A Step in Speciation, Part 2

A genetic look at EnsatinaSummarized from A Closer Look at a Classic

Ring Species – link to this resource is on Moodle.

The story so far…

• We’ve looked at Robert Stebbins’ data from the 1940s and 50s on Ensatina escholtzii subspecies in California.

• The observational data led to a new question: do these Ensatina populations form a ring species?

The work of Tom Devitt

• It wasn’t until DNA analysis became available that the Ensatina question could be addressed again.

• Tom Devitt and his team looked at DNA in Ensatina from the west coast to see what new information they could find.

Why mitochondrial DNA?

• Turns out that Ensatina has a huge genome, roughly six times larger than the human genome, and with multiple gene repeats.

• Mitochondria have their own DNA apart from an organisms nuclear DNA. Mitochondrial DNA is shorter and easier to sequence.

Results: It’s complicated!

• The current interpretation shows that Ensatina groups are more complex than previously thought.

• Some subspecies may need divided – or perhaps these aren’t subspecies at all.

What about hybrids?• If Ensatina forms a ring,

can species in southern California, where the ends meet, hybridize?

• In the 60’s, one of Stebbins’ students had found some Ensatina in this area that appeared to be hybrids.

• However, the two species don’t consistently hybridize in all places where they are found together. Why?

Hybrid hypotheses

• Devitt formed three hypotheses to explain the inconsistent hybridization:– The two subspecies have difficulty recognizing

each other as mates, due to different coloration or different pheromones (behavioral isolation).

– The two subspecies are reproductively incompatible (post-mating isolation mechanisms).

– The two have different habitat preferences, so they rarely meet (ecological isolation).

Post-mating isolation?• Dewitt is trying the

post-mating isolation hypotheses by bringing the two subspecies together in the lab.

• Results so far are inconclusive. Only a few Ensatina bred, but was it because they typically do not interbreed, or because conditions were not right?

Ecological isolation?

• The ecological isolation hypothesis is being tested with the aid of satellite photos of the habitat.

• Dewitt hopes to find out how much intermixing is possible between the two subspecies.

Your Task:

• Between now and next class meeting:– Download and print the A Step in Speciation, Part

2 worksheet.– Click on the link to A Closer Look at a Classic Ring

Species. – Read the material on the web site. – Answer the questions on the worksheet.– Bring these to class to discuss next time.