The Cambrian Explosion and Beyond by Id0uy0

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									The Cambrian Explosion and
        Beyond

         Chapter 17




                             1
Fig. 2.18 The geological time scale




                                      2
      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
                                                        3
       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
                                                  4
                 Kimberella
http://www.ucmp.berkeley.edu/vendian/kimberella2.html




                                                        5
               Dickinsonia
http://www.ucmp.berkeley.edu/vendian/dickinsonia.jpg




                                                       6
        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)
                                                      7
The first
animals:
phylogeny and
fossils (Fig.
17.12)




                8
          Macroevolutionary Patterns

•   Adaptive radiation
•   Punctuated equilibrium versus gradualism
•   Extinction
•   Taxon survivorship curves
•   Mass extinctions
•   The Cretaceous – Tertiary (K-T) impact extinction
•   Anthropogenic extinction


                                                    9
                  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 (?)

                                                                      10
Darwin‘s Finches (Fig. 3.4)




                              11
Adaptive radiation (Fig. 17.13)




                                  12
           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
                                                      13
                    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
                                                                        14
Patterns of morphological change: punctuated
   equilibrium and gradualism (Fig. 17.15)




                                               15
          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




                                                             16
       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




                                                                17
Punctuated change in cheilostome Bryozoa (Jackson
         and Cheetham 1994) (Fig. 17.16)




                                                18
                     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
                                                          19
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
                          20
                  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
                                                    21
Distribution of species extinction intensities
          (Raup 1994) (Fig. 17.20)




                                                 22
Patterns of extinctions of families through time
          (Benton 1995) (Fig. 17.21)




                                                   23
        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




                                                         24
Survivorship curves for genera and families
      (Van Valen 1973) (Fig. 17.22)




                                              25
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

                      26
Geographic range affects the survivorship of bivalve
and gastropod species (Jablonski 1986) (Fig. 17.24)




                                                   27
      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
                                                           28
   Iridium
 anomaly at
   the K-T
  boundary
(Fig. 17.25 b)




                 29
Location and shape of the Chicxulub crater (Schultz
             and D‘Hondt 1996) (Fig. 17.26)




                                                  30
    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 ?


                                                  31
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   32
            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:
      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
                                                             33
          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

                                                           34
  Exinction of
forest birds on
  the island of
 ‗Eua (Tonga)
(Jablonski and
   Steadman
     1995)
  (Fig. 17.29)




                  35
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
                          36

								
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