The emergence of behavioural isolation between populations under divergent selection can be crucial for ecological speciation, but the mechanisms underlying such isolation are poorly understood. Several experimental evolution studies have shown that positive assortative mating (preference for similar mates) can arise rapidly in Drosophila laboratory populations reared in different stressful conditions, while other studies failed to confirm this effect. Here, we present the results of an evolution experiment in which outbred lines of Drosophila melanogaster were reared for 1-2 years on one of the three different diets (standard, starch based or high salt). We show that nonrandom mating arose in some, but not all lines, and that the manifestations and possible interpretations of this nonrandomness depend strongly on the type of tests used to assess mating preferences. More specifically, multiple-choice four-fly tests revealed positive assortative mating (prevalence of homogamic matings) in some starch-adapted and salt-adapted lines when paired with a control line reared on the standard diet, but competitive three-fly tests rather revealed competitive advantage of control males and females over the flies reared on stressful diets. The results imply that divergent adaptation can result in differences in mating propensity or competitive ability, which, in turn, may either facilitate or hamper speciation depending on the relative frequency of high- vs. low-competition settings in natural habitats of the diverging populations. The results also emphasize the importance of using diverse tests for assessing mating structure in natural and laboratory populations.

Quantum chemistry was used to simulate a fragment of α-glycine crystal consisting of 5 and 6 glycine molecules. The α-glycine crystal fragment model was constructed using X-ray diffraction data contained in the Cambridge Crystallographic Database (CCDC). Theoretical IR spectra were obtained. It was shown that in a fragment of a crystal, glycine molecules in zwitterionic form form N-H...O hydrogen bonds of different strength, which correspond to different frequencies of stretching vibrations νst (N-H) of the NH3+ group. Full optimization of glycine crystal fragments consisting of a small number of molecules, regardless of the method of calculation, leads to the agglomeration of the structure. The system forms the maximum possible number of the most durable hydrogen bonds, thereby lowering the overall energy of the system, which leads to disruption of the crystal structure of the fragment, distortion of the parameters of hydrogen bonds in the crystal, and, consequently, to an unsystematic shift in the frequencies of N-H vibrations. To avoid this process, further modeling was performed by partially optimizing the geometry of the crystal fragment, with the positions of heavy nitrogen atoms, carbon and oxygen atoms being fixed, and only the positions of hydrogen atoms involved in the formation of hydrogen bonds were varied. Using the DFT/B3LYP/6-311G** method, the energies of donor–acceptor interaction in a fragment of a crystal, as well as its oscillation frequencies, are calculated. The calculated frequencies are consistent with the three experimental frequencies in the indicated X-ray region. A comparison was made between the calculated and experimental frequencies of the stretching vibrations of the N-H group of the NH3+ group. According to the geometric characteristics of the HB and the energy of donor-acceptor interaction between the lone electron pairs of oxygen atoms and the *(N-H) loosening orbitals, it was found that the hydrogen bonds of N-H...O between molecules in one layer are stronger than between molecules in different layers of a crystal.

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