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Research Profiles : Aloha, spider style! :

A surprising coincidence?

Do you notice anything surprising about the relationship between this evolutionary tree and the order of islands in the archipelago?

Cladogram showing happy face spider relationships and how the spiders match up with the islands

The physical order of the islands and the tree's branching pattern match up! That's a bit like drawing numbers out of a hat one at a time and getting the numbers 1 through 4 in the exact correct order — it might happen by chance, but not very often. Is this correspondence between island geography and the evolutionary tree a coincidence, or is there some other explanation?

The correspondence is not a coincidence, but in order to understand why, you need to know a little bit about the formation of these islands. The islands in the Hawaiian archipelago are arranged linearly from oldest to youngest. Kauai is the oldest island, Oahu the next oldest, and the large island of Hawaii is the youngest.

Map of Hawaii showing ages of the islands at selected points

This linear formation is the result of the movement of tectonic plates over an active fissure at the bottom of the ocean that constantly emits magma. This magma cools, hardens, and creates the land we see on each island. Click the "go" button below to see a simplified version of how this process works.

The evolutionary tree suggests that the "oldest" (original) group of spiders evolved on the oldest island of Oahu. As new islands formed, individuals from this original population colonized subsequent islands in a "hopscotch" manner. The youngest islands of Maui and Hawaii were colonized last and harbor the "youngest" populations of spiders. Click the button below to watch this process in action.

Aloha, spider style!
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Learn more about plate tectonics on the Understanding Global Change site.

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