Evolution of Biodiversity – Chapter 7
Long term patterns
 Changes in biodiversity caused by originations and extinctions of taxa over
geologic time
 Analyses of diversity in the fossil record requires procedures to correct for
biases caused by the incompleteness of the fossil record
How does diversity change over time?
 Paleobiologists adapt models of population growth to explain changes in
taxonomic diversity
 Rather than being affected by births and deaths of individuals, the change in
number of taxa is affected by origination and extinction of taxa
Trends in taxonomic diversity
 Diversity has increased in a number of different taxa
Marine animals
Insects
Land plants
Terrestrial vertebrates
Turnover
 Taxa with high rates of origination have high rates of extinction
 These taxa fluctuate greatly and are prone to extinction
Rate of origination has declined
Rate of extinction has declined
Extinction
 Extinction is caused by failure of organisms to adapt
 Populations that have been reduced in size are less able to adapt
 Fewer mutations
 Less variations
Why are extinction rates declining?
 This is not what we would predict
 Extinction probability should be constant
 Extinction resistance is not selected for when environmental conditions that
cause extinction vary
Taxonomic survivorship curves
Actual survivorship curves
Red Queen Hypothesis
 The red queen and Alice find themselves running to keep still
 “…they were running hand in hand, and the Queen went so fast that it was all
she could do to keep up with her...The most curious part of the thing was, that
the trees and the other things around them never seemed to changed their
places at all...”
Red Queen Hypothesis
 Van Valen (1973) hypothesized that each species has to evolve (“run”) as fast
as possible just to survive (“stay in the same place”) because its competitors,
predators and parasites also continue to evolve.
 There is a constant chance that it will fail to survive
Then why are they declining?
 Increase over time in average number of species per family would lower the
extinction rate
 It would take longer for all the species of the same family to go extinct
 Higher taxa that are more prone to extinction were eliminated early
Mechanisms causing extinction
 Anagenesis (a matter of our definitions)
 Biological
 Small, specialized populations
 Competition
 Area effects (species-area relationships)
 Tend to be smaller and more localized
 Physical
 Sea level change
 Climate change
 Asteroid/comet impacts
 Can be intense and global
Sources of catastrophic extinction
 External – asteroids, comets
 Internal
 Volcanism
 Catastrophic chemical turnover (oceans)
 Humans
Mass extinctions
Mechanisms causing extinction
 Anagenesis (a matter of our definitions, not biology)
 Biological
 Small, specialized populations
 Competition
 Area effects (species-area relationships)
 Tend to be smaller and more localized
 Physical
 Sea level change
 Climate change
 Asteroid/comet impacts
 Can be intense and global
Cretaceous-Tertiary extinction
 Independent geological evidence for asteroid
 Consequences
 Local earthquakes, fires, volcanism
 Tsunami
 Global changes in climate
 Darkness
 Altered biogeochemistry
 60%-80% of all species became extinct
 Tropical coral reefs disappeared
Did K-T impact remove dinosaurs?
 Diversity is a function of balance between speciation and extinction
 Extinction > speciation for many groups prior to impact
 Temporal resolution is difficult to test
 Rate of loss was extremely high at K-T boundary, however
Chicxulub crater
Permian-Triassic extinction event
 Up to 90% species loss
 Ocean anoxia? (may also have preceded the Cambrian explosion)
 Evidence (Isozaki, Science 276: 235 – 1997)
 Study pelagic cherts
 Chemical signature of oxygen lacking in sediments of 20 million years at PT boundary
 Conclusion: anoxia of oceans (caused by global warming after volcanic
eruption)
Permian mass extinction
Human contributions to extinction?
 Habitat loss
 Overexploitation
 Transportation of exotic, invasive species
Human activity: Habitat loss
Mass extinctions were selective
 For example, survivorship of gastropods and bivalves was greater for taxa with
planktonic larvae througout the Creteaceous, but at K-T extinction, extinction
of planktonic and non-planktonic forms was equal
Shaping of biota by mass extinctions
What factors lead to diversification?
 Release from competition
 Ecological divergence
 Co-evolution
 Provinciality
Release from competition
 Many instances where extinction of one group led to proliferation of another
group
 Gymnosperms declined while angiosperms rose
 Mammals rose after dinosaurs were extinct
Competition
 Later groups may have caused the extinction of earlier groups – direct
competition between groups
 Competitive displacement
 Existing taxon may have prevented an ecologically similar taxon to proliferate
 Incumbent or competitive replacement
 Mistake in your text!
Angiosperms replaced gymnosperms
Incumbent replacement model
 Mass extinction event reduces Clade 1 more than Clade 2
Ecological divergence
 Key adaptation or key innovation
 An adaptation that enables an organism to occupy a substantially new
ecological niche
Key adaptation in Sea Urchins
Replicated sister group comparison
 Compare ancestral group to group with key innovation to test if diversification
is due to innovation; use more than one clade
Insect and angiosperm coevolution
Provinciality
 Provinciality is the degree to which the biota is partitioned into geographic
regions
 Caused by changes in distributions of land masses
Logistic growth model
dN/dt = [S0(1-N/Nmax)-E]N
Equilibrium number of species
 If dN/dt = 0, then N* = (1-E/S0)Nmax
 Equilibrium number of species will be less than carrying capacity
Exponential and logistic growth