WEEK 4 - SAMPLE LAB EXAM QUESTION
- QUESTION:
Echinoderms are represented by five living classes. The crinoids are considered to be most similar to ancestral echinoderms. Although the fossil record is inconclusive, it appears that the Class Crinoidea separated from the other four classes early in the evolutionary history of this phylum. The remaining four classes separated from each other during a brief period roughly 500 mya.
Dr. Mike Smith of SFU has investigated the phylogeny of these four classes by examining their mitochondrial DNA. All seastars and brittle stars share a mitochondrial DNA inversion that is not present in any sea urchins and sea cucumbers. Dr. Smith has investigated the mitochondrial arrangement in crinoids. Do you expect crinoids show the mitochondrial inversion characteristic of seastars and brittle stars? Explain.
- ANSWER:
The crinoids would be expected to show the same mtDNA arrangement characteristic of sea urchins and sea cucumbers. The fact that all seastars and brittle stars share the inversion demonstrates that the inversion occurred after the evolutionary line leading to seastars split from the evolutionary line leading to sea urchins, but before the seastar and brittle star lines split from each other.
The Class Crinoidea would have split from the line leading to the other four classes before the split between the seastar/brittle star line and the sea urchin/sea cucumber line.
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