Volume 6, Nº 3 (2016)

The apparent morphological lacunae dividing the macrotaxa of modern birds (orders of Neornithes) present a serious impediment to the construction of macroevolution models. However, the discovery and detailed investigation of multiple groups of fossil birds in recent decades contributed to the elucidation of the process of the formation of modern higher taxa and allowed for analysis of the macroevolution process that occurred in this group of vertebrates during the Cenozoic period. The present article is a concise review of the “transitional” taxa with a focus on representatives of the Galloanseres clade. Almost all of the transitional bird forms exhibit a mosaic morphology, although certain stable morphological traits or combinations of traits occur in different groups and may represent either a primitive or an advanced feature for a specific group. The stability of these morphological features is due to the correlation between ontogenetic processes, whereas the independent evolution of individual morphological characteristics is related to the modular organization of ontogeny. The problem of the emergence of the morphological type of modern Passeriformes is discussed with modular organization of the birds’ feet as an example. Mosaicism of the features in the transitional forms is regarded as the only macroevolutionary pathway possible, given the restraint on parallel optimization of multiple systems of the body. Consequently, stem taxa with mosaic morphology are regarded as ancestors of crown taxa. Complete filling of the lacunae between taxa does not occur during macroevolution, since the “transitional” taxa are separated from the respective descendant and ancestor taxa by lacunae as well. Notably, the evolutionary formation of some higher taxa of birds started with the formation of advanced morphology of the jaw apparatus and the entire skull, while the postcranial skeleton initially remained plesiomorphic.

Although diverse complexes of chromosome races are of rather rare occurrence in mammals, this does not reduce their importance to an insignificant phenomenon unworthy of studying as a unique case without direct analogy. Moreover, these complexes represent virtually ideal models for estimation of the impact of hybridization on the process of microevolution. The chromosome races are characterized by an almost zero level of molecular genetic differentiation and well-defined distinctions between them, which are usually associated only with chromosome rearrangements. This review shows the valuable contribution of the studies on Sorex araneus and Mus domesticus chromosome Robertsonian systems to our understanding of the varied impacts of hybridization on the speciation process. It particularly promotes a better understanding of evolutionary phenomena such as the “reinforcement” of reproductive isolation in secondary contact zones between divergent populations, speciation without geographic separation (“divergence with gene flow”), and “zonal raciation.”

Speciation is a microevolutionary process that starts with intraspecies differentiation, followed by the establishment of reproductive isolation. One of the intrinsic isolating factors causing physiological isolation is assortative mating. As a result of many years of observations of the natural populations on the territory of the South Urals (the Bashkortostan Republic) and long-term laboratory experiments, we revealed assortative mating in populations of the potato beetle Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae). The assortativity of mating manifested itself as a statistically significant deviation from the normal distribution of the frequencies of imago pairs belonging to three morphological types, which are defined by the degree of integument pattern melanization, namely, the achromists (A), melanists (M), and intermediate type (I). In copulo, we have not detected mating between achromist males (A) and melanist females (M), with the active choice in this case belonging to males. Under the laboratory conditions, with a sampling of 40 artificially formed pairs, we revealed statistical differences in the longevity and breeding performance of imagoes belonging to different morphological types. Achromists and melanists were characterized by a statistically higher longevity, almost two times higher than that in individuals of the intermediate type. Females of the intermediate type showed statistically higher fecundity in the homonomous crosses than achromists and melanists. It was demonstrated that the viability of the progeny also differed statistically in different pairs. The highest viability was detected for the progeny obtained from the type A and type I females. The observed differences indicated the existence of different reproductive strategies in studied species populations. In the S laboratory line of the common house fly Musca domestica L. (Diptera: Muscidae), we detected individuals with different reproductive strategies associated with longevity. The maximum longevity in two inbred lines, Sh28 (short-living) and L2 line (long-living), isolated from the S line showed a statistical, almost twofold difference. Mass reproduction time is confined to the first or second week of the imago stage in the short-living fly line; it is shifted to the third or fourth week in long-living flies. In reciprocal crosses between the lines, we detected a dramatic decrease in the fecundity of the females and a decrease in the viability of the hybrid females. The results confirm the essential role of assortative mating and frequency-dependent selection in the maintenance of the intrapopulation diversity.