130 



THE SPECIES PROBLEM WITH FOSSIL ANIMALS 



phylogenetic tree, is shown in Fig. 2. The branches of this tree 

 represent ancestral-descendent population sequences replacing 

 one another through time as they undergo morphological and 

 genetic divergence. For the sake of simplicity, it is assumed that 

 the organisms involved are sexual, biparental, and that complica- 

 tions due to partial genetic barriers and Rassenkreise do not exist 



12 



13 



II 



rf^t 



Divergence- 



Fig. 2. A simplified theoretical phylogeny. At the indicated stratigraphic 

 horizons (A, B, C, D) transient species (1-13) are discrete evolutionary 

 units with no morphological or genetic overlap. In rare instances, where 

 the record of a line of descent is nearly continuous, segments of evolving 

 lineages may be designated as successional species. (After Newell, 1956.) 



at levels A, B, C, and D. All the individuals living at time A arc 

 capable of interbreeding. By time B speciation has occurred so 

 that populations 2 and 3 arc no longer capable of genetic inter- 

 change. Five similar branchings are recorded in the diagram. 



Given the simplified conditions of this model, contemporaneous 

 organisms arc always distributed among discrete, reproductively 

 isolated groups. For reasons discussed below, palcontological 

 samples almost always consist of the remains of individuals who 

 lived dining a fraction of geologic time so short that evolutionary 



