quinta-feira, maio 07, 2009

Entrevistas aos bochechos: 4ª pergunta ao Paul Upchurch

RA: Yeah, one of the methodological tools that came out from this sort of ideas were area cladograms, wasn’t it?

PU: Area cladograms are an older idea, they go back to the original work done in the 1970’s. But, there is a fundamental problem with an area cladogram which is that by definition a cladogram shows branching structure. And that is fine when you are dealing with organisms that have a nice branching structure phylogeny. The problem is that an area cladogram isn’t describing the evolution of organisms, it is describing the relationships between geographic areas. And geographic areas do not have to obey a branching pattern. They can break up from each other, but they can also collide…they can reconnect. That means that the history of geographic areas is a complext network and not a simple branching pattern. So, the area cladogram by itself is a good idea, but not sufficient to describe biogeographic history. Where the time slicing idea comes in is that you continue to produce area cladograms, but each area cladogram is different for each point in time. By looking how each area cladograms change from each point in time, to the next, to the next, to the next…those changes tell you about the network-like history
Between the areas. So, to give you an example, in the Early Cretaceous we find that South America and Africa cluster together in the area cladogram, we’d say Australia would be more distant – as it was – and then in the Late Cretaceous we find actually that South America and Australia are clustering together with Africa being further away. What we see is a change in the area cladogram structure and, what that means is that between the Early Creataceous and Late Cretaceous that has been some change in biogeography, that cannot be expressed by one area cladogram but by two in sequence. And so, what we are really doing is trying to express this really complex network of area relationships as a series of area cladograms, rather than one area cladogram

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