ch. 7—evolution and the fossil recordfaculty.chas.uni.edu/~groves/ehch07lecture.pdfbody size to...
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Earth History, Ch. 7 1
• Evolution (popular definition) = descent
with modification
• Evolution (technical definition) = change in
gene frequencies or gene combinations in a
series of populations, brought about by
natural selection
Ch. 7—Evolution and the fossil
record
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Earth History, Ch. 7 2
• The basic tenets of evolution
– In natural populations, more offspring are produced than can be sustained in the environment
– Mutations and genetic recombinations are the sources of variability among individuals
– Natural selection results in the differential survival of variants
– Variation is heritable: therefore, the more successful variants preferentially will pass on their genes to following generations, and through time the overall composition of the gene pool will shift
Evolution and the fossil record
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Earth History, Ch. 7 3
Extinction
• Extinction may result from one or more of these causes, if taken to extremes
– Predation
– Disease
– Competition from other species
– Change in physical environment or ecosystem
• Rates of extinction vary by type of organism, for example
– “lifespan” of mammal species is 1 to 2 million years (extinction rate of 50–100% every million years)
– “lifespan” of bivalve species is ~10 million years (extinction rate of 10% every million years)
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Earth History, Ch. 7 4
Extinction
• Mass extinctions = intervals during which unusually large numbers of taxa suddenly become extinct (e.g., 40% or more genera)
• Causes of mass extinctions may be extraterrestrial or a combination of earthly factors
• Five major mass extinctions in Phanerozoic history
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Earth History, Ch. 7 5
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Earth History, Ch. 7 6
Origination
• Evolutionary radiation = episode of rapid evolutionary expansion (production of large numbers of new taxa)
• Radiations usually occur shortly after the origin of a new major taxon
• Radiations may follow extinctions—as ecologic “replacements”
• Radiations may follow an adaptive breakthrough
– The appearance of some key feature that allows ecologic and morphologic diversification
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Earth History, Ch. 7 7
Origination
• Example: radiation of hexacorals in
Triassic Period
– Radiation quickly followed the extinction of
Paleozoic rugose and tabulate corals (vacant
niche)
– Radiation was facilitated by adaptive
breakthrough: the ability to quickly secrete
large skeletons using relatively little CaCO3
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Earth History, Ch. 7 8
Triassic hexacorals
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Earth History, Ch. 7 9
Adaptive radiation of hexacorals in middle Triassic time(in the aftermath of the end-Permian mass extinction)
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Earth History, Ch. 7 10
Extinction & origination
• Organisms that have high rates of origination may
also have high rates of extinction, making them well
suited as guide fossils—ammonoid cephalopods
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Earth History, Ch. 7 11
• Convergence and iterative evolution
– Marsupial/mammal and foram examples
• Evolutionary trends
– Cope’s rule
– Evolution of the whales
– Evolution of the horses
• Phyletic gradualism vs. punctuated equilibria
Patterns of Evolution
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Earth History, Ch. 7 12
Evolutionary convergence
• Convergence = the evolution of similar
form in two or more distinct biologic groups
– Example: similarity between marsupials and
placental mammals
• Unrelated or distantly related groups
typically converge on particular forms that
have high adaptive value
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Earth History, Ch. 7 13
Convergence
Although marsupials and placentals mammals have a common ancestor, there has been no genetic communication between the two groups sincethe breakup of Pangaea, during the Mesozoic Era.
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Earth History, Ch. 7 14
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Earth History, Ch. 7 15
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Earth History, Ch. 7 16
Iterative evolution
• Iterative evolution = the repeated evolution of a
particular form from the same ancestor, but at
different times
• Ancestral form is usually a stable (morphologically
conservative) and long-ranging taxon
• Similar descendant forms arise periodically by
chance? [Or, genetic regulation may allow variation
in only one direction?]
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Earth History, Ch. 7 17
Iterative evolution
A
B
C
tim
e
morphology
A second descendant, C,
which may be nearly identical
to B, originates from A long
after the extinction of B.
Stable ancestral taxon A
gives rise to slightly more
complex descendant B.
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Earth History, Ch. 7 18
Iterative Cenozoic planktonic foram radiations
Quaternary
Pliocene
Oligocene
Miocene
Eocene
Paleocene
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Earth History, Ch. 7 19
Evolutionary trends
• Cope’s rule = the general tendency for
body size to increase during the evolution of
a particular group of organisms. Examples:
– Whales probably originated from even-toed
ungulates ~50 mybp
• Progressive modification of appengages
• Progressive adaptation to marine environment
• Progressive increase in size
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Earth History, Ch. 7 20
Evolutionary trends (cont.)ti
me
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Earth History, Ch. 7 21
Whale
ancestor
(note hoof-like
nails and position
of nostrils)
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Earth History, Ch. 7 22
Whale
ankle bones
A = newly discovered bone
B = ankle of extinct, carnivorous
ungulate
C = ankle of even-toed ungulate
(like a hippo)
AB C
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Earth History, Ch. 7 23
Evolutionary trends (cont.)
• Horses
– Originated ~ 55 mybp
– Earliest forms were dog-size, with four toes on
front feet and three toes on rear feet, and small
molars
– Modern horse is large, with a single hoofed toe
on each foot, and complex molars for grinding
grasses
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Earth History, Ch. 7 24
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Earth History, Ch. 7 25
Modes of evolution
• Phyletic transition
– Gradual, stepwise evolution
• Punctuated equilibria
– Long periods of stability “punctuated” by short
bursts of evolutionary change
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Earth History, Ch. 7 26
Types of evolutionary changeti
me
Gradual, or phyletic transition
in response to directional
environmental change
Speciation, as a result of
geographic isolation and
then reproductive isolation
gene pool gene pool
modif
icat
ion o
f a
spec
ies
thro
ugh t
ime
splitting of one
species into two
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Earth History, Ch. 7 27
Phyletic
transition
Gradual evolution of the
Jurassic oyster, Gryphaea.
Note increase in size as
well as progressive flattening
of the lower valve.
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Earth History, Ch. 7 28
Punctuated
equilibrium
Evolution of bowfin fishes.
Note long period of stability
(Cretaceous–late Eocene),
then rapid speciation followed
by another long period of
stability (late Eocene–present).