very old because in order for some processes
to occur, such as mountain formation or canyon formation, it would require enormous amounts of time.
b. According to this theory, things change slowly, incrementally over time. This would mean that organisms
are constantly evolving at a slow, steady pace.
c. This type of evolution occurs in circumstances where the selective pressure isn’t very strong. If an 1
species is out competing another only slightly, then they are not surviving/reproducing at an extremely
higher rate. This would mean that it would take a long time before the gene pool to reflect an extreme
greater percentage of their genes.
2. Punctuated Equilibrium
1. In this method, long periods of stability (this is the equilibrium) are interrupted suddenly(this is the
punctuated) by a major disruption(such as an asteroid hitting the earth) that causes a mass extinction of
existing species to occur. Once all disruption has calmed down(usually after several years), a mass
evolution of new species will occur to occupy all the new open niches that were created due to the mass
extinction. (These punctuations usually mark/cause the end of an era.)
a. Adaptive Radiation - Evolution of many diverse species from one common ancestor.
i. Typically when few organisms occupy new area or after a natural disaster. Generally,
the variations of a population that do not win in competitions die out. However, in
situations of occupying new land or following a natural disaster, there are many
unfilled niches; therefore, these variations that aren’t originally successful fill other
available niches. Because they have different environmental pressures in these new
niches and they are not competing with others in the population, they begin to rapidly
evolve.
2. This type of evolution occurs whenever 1 species has completely out-competed other species for
extended periods of time. There is little to no evolution seen during these times of dominance because
this species is basically already perfectly adapted to the environment; therefore, all new mutations
(remember mutations will always happen) will be inferior to the phenotypes already present. These new
mutated individuals will therefore not survive as long or reproduce as much as what is already present.
Then, some type of disaster causes environmental change that causes this species to no longer
dominate.
3. Examples:
a. Snowball Earth caused the end of the Pre-Cambrian era. 7/8 of the earth was covered by ice.
It took millions of years to thaw out. Most organisms died. Those that survived were around
deep sea thermal vents, where it was warm enough to support life. Once the ice melted, the
Cambrian explosion of species occurred to start the beginning of the Paleozoic Era (called the
Age of Fish).
b. Pangaea, the super continent, caused the end of the Paleozoic era. This coming together of all
the continents caused the earth’s water to be dramatically displaced. The interior swamps and
oceans disappeared and over time became vast deserts. Most aquatic and terrestrial animal
and plant species went extinct due to loss of water. Those that survived were around the edge
of the supercontinent or in the one big ocean. This mass extinction allowed for the mass
explosion of new reptile species and desert plants. This began the Mesozoic Era (called the
Age of Reptiles).
c. The Asteroid that hit the earth 65 million years ago caused the end of the Mesozoic Era and
the extinction of the dinosaurs and many plant species. It caused the sun to be blocked out by
soot and ash for years. The planet became very cold. The organisms that survived were
mainly Mammals, because of their warm fur. Some reptiles, amphibians, and fish survived
too. Also some plants. Once the sun returned to the entire earth, we see the mass explosion of
mammals and the beginning of the Cenozoic Era (called the Age of Mammals).
Geologic Time and Processes (8.3) – Part 3: Part B (Chemical evolution – the beginning of life)
I. Conditions of Early Earth
a. Thick with water vapor and compounds released by volcanoes
b. Cooled and water vapor became oceans
c. NO OXYGEN – because there was no photosynthesis
a. Photosynthesis was complex so it took a while for these organisms to occur
d. Prokaryotic Life forms evolve about 3.5 Billion years ago
e. Eukaryotic Life forms evolve about 2.7 billion years ago (Remember the Endosymbiant Hypothesis?)
f. Oxygen Catastrophe or Oxygen Revolution
1. The rapid rise in atmospheric free oxygen occurred about 2.7 Billion years ago by the evolution of
Cyanobacteria in the Earth’s water bodies. (“cyano” means “blue-green”) because they were
performing photosynthesis.
2. This caused a mass extinction of anaerobic (lacking oxygen) organisms. Oxygen was deadly to them.
II. Chemical evolution versus organic evolution
a. What we have studied this entire unit is Organic evolution (the evolution of living things). This theory requires
that all living things come from something living before them. Even so, something had to be the very FIRST
living organism.
b. Chemical evolution – evolution of inorganic compounds to organic compounds. Basically, chemical evolution
tries to explain how the first life form could have formed from nonliving material.
III. Theories explaining chemical evolution.
a. Stanley Miller and Harold Urey – “lighting experiment”. Their experiment shows how the INGREDIENTS of life
could have been formed.
a. Showed that amino acids and other organic molecules could be formed under conditions believed to
simulate those of early Earth.
b. Lack of Oxygen allowed creation (O is oxidizing agent and disrupts chemical bonds)
c. Created early earth environment and showed that lightning could create life
d. Early environment now not believed to have been composed of same compounds used in Miller’s
experiment
e. It has since been discovered that the ratio of elements in their experiment was not exactly like what have
been on early Earth.
b. There have been amino acids found on asteroids that have hit Earth. This could have been the source for the
original organic materials.
c. There are theories that life began in deep thermal vents where the extreme heat provides the energy for such a
process.