Fundamentals of Evolution - Korzh O.P. - 2006

Part I. MICROEVOLUTION

Chapter 6. ISOLATION

6.1. Evolutionary Significance of Isolation

Favorable trait combinations can become established provided that panmixia is restricted. Such restriction can be either complete or partial and arises As a result of isolation in one form or another.

Isolation refers to the spatial, physiological, or morphological Separation of individuals or groups of individuals from one another, which hinders or even completely prevents interbreeding between them.

From a genetic perspective, the most crucial characteristic of isolation is the restriction of panmixia, which at its maximum extent leads to the complete exclusion of free interbreeding. It is of great evolutionary significance by establishing intragroup barriers and is considered another elementary evolutionary factor. By limiting Gene flow between two populations, isolation determines the distinctiveness of their gene pools and the direction of Selection, thereby actively contributing to the diversification of life forms.

Territorial or biological restrictions on interbreeding persist over a relatively long period and tend to intensify. Beginning as a partial limitation on the freedom of interbreeding due to the continuous migration of individual organisms, it is subsequently reinforced by more substantial barriers as differences between the separated populations develop. Sooner or later, this process culminates in the complete impossibility of interbreeding.

Isolation in itself does not create new genotypes or intraspecific forms. Its significance lies in the fact that it fixes and amplifies the Initial Stages of genotypic differentiation, thereby ensuring the preservation of the gene pool Specificity of diverging forms. Consequently, by disrupting panmixia, it consolidates emerging differences in the genetic Structure of isolated population segments, accelerating The Development of heterogeneity. Isolation interacts directly with the sources of elementary evolutionary material—the mutation process and population waves.

As a rule, isolation is a long-term factor, and therefore exerts a persistently stronger pressure on a population than the mutation process, approaching The Significance of population waves. Significant isolation of groups can induce divergent directions of selection among them, determining the character of their evolution. At the same time, its effect on evolutionary material, much like that of the two preceding elementary evolutionary factors, is random and non-directional.

All representatives of modern flora and fauna have emerged as a result of Various Forms of isolation. The greatest experiment of nature involving isolation can be considered the breakup of the single supercontinent Pangaea at the beginning of the Phanerozoic and The formation of the modern continents.

Immediately after the separation of South America from ancient Gondwana, the mammalian fauna of this continent consisted solely of representatives of three main groups: marsupials (descended from primitive opossums), xenarthrans (mainly armadillos), and condylarths (primitive generalized ungulates). This faunal complex was unique, as the quantitative and qualitative composition of major mammalian groups differed significantly on other continents.

North America also harbored such fundamental lineages as marsupials (in significantly lower diversity) and ungulates. Due to the isolation of South and North America at that time, the latter evolved in different directions on these two continents. The most prominent differences lie in the fact that North America also possessed eight other mammalian genera unrelated in origin to either marsupials or ungulates. Consequently, in the early Cenozoic, North America had a more diverse and balanced fauna compared to South America. This predetermined the specific evolutionary trajectories of the respective continental faunas, which were further intensified by their subsequent reunification.

Thus, isolation not only limits the initial diversity of forms—which ultimately shapes The structure of specific biocenoses—but also promotes the divergent evolution of identical organismal groups. For the final delimitation of forms, the duration of isolation is of paramount importance. Even today, these processes are most clearly observable in insular systems, whose biota is invariably characterized by deficiencies (the absence of major representatives typical of standard faunas or floras) and a high degree of endemism.