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 & reversalse.g. late Cenozoic mammals : 12 darwins for

short periods

Fluctuations in RateGingerich : 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

wid

th

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

time

mor

phol

ogy

Phylogenetic Rate = Taxonomic Rate

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

time

mor

phol

ogy

↑ 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

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 intermediariesSimpson:• 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