Selected  Letters  Between  Steve  Gould,  Tom  Schopf,  and  Dave  Raup  about  the  “MBL  Model”  

 This  packet  contains  letters  written  between  1972  and  1981  by  members  of  the  group  that  authored  the  so-‐called  “MBL  model”  papers  described  in  the  Gina  Kolata  article  in  Science.    I  apologize  about  the  scan  quality  on  some  of  them,  but  collectively  they  give  a  nice  insight  into  the  background  discussions  about  the  model  and  its  potential  implications,  as  well  as  a  view  of  the  personalities  involved.    For  a  more  detailed  history  of  the  MBL  model  see  either  the  Sepkoski  or  Huss  papers  in  the  Further  Readings  section  for  this  topic,  but  here  is  a  brief  summary  of  the  context:  In  1971,  Tom  Schopf,  a  young  invertebrate  paleontologist  at  the  University  of  Chicago,  organized  a  symposium  at  the  Geological  Society  of  America  meeting  on  “Models  in  Paleobiology.”    Schopf,  like  his  friends  Steve  Gould  (recently  appointed  at  Harvard)  and  Dave  Raup  (then  at  the  University  of  Rochester),  wanted  to  inject  what  Schopf  called  “greater  analytical  rigor”  into  the  practice  of  paleontology,  which  they  viewed  as  too  descriptive  and  conservative.    Schopf  asked  Gould  to  give  a  talk  on  “Models  of  Speciation,”  and  with  co-‐author  Niles  Eldredge  Gould  presented  the  first  version  of  what  would  be  the  original  paper  on  “Punctuated  Equilibria”  (published  in  the  symposium  volume  in  1972).    Immediately  after  the  symposium,  Schopf  decided  to  take  advantage  of  the  momentum  he  had  created  by  organizing  a  weekend  brainstorming  session  at  Woods  Hole,  where  he  had  an  affiliated  research  position,  to  consider  further  applications  of  models  to  paleontology.    He  invited  Gould,  Raup,  and  the  young  theoretical  ecologist  (and  E.O.  Wilson  student)  Dan  Simberloff,  and  the  group  spent  several  days  trying  to  figure  out  how  to  derive  quantitative  models  from  empirical  data  sets  like  the  Treatise  on  Invertebrate  Paleontology.    Simberloff  had  been  invited  because  of  his  experience  with  quantitative  ecological  models,  and  Raup  was  the  computer  guru  of  the  group,  having  written  a  series  of  papers  in  the  late  1960s  in  which  he  used  computers  to  simulate  the  morphology  of  coiled  shells.    However,  after  two  days  with  little  success,  the  group  concluded  that  the  fossil  data  they  were  working  with  was  too  fragmentary  and  incomplete  to  work  with.    In  desperation,  Raup  came  up  with  the  idea  of  creating  a  computer  simulation  in  which  evolutionary  phylogenies  were  generated  and  randomly  subjected  to  probabilities  of  branching  or  termination  in  order  to  see  what  evolution  might  look  like  in  a  world  where  the  success  of  lineages  was  not  determined  by  selection.    This  was  explicitly  considered  a  thought  experiment,  since  the  null  hypothesis  was  that  we  do  live  in  a  Darwinian  world  governed  by  natural  selection.    After  the  first  meeting,  Raup  wrote  a  very  simple  program,  and  the  initial  paper  (published  in  1973)  described  the  results.    Those  results  are  discussed  in  the  Kolata  article,  but  very  briefly,  the  authors  were  impressed  by  the  fact  that  the  simulated  lineages  looked  surprisingly  similar  to  actual  evolutionary  phylogenies.    This  suggested  that  existing  patterns  of  evolution  could  have  been  produced  by  stochastic  (random)  factors  alone,  but  the  authors  did  not  argue  that  they  actually  were.    After  this  initial  meeting  and  paper,  Schopf  convened  the  group  again—this  time  including  Jack  Sepkoski  (my  dad),  who  was  a  young  graduate  student  working  with  Gould—to  plan  several  follow-‐up  papers.    The  results  were  a  paper  written  by  Gould  and  Raup  in  which  the  simulation  was  extended  to  include  the  evolution  of  morphological  traits,  and  the  group  collaboration  concluded  with  a  paper  broadly  comparing  results  of  the  simulation  runs  with  data  from  the  actual  fossil  record.    These  papers  can  be  found  in  the  Recommended  Readings  section  on  the  website.    During  this  time,  members  of  the  group  developed  differing  interpretations  of  their  results.    Initially,  Schopf,  Gould,  and  Raup  were  highly  impressed  by  the  potential  for  the  model  to  suggest  

