Because of the bias of the fossil record, we can’t yet figure out if lots of groups are starting outside of the tropics and moving into the tropics — but we can look at the flipside of this issue: whether species that first evolve inside the tropics tend to spread outwards and invade other climates.

To do this, we’ll focus on genera whose fossils first appear in tropical rocks. To figure out if they spread out of the tropics, we’ll look at where these organisms are found on Earth today. Are they now found in non-tropical climates as well?

The graph below focuses only on bivalve genera that started in the tropics 10,000 to 1.8 million years ago. It shows how far north or south of the equator those genera can be found today. Some of them are still found only in the tropics (represented by the left side of the graph) and some of them are now found outside of the tropics as well (represented by the right side of the graph).

To make sure that you understand this graph, click through and answer these questions:

• How many genera are represented by the bar labeled A?
• The bar labeled A is positioned along the x-axis (the horizontal axis) at 35º north or south. What does this mean?
• According to this graph, how many of the genera shown on this graph originated in the tropics and never left them?
• Does the graph above support the idea that genera that first evolve in the tropics tend to spread into other climates? How does the graph above support your thinking?