skull. This is known as nasal drift. The nostrils of modern cetaceans have become modified into blowholes that allow them to break to the surface, inhale, and submerge with convenience. The ears began to move inward as well, and, in the case of Basilosaurus, the middle ears began to receive vibrations from the lower jaw. Today's modern toothed whales use their melon organ, a pad of fat, for echolocation. Radiation events There are three major radiation events that mark diversification and speciation in the evolutionary history of Cetacea. The first occurred around the middle Eocene (40 Mya) when these early cetaceans abandoned riverine and shallow coastal habitats, setting the scene for Protocetidae – the first fully marine cetacean. With the oceans and its nutrients at their disposal, rapidly diversifying protocetids were also responsible for the first major geographic expansion, dispersing throughout North Africa, Europe, and North America. The second of three major radiation events occurred near the start of the Oligocene (~34 Mya) when Neoceti diverged from Basilosauridae. This radiation event concurrently occurs with the breakup of Gondwana and the opening of the Southern Ocean, wildly changing ocean ecosystems, productivity, and temperature gradients. The timing of this second radiation event is not coincidental, as the following diversification of cetaceans was likely due to new ecological opportunities the change in oceans gave them. The final major radiation event, occurring throughout the middle Miocene and into the Pliocene (12 Mya to 2 Mya), was not due to a specific event but is associated with widespread generic expansion of odontocetes and mysticetes. Some modern genera of cetaceans began to emerge, including Balaenoptera, a genus of rorquals that includes the blue whale. Delphinidae, ocean dolphins, also arose during this radiation event in the late Miocene. Ongoing evolution Culture Culture is group-specific behavior transferred by social learning. Tool use to aid with foraging is one example. Whether or not a dolphin uses a tool affects its eating behavior, which causes differences in diet. Also, using a tool allows a new niche and new prey to open up for that particular dolphin. Due to these differences, fitness levels change within the dolphins of a population, which further causes evolution to occur in the long run. Culture and social networks have played a large role in the evolution of modern cetaceans, as concluded in studies showing dolphins preferring mates with the same socially learned behaviors, and humpback