Development of Theoretical Views on Viviparity

This article discusses theoretical aspects of studies on viviparous organisms and provides new approaches to understanding the evolution of viviparity. Viviparity is considered here not as a “great biological advantage,” but as the simplest way of birth, that in terms of biodiversity is in the minority; it is realized only in cases in which, due to various reasons, laying eggs is impossible or poorly compatible with newly acquired morphological and/or physiological characteristics. Aberrations of the reproductive sphere, changes in its ancestral state and changes leading to viviparity, can be divided into three main groups: (1) the appearance of oogamy in the ancient colonial protists and multicellular organisms before the appearance of any mechanisms for extrusion of immobile oogamete (or zigote); (2) evolutionary conversions from external to internal fertilization in organisms that have no specialized oviducts with accessory glands and spermathecae; (3) abnormalities in well-developed female reproductive systems: the loss of insect reproductive organs (in cases of paedogenesis, neoteny, or paedomoprhosis), larval meiosis, change of fertilization area from ectodermal parts of oviducts to intragonadal fertilization. All the above mentioned evolutionary rearrangements are considered from the point of view of an analytical review of viviparity in the largest biological taxa from archaic multicellular organisms to the most advanced groups of invertebrate and vertebrate animals.