Transcript Lecture 18: Rates of Evolutionary Change

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 
1) Phylogenetic Rate
• Morphological Rate
• rate of change of character or group of characters in
a lineage
• (anagenesis)
Rate = Change/ Unit Time
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
Evolution of Equine Lineage
Horse Teeth
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
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
Fluctuations in Rate
width
Gingerich : rate of evolution 1/ time measured
Short term fluctuations cancel out
e.g. beaks of Darwin’s finches
e.g. changes in radiolarian tests
time
Character Types
• characters evolve at diff’t rates
(mosaic evolution)
• rate of change is not constant
• conservative characters: canalized; general
adap’ns
• derived characters: specialized, rapid evol’n
Rates of change & population
genetics
• Given: variance in character, estimate of
heritability (hN2),  in mean over t gen’ns: can
estimate strength of directional selec’n req’d
• i.e. proportion of pop’n that fails to reproduce
in order to produce observed changes
• contrast: weak, stabilizing selection, but pop.
size small enough that drift will produce change
Horse Example
• Assume (hN2) = 0.5
• 2 selective deaths / 106 individ / generation
(selection)
• population size of < 104 individuals (drift)
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
Cladogenesis & Anagenesis
• Speciation at t1 & t2
• a & c contemporary
• b goes extinct
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
Phylogenetic Rate = Taxonomic Rate
morphology
• rapid rate of morphological change leads to
high rate of taxonomic replacement
time
↑ Taxonomic Rate  ↑ Phylogenetic Rate
morphology
• high rate of turnover; little morphological
change
time
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
Living Fossils
• oldest living species: Triops cancriformis
(tadpole shrimp)
• unchanged since Triassic! (180 mya)
Coelacanth
Cycad
Recent Taxa
• rapid evolution
• poor fossil record
• typical of Adaptive Radiations:
Elaphus
Primelaphus
Loxodonta
Mammuthus
~ 1 my (during Pliocene)
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?
Quantum Evolution
Problem: new taxa without fossil intermediaries
Simpson:
• rapid, substantial evol’nary 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 evol’n
& diversificat’n of deer, camels, antelopes
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. sel’n