that  many  evolutionary  processes  might  actually  be  stochastic.    Schopf,  in  particular,  developed  a  vision  of  a  “stochastic  paleontology”  in  which  species  or  higher  taxa  could  be  treated  mathematically  like  molecules  interacting  randomly  in  a  volume  of  gas.    As  time  went  on,  however,  Raup  and  Gould  became  less  sure  about  this  metaphor.    The  letters  give  a  flavor  of  the  debates  that  went  on  between  the  three.    Ultimately,  a  paper  by  Steve  Stanley  and  others  showed  that  the  original  MBL  model  suffered  from  scaling  problems,  and  was  not  a  good  simulation  of  actual  evolutionary  history.    This  led  Raup  and  Gould  (the  latter  more  reluctantly)  to  withdraw  most  of  their  enthusiasm  for  the  model.    Schopf,  however,  stuck  to  his  guns  until  his  untimely  death  in  1984,  and  continued  to  promote  his  vision  of  a  stochastic  view  of  life  (see  Further  Readings).    Although  the  MBL  model  did  not  revolutionize  paleontology  as  its  authors  originally  hoped,  it  had  a  significant  influence  on  the  development  of  what  many  paleontologists  now  call  “paleobiology,”  and  in  particular  on  the  evolving  views  of  Gould.    As  I’ve  argued  elsewhere,  it  may  have  been  the  initial  basis  for  Gould’s  understanding  of  the  role  of  contingency  in  the  history  of  life,  and  it  sparked  continuing  debates  about  the  interplay  of  chance  and  determinism  in  evolution  (Dan  McShea  and  Robert  Brandon’s  2010  book  Biology’s  First  Law  is  essentially  a  philosophical  reconsideration  of  the  MBL  experiment).    The  MBL  model  also  signaled  the  importance  of  computer  modeling  for  paleontology,  and  inspired  Raup  and  Sepkoski  to  develop  approaches  to  model  and  analyze  fossil  data  to  explore  patterns  of  diversification  and  evolution  in  the  history  of  life  that  have  influenced  the  entire  field.    Gould  drew  on  this  further  work  in  his  theoretical  explorations  of  macroevolution  during  the  1980s  and  beyond,  as  seen  for  example  in  his  paper  “The  Promise  of  Paleobiology  as  a  Nomothetic,  Evolutionary  Discipline.”    All  of  the  papers  below  are  available  on  the  seminar  website.    Recommended  additional  reading:  -‐  Raup,  Schopf,  and  Gould,  "Stochastic  Models  of  Phylogeny  and  the  Evolution  of  Diversity"  [The  original  MBL  Model  paper,  published  in  1973]  -‐  Raup  and  Gould,  "Stochastic  Simulation  and  the  Evolution  of  Diversity  -‐  Towards  a  Nomothetic  Paleontology"  [A  later  paper  in  the  MBL  series,  perhaps  the  most  important,  showing  how  a  stochastic  model  could  influence  the  evolution  of  morphological  diversity]  -‐  Gould,  "The  Promise  of  Paleobiology  as  a  Nomothetic,  Evolutionary  Discipline"  [One  of  two  manifestos  published  by  Gould  in  the  same  issue  of  the  journal  Paleobiology  in  1982,  this  paper  brings  together  Gould's  vision  of  a  "nomothetic"  paleontology  with  his  emerging  hierarchical  thinking.  A  contentious  classic  in  the  field.]    Optional  for  further  exploration:  -‐  Sepkoski,  "“Towards  a  Nomothetic  Paleontology”:  The  MBL  Model  and  Stochastic  Paleontology"  [The  chapter  from  my  book  that  tells  the  story  of  the  MBL  model  in  considerable  detail,  with  attention  to  the  internal  divisions  and  arguments  within  the  group  of  authors]  -‐  Huss,  "The  Shape  of  Evolution:  The  MBL  Model  and  Clade  Shape"  [John  Huss'  excellent  short  summary  of  historical  and  philosophical  issues  surrounding  the  MBL  Model]  -‐  Gould,  "Generality  and  Uniqueness  in  the  History  of  Life"  [Gould's  summary  of  the  MBL  papers  for  an  audience  of  biologists-‐-‐fairly  technical]  -‐  Gould  et  al.,  "The  Shape  of  Evolution:  A  Comparison  of  Real  and  Random  Clades"  [The  last  MBL  collaborative  paper,  in  which  the  authors  test  the  model  against  real  data.  This  paper  led  to  the  fairly  devastating  critique  by  Stanley  et  al.]  -‐  Stanley  et  al.,  "Natural  Clades  Differ  from  "Random"  Clades:  Simulations  and  Analyses"  [The  Stanley  et  al.  critique  of  the  MBL  Model]  -‐  Schopf,  "Evolving  Paleontological  Views  on  Deterministic  versus  Stochastic  Approaches"  [Schopf's  broad  methodological  defense  of  stochastic  paleobiology]      

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