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Evolution
Edited by L. Bridge
May 2015
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Earth’s Age
Earth forms
Earliest life forms
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The Concept of Evolution
• In biology evolution is simply defined as biological
changes that have occurred in living organisms since the
beginning of life.
• Evolution is “descent with modification”, which is possible
because of the changeability/malleability of the DNA code.
– “Modifications” = random mutations can constantly change
genotypes/phenotypes from generation to generation = novel
variations of traits in the population
• Evolution assumes a certain relatedness (“common
ancestry”) between organisms.
Evolution “for dummies 9:58
http://www.youtube.com/watch?v=SeTssvexa9s
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Phylogeny: the study of the evolutionary
history of groups of organisms
Plant evolution
(cladogram)
Like a “family tree”
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Primates
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Fossils as Evidence of Evolution
– Hard body parts are preserved
in most cases
trilobite
– Often embedded in sedimentary
rock
– Deposited in layers called strata
• Each stratum is older than the one
above and younger than the one
below
– “Transitional fossils”
• Especially significant finds;
represent evolutionary links
between groups
However fossilization is a relatively
uncommon occurrence, usually
requiring hard body parts and death
near a site where sediments are
being deposited, the fossil record
only provides sparse and intermittent
information about the evolution of
life.
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Transitional Fossils
example: Archaeopteryx
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Vertebrate groups
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New Fossils Of Extremely Primitive 4-Legged Creatures Close The
Gap Between Fish And Land Animals
ScienceDaily (June 27, 2008)
— New exquisitely
preserved fossils from
Latvia cast light on a key
event in our own
evolutionary history, when
our ancestors left the
water and ventured onto
land. Swedish researcher
Per Ahlberg from Uppsala
University and colleagues
have reconstructed parts
of the animal and explain
the transformation in the
new issue of Nature.
Tiktaalik
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Geological Evidence of Evolution
• Geological Timescale
– History of Earth is divided into eras, then periods, and then
epochs
– Based on dating of fossil evidence
– Relative dating method – determines the relative
order of fossils because the fossil-containing
sedimentary rocks occur in layers. Top layers are
younger and thus the fossils in them are younger.
Thus it is a matter of “What came first, second, third”
– Absolute dating method - radioactive carbon dating
(radiometric dating): Makes use of radioactive
elements that decay into other more stable elements
according to a strict timetable.
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Radioisotope Dating
• Ex: Radioactive 14C (In Organic Matter) changes into 14N
(1/2 of 14C will change into 14N in 5,730 yrs)
• Assuming that organic matter always begins with the same
amount of 14C.
• Goal is to compare the 14C radioactivity of the fossil to that
of a modern sample of organic matter. The amount of
radiation left in the fossil can be converted to the age of the
fossil.
• Other similar methods are used to date rocks that are
thought to be billions of years old.
ex: Starting with 8g of 14C, down to 1g; how much time has elapsed?
ans: 8  4  2  1 = 5730 + 5730 + 5730 = 17,190 yrs
27-11
Commonly used radioisotopes for dating:
Parent
Daughter
Change in…
Carbon-14
Nitrogen-14
5730 years
Uranium-235
Lead-206
4,470 million yrs
Potassium-40
Argon-40
1,208 million yrs
Thorium-232
Lead-208
14,010 million yrs
Rubidium-87
Strontium-87
48,800 million yrs
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Geological/Fossil Evidence of Evolution
• Background extinctions
• “Mass Extinctions”
– Large numbers of species become extinct in a short period of time
• Remaining species may spread out and fill habitats left vacant
– Five Major Extinctions have occurred
– It is proposed that many mass extinctions have resulted from
extra-terrestrial events, volcanism, atmospheric fluctuations, global
warming, cooling (ice ages), sea levels, etc.
• However, a current SIXTH one is in progress due to human
encroachment
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The last major mass extinction
• The K-T Event
– 65 million yrs ago
– Killed off the dinosaurs, among others
– Marked the end of the Cretaceous period,
beginning of Tertiary
• Clay from that period is high in iridium, an
element in meteorites
• Proposed that meteorites hit Earth and dust
filled the atmosphere
– Blocked sunlight, plants died
– One reason why we don’t see some of the
evolutionary ancestors today is because of
mass extinctions.
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Biogeographical Evidence of Evolution
Biogeography is the study of the geographic
distribution of species throughout the world
– The Earth has six biogeographical regions
• Each has its own distinctive mix of species
– Barriers prevented evolving species from migrating to other
regions
– Continental Drift• The positions of continents and oceans has shifted
through time
• The distribution of fossils and existing species allows us
to determine approximate timeline
• Example: oldest camel fossils 45-40 mya in N. America!
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Continental Drift
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Distribution of Large
Cats
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Evidence of Evolution: Anatomical Evidence
• Common descent
offers explanation for
anatomical similarities
• Homologous
Structures
– Same underlying
structure, adapted for
different functions,
indicating a common
ancestor
• Ex: human arm and
whale forelimb
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Analogous Structures
Same basic function but
different origins.
Underlying anatomy is
different, thus unlikely
to have evolved from
common ancestor
ex: wing of bird and wing
of an insect
Not a good indicator of
relatedness
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Anatomical Evidence: Vestigial Structures
Anatomical structures fully functional in one group and reduced,
nonfunctional in another
Ex: Modern whales have a pelvic girdle and hind leg bones
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Probable origin of appendix
Gastric caecum in herbivores
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Evidence of evolution
: Embryological Development
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Biochemical Evidence
of Evolution
– All organisms use same basic biochemical
molecules
• DNA coding: A,T,C,G
• Amino acid sequence of proteins
– Many developmental genes are shared
(homeotic)
– Degree of similarity between DNA base
sequences (and amino acid sequences of
similar protein) indicates the degree of
relatedness
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Compare a homologous (shared) gene sequence.
This can be used to quantify similarities/differences.
•
•
•
•
•
Canis lupus familiaris
Canis lupus lupus
Canis lupus dingo
Vulpes vulpes
Canis rufus
Provides verifiable data.
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…Or can use a shared protein sequence for
comparison
Muscle myosin
sequence
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The Molecular Clock
• Based on the molecular clock
hypothesis (MCH), this relates the
amount of time since two species
diverged to the number of molecular
differences measured between the
species' DNA or protein sequences or
proteins.
• It is sometimes called a “gene clock” or
“evolutionary clock”.
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The Molecular Clock
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