DISCONTINUITY IN EVOLUTION:
        HOW DIFFERENT LEVELS OF ORGANIZATION IMPLY PRE-ADAPTATION

Although sequences  of  nucleotide bases  in DNA and amino acids  in proteins 
appear to mutate  at  approximately  constant rates, evolution seems  to have 
been anything  but  steady.  In  fact,  most  dramatic  changes in  evolution 
occurred rather abruptly. Gould claims that in order to explain  such radical 
changes one should abandon the adaptationist  or  "ultra-darwinist" paradigm. 
The idea that all change is gradual  and continuous is in fact  a consequence 
of interpreting phenotipic changes  as mostly  adaptations. Gould proposed to 
classify novel phenotipic traits in  three different  categories: adaptations 
(structures that have a new function),  preadaptations  (structures built for 
one purpose and then adapted  to another),  and  exaptations  (structures not 
built as adaptations at  all  but  later  adapted  for  some function) and he 
claims that the last two categories must greatly exceed adaptations in number 
and importance.
Of course,  no  biologist  has  ever  advocated the total equivalence between 
retained  changes  and  adaptations.  Adaptive  changes in some trait entails 
correlated alterations  of  other  traits.  The  architecture  of genetic and 
embryological  systems defines channels  of possible change. Selection may be 
required  to  push  an organism down a channel but the channel itself, though 
not an adaptation,  acts  as  a  major  determinant of the direction taken by 
evolution. The main  questions  then  are the relative importance of adaptive 
versus  non-adaptive  changes  and  a  better understanding of the mechanisms 
underlying  both  types of change. Using a computational model we will try to 
show  that  non-adaptive evolutionary changes  may largely outnumber adaptive 
ones. This  fact  appears mainly  due to the hierarchical organization of the 
simulated organisms. We  will  also  show  how  some  of  these  non-adaptive 
(neutral)  changes  may  subsequently  become  the  basis for further changes 
which  do  prove  adaptive  and  therefore  how  preadaptation  phenomena may 
arise producing sudden evolutionary changes in the behavior of organisms.