The Effect of Vicariance and Dispersal on the Evolution of Ratites.

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The effect of vicariance and dispersal on the evolution of ratites. Introduction The ratites are an interesting clade within Aves, and one that caused problems for biologists and biogeographers for many years. This is a group of large, flightless birds that are distributed across all of the southern continents, separated from each other by thousands of miles of oceans. How do species incapable of dispersing across these geographic barriers display such a pattern? It wasn’t until Alfred Wegener (1912) presented his theory of continental drift that a reasonable explanation was available. As this theory gained acceptance, although slowly at first, through the 20th century geologists were able to gather evidence to produce maps showing the arrangement of the Earth’s major continents at different periods of its history. These paleo-maps are of great importance for those who study evolution, as the presence and break-up of so called ‘super-continents’ in the past offered an explanation to the disjointed distributions of many terrestrial animals, both extant and fossils. Scientists pre-Wegener postulated that sister clades somehow travelled across large oceans, via land-bridges that are geologically improbable, to explain their presence in both Africa and S. America. What was now a possibility is that these animals were simply on the opposite sides of one continent as it split down the middle to form the Atlantic Ocean. This process became known as vicariance. Of particular importance for those interested in ratites, was the presence of the super-continent Gondwana. Made up of what are now South America, Africa, Antarctica, Madagascar, India, Australia and New Zealand, Gondwana began to split in the late Cretaceous, a time when the clade Aves began to diversify. The distribution of recently extant ratites could be explained by vicariance biogeography, and the break-up of

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