Download Ch. 7—Evolution and the fossil record

Ch. 7—Evolution and the fossil
record
• 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
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Evolution and the fossil record
• 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
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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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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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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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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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Triassic hexacorals
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Adaptive radiation of hexacorals in middle Triassic time
(in the aftermath of the end-Permian mass extinction)
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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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Patterns of Evolution
• 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
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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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Convergence
Although marsupials and placentals mammals have a common ancestor,
there has been no genetic communication between the two groups since
the breakup of Pangaea, during the Mesozoic Era.
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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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C
Iterative evolution
A second descendant, C,
which may be nearly identical
to B, originates from A long
after the extinction of B.
time
B
Stable ancestral taxon A
gives rise to slightly more
complex descendant B.
A
morphology
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Paleogene
Eocene
Paleocene
globorotalid
conical
Earth History, Ch. 7
pulleniatine
orbuline
hastigerine
conical
globorotalid
turborotalid
globigerine
Oligocene
turborotalid
orbuline
hastigerine
Miocene
globigerinoid
Pliocene
globigerinoid
Neogene
Iterative Cenozoic planktonic foram radiations
Quaternary
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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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time
Evolutionary trends (cont.)
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Whale
ancestor
(note hoof-like
nails and position
of nostrils)
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Whale
ankle bones
B
A
C
A = newly discovered bone
B = ankle of extinct, carnivorous
ungulate
C = ankle of even-toed ungulate
(like a hippo)
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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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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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modif
ic
specie ation of a
s thro
ugh ti
me
time
Types of evolutionary change
splitting of one
species into two
gene pool
gene pool
Gradual, or phyletic transition
in response to directional
environmental change
Speciation, as a result of
geographic isolation and
then reproductive isolation
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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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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).
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