«A Review by Michael Tubby 43407116 1. Abstract The modern Antarctic environment is typified by extremes in temperature and light. However extreme ...»
Evolution in Antarctica has produced many novel traits, but has also followed large scale patterns. At present the Antarctic is relatively depauperate in vertebrate diversity, yet supports large populations. Compare this with the tropics which support small populations but are rich in vertebrate species. This is a reflection on the harsh Antarctic environment. Animals are required to become so specialised, and environmental stresses are so high that any creature not perfectly suited to Antarctic conditions is eliminated. The result is a low species turn over rate and minimal interspecific competition. Antarctic vertebrates find good adaptations and stick with them, while in the tropics animals are engaged in a rapid and continuous arms race as they try to out-compete rival species. Antarctic species will tend to evolve slowly while maintaining high levels of specialisation, tropical species will evolve rapidly in an environment which favours adaptability and generalisation. Compare two bird families: the tropical family psittacidae (parrots) and the polar family spheniscidae (penguins). Spheniscids have managed to radiate into between 17 and 20 extant species. Psittacidae incorporates closer to 330 species (Fain & Houde 2004). The evolutionary consequences of extreme Antarctic conditions are bradytelic, R-selected vertebrate taxa.
Antarctica’s current vertebrate fauna assemblage is far from typical. For the majority of natural history conditions were much milder and fostered a much more diverse vertebrate ecosystem. Extinct fauna lacked adaptations to extremely low temperatures and were not as isolated from the other continents which constituted Gondwana (Warner, Rich & Vickers-Rich 1997). However, extreme seasonality did occur, and some adaptations may be seen in the fossil record. Reptiles appear to have dominated the mega fauna during the Mesozoic and displayed adaptive traits convergent with traits exhibited by extant species living in temperate climates, such as hibernation in the grizzly bear, Ursus arctos horribilis. However, the bones of Dinornis sp. also posses cortical rings, or LAG’s, and are not believed to have hibernated (Turvey, Green & Holdaway 2005), highlighting the patchy nature of the fossil record.
Birds and mammals only make fleeting appearances in Antarctica’s fossil record, yet it can be assumed that truly terrestrial forms were present on the continent at least until the Late Eocene. These earlier forms are not ancestral to the modern mammal and bird taxa, and so it can be readily assumed that they lacked any cold climate adaptations, resulting in their subsequent local extinction. All extant Antarctic pinnipeds, cetaceans and spheniscids are migrant species, originating from warmer regions. This exposes another trend in Antarctic vertebrate assemblages, all extant forms are the result of a recent migration, no group has an extensive local history except for the notothenioids (Bargelloni et all. 2000). Cetaceans radiated down from the Tethys and New Zealand, the pinnipeds originated in Eurasia and the spheniscids came from lower latitudes of the Southern Hemisphere, all within the last 40 million years (Jadwiszczak 2001). What’s more, the modern seals, whales and penguins have not diverged from their ancestral stock by and great degree. Contrast with the notothenioids, which are quite removed from all other modern teleost clades.
Just possibly, extreme Antarctic conditions favour convergent evolution over divergent evolution and also slow down the pace of speciation, instead refining a species. Indeed, an organism must already be rather well adapted to penetrate the Antarctic environment, and as such does not need to change and adapt to any great length before it is adapted enough to thrive (Watermann 1999). Convergent evolution can be seen everywhere in the extant Antarctic fauna. Seals and penguins share a unique circulatory system, and both also utilize blubber, as do whales.
Divergent evolution separates modern Antarctic vertebrates from the rest of the world, but convergent evolution endows them with the most effective adaptations for Antarctic survival.
Antarctica is an ever changing land of extremes. However such extreme conditions did not always plague the continent. Through history it has seen the rise and fall of many vertebrate faunal assemblages and helped shape the Earth’s modern fauna. In the past, when conditions were milder, it acted as a halfway house, with species passing through between continents. Eventually it developed a unique vertebrate fauna which resembles modern day temperature vertebrates, fauna which are believed to have migrated and hibernated like modern mega fauna. Some smaller vertebrates may also have adapted to extended darkness and remained active during the polar winter. Cenozoic cooling and mass extinction denuded Antarctica of all its terrestrials vertebrates, leaving only marine fishes. These fish diverged greatly from their ancestral stock and developed unique adaptations at a molecular level.
On land, after the continent had been scraped clean of life by glaciers, new marine mammals and birds colonised, evolving similar adaptations at a larger scale, morphological and behavioural. The end result is the development of a bradytelic vertebrate fauna with many convergent traits.
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9. Appendices Appendix A. Paleogeography – Antarctica’s position Figure 1. EarlyPermian Figure 2. Late Permian Figure 3. Late Jurassic Figure 4. Late Cretaceous