The study of the Southern Hemisphere is of serious importance in Earth research because this area is associated with attractive mechanisms of dispersal of living beings and geographic and thus reproductive isolation of species. In contrast to the continents of the Northern part of the planet, the Southern Hemisphere is represented by separate island fragments not connected by land: South America, Australia, Southern Africa. Since historically, there has been no communication between these territories for a long time, many of the endemics living there have become deeply isolated species, unable to mix effectively with other, even closely related, individuals from other continents. Thus, the Southern Hemisphere study allows a more accurate and reliable explanation of the mechanisms of biota formation and their evolutionary significance.
Nevertheless, it is important to clarify that, contrary to current opinions, the development of life on isolated southern continents may have been associated with intense biological activity in the northern hemisphere. Although the geographical distance is one of the most important predictors of speciation, one should not exclude the possibility that invasive species were introduced from outside: by humans during the colonization of territories or by animals during migration. In particular, species migration can be supported by the phenomenon of bipolarity, in which organisms historically inhabit both southern and northern hemispheres (Segawa et al., 2018). The extremely important point is that the bulk of these organisms do not belong to the most cold-water faunas of either hemisphere but to those living southward in the temperate zone (Villaverde et al., 2017; Biersma et al., 2017). Bipolar forms are found among seals, whales, and marine fish: anchovies, sprat, sardines, or sharks. The possibility of dispersal of individuals in both hemispheres at once may be evidence of active evolutionary migration, fixed subsequently by comfortable conditions.
Another biogeographic hypothesis of species distribution, which is typical, for example, for Chironomidae, is the vicariant explanation. Among southern mosquitoes, the most archaic forms are found in South Africa, and phylogenetically, the insects of Australia and South America are closely related (Cranston et al., 2019). This sequence of relatedness is quite consistent with the order in which the fragments of Pangaea diverged: first, there was a split into Laurasia and Gondwana, and only after that, within Gondwana, South Africa, New Zealand, and finally Australia and South America successively lost contact with each other.
References
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Cranston, P. S., Hardy, N. B., & Morse, G. E. (2019).A dated molecular phylogeny for the Chironomidae (Diptera). Web.
Grens, K. (2019). Kary Mullis, inventor of the PCR technique, dies. The Scientist. Web.
Jafari, M., Akram, W., Pang, Y., Ahmad, A., Ahmed, S., Yasin, N. A., & Dong, S. (2018). Genetic diversity and biogeography of T. officinale inferred from multi locus sequence typing approach. Plos One, 13(9), 1-18.
Segawa, T., Matsuzaki, R., Takeuchi, N., Akiyoshi, A., Navarro, F., Sugiyama, S., & Mori, H. (2018). Bipolar dispersal of red-snow algae. Nature Communications, 9(1), 1-8.
Villaverde, T., Escudero, M., Martín‐Bravo, S., Jiménez‐Mejías, P., Sanmartín, I., Vargas, P., & Luceño, M. (2017). Bipolar distributions in vascular plants: A review. American Journal of Botany, 104(11), 1680-1694.