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 x 2 - ln x 1 /change in t) Transformation : % change (removes scaling effect) e.g. 34 mm to 56 mm over 12 my ln (34) = 3.526; ln (56) = rate of change = ( ) / 12 = 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  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. = 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 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 width

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 (h N 2 ),  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 (h N 2 ) = selective deaths / 10 6 individ / generation (selection) population size of < 10 4 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 t 1 & t 2 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

time morphology  Phylogenetic Rate =  Taxonomic Rate rapid rate of morphological change leads to high rate of taxonomic replacement

time morphology ↑ Taxonomic Rate  ↑ Phylogenetic Rate high rate of turnover; little morphological change

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 PrimelaphusLoxodonta Mammuthus ~ 1 my (during Pliocene)

Problem of stasis: Fossil Deposits: 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