The Rate of Evolution

A. Punctuated equilibrium - 1972, Niles Eldridge and Steven J. Gould

1. Used fossil record to argue that gradual Darwinian evolution was wrong

2. Large evolutionary change in traits only occurs at speciation events

3. Rarely find transitional forms because speciation occurs in peripheral isolates. New forms are found coexisting only after reinvasion, and possibly replacement of original species

4. Traits experience stasis over long evolutionary periods, due either to

a. stabilizing selection
b. genetic constraint, i.e. lack of genetic variation (unlikely, since all traits show h2)

5. Occurs as a consequence of allopatric speciation or major genetic changes caused by developmental mutations or breakdown of homeostatic controls, e.g. recombination rates increase in high or low temperatures

B. Phyletic gradualism

1. Continual evolutionary change both during and between speciation events

2. Evolution proceeds at a constant rate

3. Speciation does not involve any unique evolutionary mechanisms

II. Data

A. Continuous morphological change in fossil series

1. Punctuated equilibrium

Caribbean bryozoans (p. 674)

a. little change over 5-15 MY for extant and fossil species
b. measured 15 skeletal traits
c. confirmed species designations in extant species by showing that each species had unique alleles
d. descendant and ancestral species co-occur, implicating rapid evolutionary change

Pliocene bivalves (clams)

Lungfish

a. Rapid change occurred 300 MYA, little change since then
b. Deduced change by scoring discrete character changes, such as fusion of limb bones

Horshoe crabs

a. Morphological change occurred from Cambrian to Jurassic, but then almost no change to current time
b. Lack of morphological change is not due to lack of genetic variation

i. nucleotide and amino acid sequences often change at a constant rate and can be used as a molecular clock

ii. horshoe crab comparison to crab lineage shows more genetic variation but much less morphological change, indicating genetic variation is not the reason

2. Gradualism

a. Ordovician Welsh trilobites - eight species show gradual change

b. Horse tooth evolution since Miocene

c. Foraminferans (planktonic or benthic protozoans that leave calcareous shells)

i. 8 million years of change from Gulf of Mexico cores
ii. 4 morphospecies occur sequentially
iii. ancestral forms co-occur with descendants in every case
iv. comparison of differences in disc scores from 11 shell traits over time between 2 spp shows continual gradual change

d. Radiolarian morphological evolution

B. Taxa turnover - represent net result of speciation and extinction rates

1. Species survival plots from fossil record

a. bivalve molluscs - average genus lasts 78 MY
b. mammals - average genus lasts 8 MY, speciation and extinction rates are higher for mammals than for clams

c. complex terrestrial species last shorter times than simple marine species

2. Possible reasons for differences in speciation rates

a. extrinsic: habitat heterogeneity, predation, parasitism, competition
b. intrinsic: differences in genetic variability

C. Adaptive radiations

1. Galapagos finches

a. 14 spp evolved in 500,000 years
b. 1977-78 drought caused a 4% change in bill size
c. Estimate that 12-25 such events, depending on which island population was the ancestor, is enough to turn G fortis into G magnirostris
d. This means that such droughts only need to occur every 20,000 years!

2. Cichlid fishes of African rift lakes

a. mtDNA shows that each lake is monophyletic
b. Lake Nabugobu is 4000 years old and has 3 endemic species!
c. Tremendous morphological change in feeding adaptations and body color, indicating that character displacement has occurred, possibly involving mate selection

3. Hawaiian Drosophila - 500 spp

a. exhibit tremendous variation in body size, bristle and wing patterns, etc
b. Genetic data indicate they may have evolved over 40 MY

III Conclusions

A. Evolutionary rates are extremely variable

1. Rapid evolution typically occurs in empty habitats with intense selection pressure
2. Radiations may be accompanied by key adaptations
3. Slow evolution occurs in stable habitats

B. The dichotomy between PE and PG is probably a continuum. No evidence that speciation requires unique evolutionary processes different from conventional Darwinian natural selection