The Rate of Evolution

1. Theories
   1. Punctuated equilium - 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
         1. stabilizing selection
         2. 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
   2. 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
2. Data
   1. Continuous morphological change in fossil series
      1. Punctuated equilibrium Caribbean bryozoans
         1. little change over 5-15 MY for extant and fossil species
         2. measured 15 skeletal traits
         3. confirmed species designations in extant species by showing that each species had unique alleles
         4. descendant and ancestral species co-occur, implicating rapid evolutionary change
         Lungfish
         1. Rapid change occurred 300 MYA, little change since then
         2. Deduced change by scoring discrete character changes, such as fusion of limb bones
         Horshoe crabs
         1. Morphological change occurred from Cambrian to Jurassic, but then almost no change to current time
         2. Lack of morphological change is not due to lack of genetic variation - comparison to crab lineage shows more genetic variation but much less morphological change
   2. Gradualism
      1. Ordovician Welsh trilobites
      2. Foraminferans (planktonic or benthic protozoans that leave calcareous shells)
         1. 8 million years of change from Gulf of Mexico cores
         2. 4 morphospecies occur sequentially
         3. ancestral forms co-occur with descendants in every case
         4. comparison of differences in disc scores from 11 shell traits over time between 2 spp shows continual gradual change
   3. Species turnover
      1. Species survival plots from fossil record
         1. bivalve molluscs - average genus lasts 78 MY
         2. mammals - average genus lasts 8 MY
         3. complex terrestrial species last shorter times than simple marine species
      2. Possible reasons for differences in speciation rates
         1. extrinsic: habitat heterogeneity, predation, parasitism, competition
         2. intrinsic: differences in genetic variability
   4. Adaptive radiations
      1. Cichlid fishes of African rift lakes
         1. Tankanikya - 2 MYR - 171 endemics
         2. Malawi - 1-2 MYR - 500 endemics
         3. Victoria < 1 MYR - 300 endemics
         4. mtDNA shows that each lake is monophyletic
         5. Victoria may have dried up completely 12400 years ago - 300 spp in 10,000 years or 33 yrs/speciation event!!! - little sequence variation among species ni Victoria
         6. Lake Nabugobu is 4000 years old and has 3 endemic species!
         7. Tremendous morphological change in feeding adaptations and body color, indicating that character displacement has occurred, possibly involving mate selection
      2. Galapagos finches
         1. 14 spp evolved in 500,000 years
         2. 1977-78 drought caused a 4% change in bill size
         3. Estimate that 12-25 such events, depending on which island population was the ancestor, is enough to turn G fortis into G magnirostris
         4. This means that such droughts only need to occur every 20,000 years!
      3. Hawaiian Drosophila
         1. 500 spp
         2. exhibit tremendous variation in body size, bristle and wing patterns, etc
         3. Genetic data indicate they may have evolved over 40 MY
3. Conclusions
   1. 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 - jaw modifications of cichlids
      3. Slow evolution occurs in stable habitats
   2. The dichotomy between PE and PG is probably a continuum. No evidence that speciation requires unique evolutionary processes different from conventional Darwinian natural selection