INTRODUCTION – what do students know before the tidbit?
• understand that similarities used to select among trees
• know basic vocabulary associated with phylogenies
TIDBIT
-- introduce to students. AA sequences of real proteins; numbered taxa are the same in all
groups. TWO MINUTES
-- individuals choose the best hypothesis. Vote via clickers. FIVE MINUTES
-- discuss within groups. All individuals revote. TWO MINUTES
-- reveal the taxa on tips of all three hypotheses on a ppt slide. Allow students to identify
the tips of their chosen tree. TWO MINUTES
-- reveal the second clicker result (choice of hypothesis). Should be bimodal distribution.
ONE MINUTE
-- discuss with a group that disagrees; identify possible reasons for differences. Race
between pairs of groups. [Here we expect someone to quickly realize that the different
groups have different sequences.] TWO MINUTES
-- two different genes, two different trees. Generate hypotheses about why those two genes
might yield different results. What kinds of evidence will help you distinguish among
those? Ask the "experts" (instructors). [Hopefully a student will ask "what are the proteins,
and what are their functions?"] FIVE MINUTES
-- summary slide(s). Reveal the identities of the different sequences. One ppt slide with the
sequences and their identities and expression location – hemoglobin on one side, prestin
on the other (indicate color of papers that groups have received). Functions of the two
proteins. A variant of prestin is involved in echolocation. [Try to elicit from students that to
resolve which is correct tree, maybe need more sequence data… Note that we have
genomes from these and other mammals, and data from most genes are consistent with the
hemoglobin tree.] TWO MINUTES
-- final clicker question: what evolutionary process best explains this dichotomy?
Convergent evolution in prestin, echolocation evolved twice
Convergent evolution in globin, echolocation evolved once ONE MINUTE
-- conclusion slide with learning outcomes…
Other directions one could in…
-- provide one or both of the real trees in different forms (rotated around a node); then may
get bimodal answers and can address that alternative conception.