Abstract Based on literature, a characteristic of the evolution of the steppe zone of Eurasia and the evolution patterns of grasses dominating in the steppe biocenoses is given. The hypothesis is discussed that the grasses’ divergence preceded the beginning of their dominance in the Neogene. Factors promoting the origin of phytophagous insects and hypotheses explaining the speciation of phytophagous insects are presented. The higher Diptera, Chloropidae, including grass flies of the genus Meromyza Mg., as well as the dominant species of grass ecosystems are characterized. The origin time of Meromyza using the mtDNA СОI gene locus was estimated in comparison with Drosophila, for which the time of speciation was known. The time of divergence of host plants was compared with the evolution of Meromyza flies. The strategy of feeding specialization in evolution of Meromyza flies for the closely related European species of the variegata cluster is described. It was shown that distribution, diversification, and the shift from polyphagy- to oligo- and monophagy followed the increase in the abundance of potential host plants, which corresponds to the hypothesis of speciation, known as “the oscillation hypothesis.”
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