Increase in Number of Species 207 



SUMMARY 



In bisexual organisms the dynamic nature of phylogenetic lines and 

 the environment causes an inexorable increase in the number of 

 species. Geographic isolation is the most important process in both 

 plants and animals. Ecological isolation, simple genie mutation, 

 and hybridization produce a limited amount of species fission and 

 in special cases may lead to the evolution of many new species. 

 Polyploidy in animals produces few new species but in plants is a 

 great force in the genesis of new independent phylogenetic lines. 



These processes involve two other factors of great importance: 

 time and geomorphology. 



From the viewpoint of time, the species arising through single 

 mutations (polyploid or otherwise) come into existence in one 

 generation. Species fission by other processes involves change 

 through numbers of generations, few for such rapidly evolving 

 species as those in Holocarpha or Clarkia, up to many thousands 

 for slower changing organisms. 



The relationship of geomorphology to species fission is of the 

 greatest importance. The processes involving solely genie mutation, 

 polyploidy, colonization, and perhaps certain types of hybridization 

 can occur without any change in the environment. The processes 

 involved in ecological isolation may be initiated by short-term or 

 local weather changes, but need nothing more than the annual 

 fluctuations normal to the area. Processes involving range fission or 

 range congregation, however, require geomorphic changes which 

 bring about long-term and stable changes in climatic and other 

 environmental conditions. This is the only mechanism which will 

 produce marked and sustained shifts in range boundaries and there- 

 by lead to range fragmentation or to the congregation of ranges 

 previously separated. 



These processes and mechanisms have been increasing the num- 

 ber of phylogenetic lines since sexual life originated, although only 

 a portion of the products survived. Extinctions have continuously 

 reduced the total number which came into existence. Judged from 

 information in groups having a seemingly good fossil record (Fig. 

 89 ) , at various times and in various groups this extinction produced 

 a startling decrease. How many phylogenetic lines came into 

 existence during biologic time is difficult to estimate because the 

 fossil record in most groups is meagre. About 1,500,000 species are 

 known today, 365,000 plants and the rest animals (Jones, 1951; 



