Transcript The Cambrian Explosion and Beyond

The Cambrian Explosion and
Beyond
Chapter 17
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Fig. 2.18 The geological time scale
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Limitations of the fossil record
• Hard parts – shells, bones, teeth – most likely to
be fossilized because they decay slowly and are
more durable
• In order to be fossilized, a specimen generally
needs to be covered quickly by water-borne or
wind-borne sediments (sand, mud, ash)
• Lack of oxygen is favorable for fossilization
• The fossil record consists primarily of hard parts
left in depositional environments such as river
deltas, beaches, flood plains, marshes, lakeshores,
and the sea floor — the fossil record is biased
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Early animals – Ediacaran fauna
(565 – 544 mya)
• First multicellular animals appear about 565
million years ago (mya)
• Simple, small, asymmetric or radially symmetric –
sponges, jellyfish
• Few bilaterally symmetric forms, such as
Kimberella, which has uncertain affinities, but
appears to be mollusc-like
• Dickinsonia considered by some to be an annelid
worm, and by others to be cnidarian (jellyfish)
polyp
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Kimberella
http://www.ucmp.berkeley.edu/vendian/kimberella2.html
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Dickinsonia
http://www.ucmp.berkeley.edu/vendian/dickinsonia.jpg
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The Cambrian explosion
• Cambrian period 543 – 495 mya
• “Explosive” appearance of large, complex,
bilaterally symmetric animals, segmented animals
with limbs, antennae, shells, external skeletons,
and notochords – including arthropods, molluscs,
annelids, and chordates within the period 543 –
506 mya
• Almost all living animal phyla are present
• Burgess shale fauna (Simon Conway Morris;
Stephen Jay Gould, Wonderful Life, 1989)
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The first
animals:
phylogeny and
fossils (Fig.
17.12)
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Macroevolutionary Patterns
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Adaptive radiation
Punctuated equilibrium versus gradualism
Extinction
Taxon survivorship curves
Mass extinctions
The Cretaceous – Tertiary (K-T) impact extinction
Anthropogenic extinction
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Adaptive Radiation
• An adaptive radiation occurs when a single or a small
group of ancestral species rapidly diversifies into a large
number of descendant species that occupy a wide variety of
ecological niches
• Adaptive radiations can occur when:
– A species colonizes a new region where there are no or few
competitors (i.e., lots of empty niches) – Galápagos finches,
Hawaiian Drosophila and silver swords
– A taxon acquires an important adaptation – evolution of flight in
birds
– A taxon is released from competition after extinction of a dominant
group – radiation of mammals after extinction of the dinosaurs (?)
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Darwin’s Finches (Fig. 3.4)
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Adaptive radiation (Fig. 17.13)
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Punctuated Equilibrium
• Niles Eldredge and Stephen Jay Gould, 1972
• The fossil record for some groups reveals that
morphological evolution consists of long periods
of stasis with little or no change and very short
periods during which morphological change
occurs in association with speciation
• Presented as a challenge to the “conventional”
picture of morphological evolution described by
the modern synthesis
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The controversy
• Phyletic gradualism
– New species arise by transformation of large ancestral groups
(often without splitting – anagenesis)
– Transformation occurs over all or a large part of the ancestral
species geographic range
– Transformation is even and slow
– Evolution occurs more or less at the same rate during and between
speciation events
• Punctuated equilibrium
– A small subgroup of the ancestral form gives rise to a new group
by splitting – cladogenesis
– New species originates in a small part of the ancestral species
geographic range – peripheral isolates model
– New species develop rapidly, then may replace ancestral species
– Between speciation events there is stasis
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Patterns of morphological change: punctuated
equilibrium and gradualism (Fig. 17.15)
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Explaining the fossil record
• Darwin was aware of this “problem” and he explained the
apparent discontinuities and sudden transitions in the fossil
record as being due to the incompleteness of the fossil
record
• Eldredge and Gould argued that sudden transformations are
not artifacts – speciation occurs rapidly and in small
populations and is, therefore, unlikely to leave a fossil
record
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Testing punctuated equilibrium
• Strong tests of punctuated equilibrium vs. phyletic
gradualism are difficult
• Need a complete stratigraphic sequence
• Are morphospecies biological species?
• Cryptic species?
• Need multiple specimens and populations of each species
in order to determine the range of variation within species
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Punctuated change in cheilostome Bryozoa (Jackson
and Cheetham 1994) (Fig. 17.16)
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Who wins?
