Understanding speciation requires the identification of traits that cause reproductive isolation. This remains a major challenge since it is difficult to determine which of the many divergent traits actually caused speciation. To overcome this difficulty, we studied the sexual cue traits and behaviors associated with rapid speciation between EA and WN sympatric behavioral races of Drosophila athabasca that diverged only 16,000-20,000 years ago. First, we found that sexual isolation was essentially complete and driven primarily by divergent female mating preferences. To determine the target of female mate choice, we found that, unlike cuticular hydrocarbons (CHCs), male courtship song is highly divergent between EA and WN in both allopatry and sympatry and is not affected by latitudinal variation. We then used pheromone rub-off experiments to show no effect of CHCs on divergent female mate choice. In contrast, both male song differences and male mating success in hybrids exhibited a large X-effect and playback song experiments confirmed that male courtship song is indeed the target of sexual isolation. These results show that a single secondary sexual trait is a major driver of speciation and suggest that we may be overestimating the number of traits involved in speciation when we study older taxa.

The rate constants for ring inversion (k r.i.) and bond shift (k b.s.) in 1 and 2 were determined by dynamic NMR spectrometry while the rate constants for bond shift and intramolecular charge transfer (k c.t.) were determined for 1 2-/2K+ and 2 2-/2K+. These processes were modeled by HF/3-21G(*) ab initio molecular orbital calculations of the ground states and of several transition states for 3, 4, 3 2-, 4 2-, 3 2-/2K+, and 4 2-/2K+. The results indicate that k r.i. and k b.s. are ca. 2.5 times greater (at 240 and 280 K, respectively) for 2 compared to 1 due to larger steric repulsions in the ground state of 2. Contrariwise, k b.s. and k c.t. are 1.7 and 166 times greater, respectively, at 280 K for 1 2-/2K+ than for 2 2-/2K+. These differences are attributed to less twisting and therefore greater π delocalization between the cyclooctatetraenyl rings and the aryl ring in the bond shift and charge-transfer transition states of 1 2- compared to 2 2-. The greater difference between 1 2- and 2 2- for k c.t. compared to k b.s. is postulated to result from looser ion pairing in the charge-transfer transition state relative to the bond shift transition state.

Nuclear magnetic resonance spectroscopy (NMR) is a viable alternative to current methods to introduce enzymatic reactions and monitor kinetics in the undergraduate curriculum. Using NMR to observe the invertase-catalyzed conversion of sucrose to fructose and glucose, one can gather information about the order of the reaction, as well as the maximum rate (v max) and the Michaelis constant (K M). Kinetic parameters determined in this NMR study are comparable to the results obtained through polarimetry, the method often used to study this reaction at the undergraduate level. The breadth of information that NMR provides can give students a better understanding of the changes in reactant and product concentration over time, giving a visual connection to the rate law they derive.

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