The orca, Orcinus orca, has the largest distribution of all aquatic mammals and is found in nearly all seas and oceans. Orca are social mammals that usually live in groups. These groups can vary in size. [Figure 2.1] shows an orca. There are a number of distinct types of orca. These distinct types of orca are classified as members of the same species. However, there is evidence that sympatric speciation is occurring. (a) (i) There are two distinct types of orca in the Northeast Atlantic Ocean: Type 1 and Type 2. Type 1 orca feed mainly on fish. Type 2 orca feed mainly on aquatic mammals, such as seals. Fig. 2.2 shows the locations in the Northeast Atlantic Ocean where Type 1 orca and Type 2 orca have been observed. Orca do not occur only in these areas and some groups of orca travel great distances. [Figure 2.2] (b) In the Southern Ocean, which surrounds Antarctica, there are three distinct types of orca: Type B, Type C and Type D. Fig. 2.3 shows the locations around Antarctica where Type B orca, Type C orca and Type D orca have been observed. [Figure 2.3] There are phenotypic differences between the different types of orca. Fig. 2.4 shows a diagram of a Type B orca, a Type C orca and a Type D orca. [Figure 2.4] Fig. 4.1 shows that the C282Y allele does not occur only in Ireland and is now present throughout Europe. The C282Y allele has been maintained in European populations, even though it is a cause of hereditary haemochromatosis.