• Erwin and Antsey (1995) – review of 58 tests of
punctuated equilibrium
• “Paleontological evidence overwhelmingly supports a view
that speciation is sometimes gradual and sometimes
punctuated, and that no one mode characterizes this very
complicated process in the history of life.”
• About 1/3 of the studies support a combination of
gradualism and stasis
• Time-scale effects – when the resolution of the fossil
record is on the order of 10’s of thousands of years,
morphological change may appear “sudden”, but if we had
been present during a 10,000 year period to witness it, it
might have seemed gradual
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Genetic and
morphological
change in two
arthropod
clades (Fig.
17.18)
a. Horseshoe crabs
today are almost
identical to those that
lived 150 million
years ago
b. Hermit crabs and
allies
Genetic distances
based on 16s rRNA
gene sequences
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Extinction
• Extinction is the ultimate fate of all species
• Mass extinctions
–
–
–
–
The big five
Global in extent
Involve a broad range of organisms
Rapid
• “Background” extinction
– Accounts for the vast majority of extinctions
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Distribution of species extinction intensities
(Raup 1994) (Fig. 17.20)
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Patterns of extinctions of families through time
(Benton 1995) (Fig. 17.21)
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Survivorship curves for taxa
• Leigh Van Valen (1973) showed that the
probability of extinction of a taxon was
independent of its age – a taxon does not become
more, or less, likely to go extinct as it gets older
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Survivorship curves for genera and families
(Van Valen 1973) (Fig. 17.22)
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How long
does a species
of marine
bivalve exist?
(Jablonski
1986) (Fig.
17.23)
Species with
planktonic larvae
have longer
durations than do
species with direct
development
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Geographic range affects the survivorship of bivalve
and gastropod species (Jablonski 1986) (Fig. 17.24)
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The Cretaceous – Tertiary (K-T)
extinction
• World-wide iridium anomaly at the K-T boundary dated to
65 mya
• Iridium is rare in the Earth’s crust but more abundant in
meteorites
• Based on the amount of iridium required to produce the
deposits seen at the K-T boundary, Alvarez et al. (1980)
estimated that a 10 km (6.2 mi) diameter asteroid hit the
earth
• The asteroid theory is supported by the presence of
shocked quartz crystals, glass microtektites, and a 180 km
(112 mi) diameter crater in the ocean near the Yucatán
Peninsula
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Iridium
anomaly at
the K-T
boundary
(Fig. 17.25 b)
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Location and shape of the Chicxulub crater (Schultz
and D’Hondt 1996) (Fig. 17.26)
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Effects of a 10 km asteroid impact
• Widespread wild fires
• Acid rain (from release of SO2)
• Darkness and intense cooling from blockage of
sunlight – reduced photosynthesis
• Tsunami (up to 4 km high)
• Severe earthquakes ?
• Increased volcanism ?
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Probability of extinction at the K-T boundary and geographic
range of marine bivalves (Jablonski and Raup 1995)
(Fig. 17.28)
Provinces are graphic regions that share similar flora and fauna
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Anthropogenic extinctions - 1
• Between 1600 and 1993, humans observed extinction of
486 animal species and 600 plant species (Smith et al.
1993)
• Most of these extinctions occurred in North America, the
Caribbean, Australasia, and Pacific Islands
• Currently, the taxa with the highest proportion of
endangered species include:
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Palms – 925 of 2,820 species (33%)
Gymnosperms – 242 of 758 species (32%)
Birds – 1,029 of 9,500 species (11%)
Mammals – 505 of 4,500 species (11%)
• Widespread habitat destruction is probably the greatest
anthropogenic cause of extinction at present
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Anthropogenic extinctions - 2
• Pacific island birds
– Steadman (1995) estimates 2,000 species have become
extinct over last 2,000 years – almost 20% of all bird
species
– 60 endemic Hawaiian species extinct in last 1,500 yr
– 44 New Zealand species extinct since human
colonization – including 8 species of moas, the largest
known birds
– On the island of ‘Eua only 6 of 27 land birds present
before human occupation are still living
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Exinction of
forest birds on
the island of
‘Eua (Tonga)
(Jablonski and
Steadman
1995)
(Fig. 17.29)
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Deforestation
in the Brazilian
Amazon
(Skole and
Tucker 1993)
(Fig. 17.30)
a. 1978
b. 1988
During this period of
time the annual loss of
forested area was about
15,000 km2, an area
approximately
equivalent to the state
of Connecticut per year
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