Activity: Convergence

When is a “Mussel” not a Mussel? — The common name “mussel” is used for many kinds of bivalves, most of them wider than tall, slender, and asymmetrical, with the umbo (the oldest part of the shell) at one end — the Blue Mussel and Zebra Mussel are examples. This is convergence — the evolutionary process that results in two distantly related forms looking very similar.

Blue Mussel, Mytilus edulis

Zebra Mussel, Dreissena polymorpha

Here is a recent Bivalve Tree of Life that shows the positions of the Blue Mussel (Mytilus) and the Zebra Mussel (Dreissena). Notice that they are far apart on the tree; that means that they are not closely related. That in turn tells us that “mussel shape” (also called mytiliformMytiliform:
) has evolved more than once during bivalve evolution. Why?

The most common explanation is that the mussel shape is conducive to a crowded population of bivalves that need to attach to a hard substratum, like the surface of a rock. If one end of each shell is pointed, the pointed ends can fit into tiny crevices, allowing more of the bivalves to fit in a small space. This is an ecological (rather than genetic) explanation for a common phenotypePhenotype:
The observable physical or biochemical characteristics of an organism, as determined by both genetic makeup and environmental influences; what an organism “looks like.” (adj. phenotypic)
(outward appearance).

Ask your students to think of other examples of convergent evolution, such as birds and bats, or sharks and dolphins, or dolphins and the now-extinct swimming reptiles called ichthyosaurs.