 The European population of common dolphins ( Delphinus delphis ) in the Mediterranean have differentiated into two types : eastern and western . According to a 2012 study , this seems to be due to a recent bottleneck as well , which drastically decreased the size of the eastern Mediterranean population . Also , the lack of population structure between the western and eastern regions seems contradictory of the distinct population structures between other regions of dolphins . Even though the dolphins in the Mediterranean area had no physical barrier between their regions , they still differentiated into two types due to ecology and biology . Therefore , the differences between the eastern and western dolphins most likely stems from highly specialized niche choice rather than just physical barriers . Through this , environment plays a large role in the differentiation and evolution of this dolphin species . 
