Evo-Ed: Integrative Cases in Evolution Education

Cases for Evolution Education

Population Genetics

Soft shell clams (Mya arenaria) are bivalves native to the east coast of North America. Some individual clams are resistant to a harmful neurotoxin called saxitoxin while others are not. During algal bloom events, soft shell clams ingest quantities of dinoflagellates that produce saxitoxin. If a clam is resistant, it will eat dinoflagellates and be unaffected by the saxitoxin. If a clam is not resistant, it will often succumb to the effects of saxitoxin poisoning resulting in paralysis or death.

illustration of soft shell clams

Distribution of Saxitoxin Resistance

The distribution of resistant clams is not random among populations. Resistance is more common among populations in the native (east coast) range that have a history of being exposed to algal bloom events (Connell et al, 2007). The images below show the proportion of clams in a set of populations found to be sensitive to saxitoxin (sensitivity represented in black and resistance in white; image taken from Connell et al., 2007)

Location # Sensative # Resistant
Havre-Aubert 6 0
Lawrencetown 9 4
Lepreau Basin 1 10
Essex 3 9
Orleans 2 10
Bellingham 11 1
Quartermaster 11 1

In the North American populations the proportion of clams resistant to the toxin was smaller at higher latitudes where water temperatures were cooler and bloom occurrence was rare.