There are so many ethnicities in such specific areas of Nigeria, that it makes one wonder how they got there. The majority of ethnic groups in Nigeria have such specific traits due to geographic isolation from each other. Below are specific details regarding how each ethnicity got to where it is in Nigeria.
Hausa and Fulani: The Hausa and Fulani people originated in western Africa. However, intermarriage, migration, and the adoption of the Hausa language lead them to spread to Nigeria. The Hausa and Fulani Sharia Islamic law has been adopted in 11 of Nigeria's northern states.
Igbo: The Igbo people originated in Nigeria, and moved to the Southern area due to the many natural resources found in that section. The oil resources were a major reason the Igbo people remained in the South, even after their oppression under the Sharia laws of the Hausa Fulani people.
Yoruba: Historically, Yoruba tribal people have been centered around important nodes, or urban areas controlled by an Oba, which is their word for king. This past distribution has lead to their current location, centered around bigger cities such as lagos.
Ijaw: The Ijaw people are considered to be one of the oldest inhabitants of Nigeria. The Ijaws are located in oil rich lands, quite near the land which the Igbo people possess.
Kanuri: The Kanuri people originated in Niger, a close neighbor of Nigeria. From their they migrated to southeastern Nigeria, specifically centered around the Bornu state.
In the map above, a circle means that the ethnicity originated in that area. A line pointing from one area to another shows the diffusion of an ethnicity from one place to another.
Key:
Circles; Blue: Igbo Black: Ijaw Green: Yoruba (the many small circles represent the many nodes which Yoruba people center around)