Lecture Note
Cambrian Explosion, Macroevolution, Extinction
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University:
California State University, Northridge -
Course:
BIOL 315/L | Principles of Microbiology and Lab Academic year:
2006
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Views:
327
Pages:
40
Author:
Brenna Norris
Cambrian Explosion,
Macroevolution, Extinction Recap
• Cellular life existed probably 3.465 bya
– Resembled cyanobacteria
– Likely used DNA
• Life continued as single cell organisms only
for about 3.2 billion years
• Multicellular life? Phanerozoic - visible life
• Multicellular organisms
• Appear as fossils about 565 mya
• next 40 million years
– Nearly all major phyla appear
– Called Cambrian explosion
• Paleontology is the study of ancient life
through fossils Types of Fossils
•
•
•
•
Impressions
Permineralized
Casts
Amber Fossil record
• Fossils created depending on specimen
– Durability
– Burial
– Lack of decay (usually anoxic environment)
• Fossilization rare
– More likely with marine or lowland delta
situations
• Still fossils provide good windows to the
past Geological time
• Based on layers of fossil assemblages
• Dated by radioisotopes
• Time divided into:
– Eons
• Eras
– Periods
» Epochs
• Cambrian to present called the phanerozoic
Eon
• Phanerozoic has three eras
– Paleozoic, Mesozoic, Cenozoic Paleozoic Mesozoic Cenozoic Cambrian Explosion details
• 40 my period nearly all major animal phyla
appear
• Phyla assigned based on embryonic
development and distinct adult body plans
• Several major divisions
– Diplo versus triploblasts
– Types of coelom (gut)
– Protostome v deuterostome gastrulation Diploblasts and triploblasts
• Diploblasts have two
embryonic tissue types
– Endoderm and ectoderm
– Radially symmetrical
– E.g. sponges, cnidarians,
ctenophores
• Triploblasts have three
(mesoderm layer added)
– Bilateral symmetry Triploblast coelom types
• Acoelomates
– No gut, like flatworms
• Coelomates
– Have coelom = true gut
– Derived from mesoderm
• Pseudocoelomates
– Have body cavity, but not from mesoderm
• Secondarily derived Triploblast coelomates
• Protostomes
– Mouth first gastrulation
• Deuterostomes
– Mouth after anus Faunal Cambrian diversification
Two well preserved faunas 40 my apart
• Ediacaran, first found in Australia
– Dated to 565 mya
– Soft bodied impression fossils
• Sponges, jellyfish, comb jellies
• Burgess Shale (British Columbia) Chengjiang
(China)
– Dated to 520-515 mya
– Impression fossils Dating Cambrian Explosion
• Molecular data
• Early divergence
– E.g. 1 bya
• Not 565 mya
• Suggests older fossils
may be found. Why Cambrian explosion?
• Ecological niches
– Ediacaran fauna sessile or planktonic filter feeders
– Cambrian developed grazers, predators, burrowers,
swimming
• New modes of active locomotion made possible
by rising oxygen in sea water
• Predation and locomotion favor hard body parts
– Shells
– Mineralized skeletons Macroevolution
• Microevolution
– Within population evolutionary change
• Adaptation, selection, drift etc.
• Macroevolution
–
–
–
–
Large scale changes in history of life
Adaptive radiation
Gradual change versus stasis
Extinction Adaptive radiation
• Single ancestral species diversifies into a
wide range of descendent species
– Accompanied by ecological niche
diversification
• E.g. Galapagos finches, Hawaiian Drosophila Triggers for adaptive radiation
• Ecological opportunity
– Islands with “empty” niches; few competitors
• Morphological innovation
– E.g. arthropod jointed limbs
• Allowed exploitation of land Rate of major evolutionary change:
stasis versus gradualism
• Stasis: things stay the same
– Or in “equilibrium” condition
• Punctuated equilibrium
– Idea that most evolutionary change occurs rapidly
• Associated with speciation events
• Long periods of no change between
• Gradualism
– Idea that most evolutionary change is continuous Darwin was a gradualist
• Expected evolutionary change to be slow
and continuous
– Predicts many many intermediate forms
• Many of course have been found in major groups
– But many fossil morphological species
• Appear suddenly in fossil record
• Fewer transitional forms than you might expect
• Darwin attributed stasis to incomplete fossil
record Eldridge and Gould
• 1972 published idea of “punctuated
equilibrium”
• Accepts stasis as real
– There are long periods of time during which a
species changes little morphologically
– Sudden changes “punctuate” the stasis Punctuated equilibrium v.
gradual change Timescales
• Paleontologist might say
– Speciation was “instantaneous” in fossil record
• An “instant” in geologic time could easily be
100,000 years in ecological time
– Tens or hundreds of thousands of generations
• Evidence of stasis
– Requires group with known phylogeny
– Sampling at close time scales Bryozoan stasis “Living fossils” show stasis
Modern Ginko plant and 40 my fossil. Concensus
• Both gradualism and punctuated
equilibrium occur
– Likelihood even depends on types of organisms
• Small marine forms gradual
– Foraminifera, radiolarians
• Larger organisms more often punctuated
– Arthropods, bivalves, corals etc. Why stasis?
• If stasis is real, and it
seems to be, then why?
– Ecological stability for
morphotypes
• Continued adaptation in
coevolutionary races
• Horseshoe crabs show
morphological stasis
– But still have lots of
evolutionary change Extinction
• Species and higher taxa die out
– Normal fate of species
• Background extinction
• Species longevity
– Mass extinctions Normal species
extinctions
Linear curves mean extinction rates constant within
clades
Geographically widespread taxa less likely to die off
“typical” species lifespan ca. 1 my Mass extinctions
• 5 times in Phanerozoic % extinction peaks
– Mass extinctions 60% of species died Importance of mass extinctions
• Major groups may die off
– Opens competitive vacuum
• Lots of open niche space
– E.g., dinosaur extinction allowed mammals to
diversify (adaptive radiation) So how did those dinosaurs die?
• What are mechanisms of mass extinctions?
– Environmental change
• Oxygen production by photosynthesis surely
poisoned the environment for previously dominant
anaerobic bacteria
– Meteor impacts
• Cretaceous-Tertiary boundary
– Called K-T boundary, 65 mya Asteroid impact evidence
• Iridium layer
– Iridium naturally rare on
earth, abundant in
asteroids, peaks at K-T
boundary
• Based on amount of
iridium, asteroid
estimated to be 10 km
wide Human induced extinctions
• Many think a 6th mass extinction event
underway
– Caused by humans Evidence: polynesian bird fauna
• Estimated 2000 bird species (20%) went extinct as
humans colonized pacific islands
– E.g. 60 spp. On Hawaii
• 1500 years of human habitation
– 44 species on New Zealand
– Data for ‘Eua (Tonga) ->
• Causes
– Hunting
– Introduction of rats, dogs, pigs North American Pleistocene
megafauna
• Wooly mammoths, giant ground sloths,
camels, wooly rhino
• Died off at same time as human arrival from
Beringia
• Likely due to hunting and climate change Modern human extinctions
• Since 1600, relatively few identified
– 486 animal species, 600 plants
• Major current threat habitat loss North American species status Habitat
loss:
forests
Warwickshire,
England
Costa Rica
USA
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