Evolution Biol 4802

Lecture 10, Chapter 7


Topics for today


  1. Broad patterns of changes over time
  2. Causes and effects of:




How has biodiversity on earth changed?


  1. Missing species

A.  Count families or genera

  1. Single occurrence

B.  Throw out

  1. Older species are more likely to become extinct

C.  Exclude taxa derived recently

Fig. 7.8 old (not in new, see below)

Does pattern change when bias is corrected?

1.   Exclude

A.     Rare families

B.     Single occurrence

C.    Recently derived taxa

D.    Biodiversity is still increasing over time

Fig. 7.1 (7.3 old)

Biodiversity increases in terrestrial taxa

Fig. 7.2 (7.4 old)

Are extinction rates declining?

1.   Two possible explanations

A.     Number of species increases in families over time

i.                    Takes longer for all members to go extinct

B.     Taxa prone to extinction eliminated early

i.                    Taxa with high turnover rates

Fig. 7.5 (7.6 old)

Are speciation rates increasing?

C.    Speciation tends to peak after mass extinction

Fig. 7.4 (7.5 old)

NOTE - we did not cover the next two points in 2012 - you are not responsible for this information

Model of speciation and extinct

  1. Rate of speciation is high when diversity is low
  2. Peaks when niche space is saturated and then declines
  3. Extinction probability increases as diversity increases
  4. Equilibrium number of species occurs at intersection

Not in new (Fig. 7.2 old)

Do species interactions stabilize diversity?

  1. Greater diversity results in greater the species interactions
  2. If species interactions stabilize diversity then:
  3. Relationship between diversity and the speciation rate?
  4. Relationship between diversity and the extinction rate?

Fig. 7.14 (7.7 old)

Are all taxa the same?

  1. Taxa with ­ speciation rates also have ­ extinction rates


  1. Degree of specialization
    1. Specialists are more likely to speciate

                                                              i.      Patchy distribution – isolated from gene flow

                                                            ii.      Avoid competition by partitioning resources

    1. Specialists more vulnerable to environmental change
  1. Population dynamics
    1. Small size or fluctuating size are more vulnerable
  2. Geographic range
    1. Broad range have lower risk because not extinguished by local environment change
    2. Greater dispersal (genes) therefore less speciation

Causes of extinction

  1. Failure to adapt to changes in the environment
  2. When the environment deteriorates, persistence depends upon adaptive evolutionary change
  3. Adaptive evolutionary change depends upon the presence of genetic diversity
  4. Small populations harbor less genetic diversity and, therefore, possess a reduced potential for adaptive evolutionary change
  5. Negative feedback

Do organisms become more resistant to extinction with better adaptation?

  1. Environment is always changing
  2. Age of taxa should not influence extinction rates
  3. Red Queen hypothesis: each species has to run (evolve) as fast as possible just to stay in the same place (survive)
  4. Constant probability of failure

Fig. 7.9(new and old)

What caused mass extinctions?

  1. Permian extinction
    1. 54 % of marine families
    2. 84% of genera
    3. 80-90% species
    4. Recovery took 100 my

Fig. 7.11 (7.10 old)

What caused Permian extinction?

1.   Time of massive volcanic eruptions

2.   Caused global warming

3.   Altered oceanic circulation resulting in loss of oxygen in deep waters

4.   Released vast quantities of methane resulting in positive feedback

What caused KT mass extinction?

  1. Impact of asteriod or large meteorite
  2. Crater off coast of Yucatán Peninsula
  3. Dust in atmosphere blotted out sun and reduced photosynthesis
  4. Evidence

A.  High concentration of iridium at K-T

Evidence of impact

  1. Microtektites are glass particles associated with impact sites
  2. Shock quartz with parallel lamellae

What is the impact of mass extinction on biodiversity?

  1. Clears the slate
  2. Allows new community structures to emerge
  3. Greater homeostatic control (e.g. circulatory system)
  4. Evolutionary trends that have been initiated are halted but sometimes reemerge in different taxa at a later time

Fig. 7.12 (7.11old)

What factors contribute to speciation?

  1. Opportunity
    1. Ecological space
    2. Vacant niches
    3. Escape from competition

Fig. 7.15 (7.12 old)

Two models of speciation

Fig. 7.16 (7.13 old)

Fig. 7.17 (7.14 old)

Example of replacement after extinction?

  1. Fossil turtle lineage that could not retract its head
  2. Replaced by two lineages which could retract their heads

Fig 7.19 (7.16 old)

Ecological divergence promotes speciation

  1. Modern organisms partition niches through subtle differences in resource use
  2. Increase in diversity over time suggests that resources are more finely partitioned than in more ancient times

A test of ecological divergence

  1. Does evolution of an attribute that allows exploitation of a new resource promote diversification?
  2. Compare sister groups of the same age
  3. Ancestral state is to feed on animals, fungi or detritus

Fig. 7.21 (Fig. 7.18 old)


  1. Interactions among species promote diversification
  2. Arms race

a.      Evolution of utilization of plant tissue for food by insects

b.      Evolution of plant species with defensive compounds

  1. Specialization

a.      Fig species have specialized pollinators (fig wasps) that have specialized parasitoids (nematodes)