Lecture 18: Rates of Evolutionary Change

1
Lecture 18 Rates of Evolutionary Change

• G. G. Simpson
• Tempo Mode in Evolution (1944)
• applied principles of modern synthesis
• (e.g. population genetics) to fossil record
• macroevolution microevolution writ large
• Two ways to measure evolution ?

2
1) Phylogenetic Rate

• Morphological Rate
• rate of change of character or group of
  characters in a lineage
• (anagenesis)
• Rate Change/ Unit Time

3
Rates of Evolution of Single Characters

• Haldane (1949)
• darwin change in e / my
• (ln x2 - ln x1 /change in t)
• Transformation change (removes scaling
  effect)
• e.g. 34 mm to 56 mm over 12 my
• ln (34) 3.526 ln (56) 4.025
• rate of change (4.025 - 3.526) / 12 0.042
  d

4
Evolution of Equine Lineage
5
Horse Teeth
6
McFadden (1992)

• 408 specimens
• 26 ancestor - descendent pairs
• 4 characteristics of teeth
• In general
• pointy, narrow (leaf eater) ? wide, flat (grazer)
• 26 X 4 104 estimates of evolutionary ?
• 0.05 - 0.1 darwins
• mainly positive, but also some reversals

7
Comparing Rates

• ? in size of Ceratopsids 0.06 darwins
• ? in skeletal dimensions of Passer domesticus
  after intro to N. Am. 50 - 300 darwins
• artificial selection 60,000 darwins!
• continuous fossil records show low rate masks
  frequent advances reversals
• e.g. late Cenozoic mammals 12 darwins for short
  periods

8
Fluctuations in Rate

• Gingerich rate of evolution ?1/ time measured
• Short term fluctuations cancel out
• e.g. beaks of Darwins finches
• e.g. changes in radiolarian tests

9
Character Types

• characters evolve at difft rates
• (mosaic evolution)
• rate of change is not constant
• conservative characters canalized general
  adapns
• derived characters specialized, rapid evoln

10
Rates of change population genetics

• Given variance in character, estimate of
  heritability (hN2), ? in mean over t genns can
  estimate strength of directional selecn reqd
• i.e. proportion of popn that fails to reproduce
  in order to produce observed changes
• contrast weak, stabilizing selection, but pop.
  size small enough that drift will produce change

11
Horse Example

• Assume (hN2) 0.5
• 2 selective deaths / 106 individ / generation
  (selection)
• population size of lt 104 individuals (drift)

12
2) Taxonomic Rate

• replacement of forms
• origination extinction (cladogenesis)
• Quantified
• ( taxa originate - taxa extinct) / unit time
• Or the inverse of the average duration of a
  species

13
Cladogenesis Anagenesis

• Speciation at t1 t2
• a c contemporary
• b goes extinct

14
Chronospecies

• Problem
• Fossil record taxonomy based on morpho
  characts.
• Hard to separate anagenesis from cladogenesis
• Identification of many chronospecies
• Chronospecies descendent recognized as separate
  spp.

Taxonomic Pseudoextinction
15
?Phylogenetic Rate ?? Taxonomic Rate

• rapid rate of morphological change leads to high
  rate of taxonomic replacement

16
? Taxonomic Rate ? ? Phylogenetic Rate

• high rate of turnover little morphological change

17

• Relationship b/w phylogenetic rate taxonomic
  rate depends on characters used to determine taxa
  
• Comparison of taxonomic rates
• balance of origination extinction
• e.g. Bivalvia (Pelecypoda)
• 17 genera appear in Ordovician
• 4 survive to Triassic
• average duration 78 my
• compare to Carnivora 8 my

18
Living Fossils

• oldest living species Triops cancriformis
  (tadpole shrimp)
• unchanged since Triassic! (180 mya)

Cycad
Coelacanth
19
Recent Taxa

• rapid evolution
• poor fossil record
• typical of Adaptive Radiations
• Elaphus
• Primelaphus Loxodonta
• Mammuthus
• 1 my (during Pliocene)

20
Problem of stasis

• Fossil Deposits 50 - 100 my apart
• short term changes are lost
• However, observe
• 1) long periods without change
• 2) rapid appearance of new forms
• 3) no transitional forms
• Real or Artifact?

21
Quantum Evolution

• Problem new taxa without fossil intermediaries
• Simpson
• rapid, substantial evolnary change with shift
  into new adaptive zones
• once a threshold passed in acquisition of new
  adaptation, strong directional selection shapes
  feature into new forms
• e.g. tarsus pulley in Artiodactyla rapid
  evoln diversificatn of deer, camels, antelopes

22
Hypotheses

• 1) Phyletic Gradualism
• constant anagenetic change
• speciation gradual
• transitional forms lost in fossil record
• 2) Punctuated Equilibrium
• stasis is real
• evolution occurs during speciation
• long-term trends in morphology due to spp. seln