Table 5 Design-teleological and selection-teleological explanations for the features of sharks and dolphins

(1) Why do dolphins have hydrodynamic shapes?

[DT1] Dolphins have hydrodynamic shapes in order to swim fast underwater, because organisms have the features that are necessary for their survival

[ST1] Dolphins have hydrodynamic shapes in order to swim fast underwater, because the latter confers an advantage; consequently, this feature has been selected for doing this and has been maintained in their lineage

(2) Why do sharks have hydrodynamic shapes?

[DT2] Sharks have hydrodynamic shapes in order to swim fast underwater, because organisms have the features that are necessary for their survival

[ST2] Sharks have hydrodynamic shapes in order to swim fast underwater, because the latter confers an advantage; consequently, this trait has been selected for doing this and has been maintained in their lineage

(3) Why don’t dolphins have gills?

[DT3] Dolphins do not have gills, but have lungs in order to get more oxygen directly from the atmosphere, because organisms have the features that are necessary for their survival

[ST3] Dolphins do not have gills because this feature was not maintained in their lineage and because lungs evolved in their terrestrial ancestors

(4) Why do sharks have gills?

[DT4] Sharks have gills in order to breathe underwater, because organisms have the features that are necessary for their survival

[ST4] Sharks have gills in order to breathe underwater, because the latter confers an advantage; consequently, this trait has been selected for doing this and has been maintained in their lineage