Origination and Extinction

1) From my earlier lecture we saw that the orgination of species or increase in diversity appeared to be related to specialization of morphology.

2) Thus, the cambrian explosion could be interpreted to be due to the rapid filling of a large number of empty "niches". This is the origination effects.

3) The balance between origination (e.g., diversity) and extinction.

4) From the patterns of the fossil record can we infer something about process.

5) Finally I will talk about biases in the fossil record.


Review of Origination and a Graphical Model

 

Back to the Arthropods:

  1. Increase in Tagmosis over time is sigmoidal
  2. Increase in Diversity over time is also sigmoidal.

This sigmoidal shape can be explained in terms of the rate of origination and extinction of taxa. A simple graphical model will suffice (board).

O = orgination, E = extinction, = the equilibrium number of species.

From this model, we expect that O = E if negative feedback process is in effect.

As diversity increases, O declines and/or E increases

Is diversity regulated? A simulation Model

Lets take the model of origination and extinction and apply it to the question of regulated diversity. Raup et al. 1973 constructed a simulation model.

Getting from the random phylogenies to the diversity map. (overhead).

Raup et al. (1973) tested the hypothesis that the patterns in the fossil record are due to diversity regulation about an equilibrium. They tried to simulate the pattern. (overhead)

What they could not simulate

1) coelocanth effect - lobe-finned fish (tuatara -- "primitive" reptile)

2) mass extinctions -- simultaneous dissappearance of clades (5/17)

3) very rapid radiations -- explosion of taxa onto the scene (therapsids)

Coelocanth Effect: Could be due to endemisms and isolation -- biogeography

Very Rapid Radiations:

Adaptive zones -- A group of similar ecological niches different from those occupied by other groups

Key innovations -- critical new adaptations that enable an organism to use resources from which it was previously barred (or frees up the morphology to evolve to other purposes.

Ecological replacement -- perhaps one group replaces the other group due to superior adaptations Radiations after Mass Extinctions (empty niches)

Logical fallacy: just because A->B (e.g., event B always follows A) does not imply that A caused B.

Patterns of Extinctions:

A little background:

Classification system Phylum - Class - Order - Family - Genus - Species

Geological Time Scale C O S D M P P | T J C(K) | T Q

Background extinction rate versus mass extinction

Mass extinction typically occur at the end of a geologic period. They are a component by which the geological period are defined (the other is radiations).

The geologic period expressed in terms of Radiations and Mass Extinctions:

Mass Extinctions versus Background Extinctions

Raup and Sepkoski 1982 mass extinctions. (overhead) above background

Why does the background extinction of families decline up to the present?

 

Progressive Adaption -- species are becoming well adapted to environment

versus a dynamical explanation

-- # of species in given families has been increasing

-- families with a few species are more likely to go extinct

Another argument against a purely Progressive adaptation

Van Valen 1973 and the Red Queen Hypothesis (overhead)

Within a taxonomic group the number of species declines linearly with time.

Constant probability of extinction

Therefore, older taxa are no more likely to survive than younger taxa.

As a group evolves, it becomes no more or less resistant to extinction

EXPAINED: Biotic environment is deteriorating (competition, predation). Species have to continually evolve just to keep up in the race.

What about mass extinctions (causes)?

Example 1: Asteroid impact meets the Iridium Anomoly at the K-T boundary

Evidence of worldwide layer of terrestrially rare Iridium (which is common in extraterrestrial sources. Alvarez et al 1980

1) Impact site has been identified in the Yucatan peninisula.

2) Evidence of Deluge in texas and along Louisiana

3) Molecular evidence. Deluge took out the biota of the Caribean Islands re-established after K-T boundary.

Example 2: Interstellar explanations

26 MYR periodicity to extinctions

Interestingly the milky way galaxy has a period of rotation of ~50 MYR.

Example 3: Pleistocene extinctions

Example 4: Current extinctions

A. 1. current extinction based on bird/mammal rates =
 
B. Humans and K
1. graph of our growth over time
2. K previously det. by disease, food supply, water avail.
3. technology = raise K
a. domestication
b. cultivation
c. medicine
d. technology & farming
4. now K will be determined by ecosystem's ability to handle waste