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
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
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)
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
Earth History, Ch. 7 5
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
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
Earth History, Ch. 7 8
Triassic hexacorals
Earth History, Ch. 7 9
Adaptive radiation of hexacorals in middle Triassic time(in the aftermath of the end-Permian mass extinction)
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
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
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
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.
Earth History, Ch. 7 14
Earth History, Ch. 7 15
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?]
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.
Earth History, Ch. 7 18
Iterative Cenozoic planktonic foram radiations
Quaternary
Pliocene
Oligocene
Miocene
Eocene
Paleocene
glo
big
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big
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has
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orb
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pull
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big
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boro
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dglo
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d
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ene
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
Earth History, Ch. 7 20
Evolutionary trends (cont.)ti
me
Earth History, Ch. 7 21
Whale
ancestor
(note hoof-like
nails and position
of nostrils)
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
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
Earth History, Ch. 7 24
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
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
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.
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).