Biogeography - Gene 3000 Biogeography(cont'd Two hypotheses...

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Gene 3000 02-15-07 Biogeography (cont’d) Two hypotheses that explain geographic distribution of organisms Dispersal – range expansion Vicariance - fragmentation of an organisms geographic distribution by external factor(s); ex. Extinction of geographically intermediate populations; division of land mass into two or more parts by a body of water Disjunct distribution (a break in the distribution of an organism) Example – Pine, Sweet gum, and Blue Beech trees distributed in south east US and Mexico/Central America, with a break in the middle for each species o If this is explained by dispersal, the trees evolved in one region, then each tree moved independently to the other (3 events) o If this is explained by vicariance, the distribution of all three trees would have originally been continuous, the break could be explain by one event (such as drying out) that wiped out all three species in that region (1 event) o Following the rule of parsimony, vicariance would be the hypothesis most likely to explain this situation because these taxa are unrelated; if this situation only concerned one type of tree, both hypotheses would be equally likely Example – Land mass distribution 190-20 MYA (fusion and fission events) and the radiation of birds and mammals 65 OR 100 MYA o If the fossil record date is correct (65 MYA) is dispersal or vicariance more likely to explain the current distribution of mammals and birds?
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  • Spring '07
  • Arnold/Anderson
  • organisms geographic distribution, Species Interactions Co-evolution, water Disjunct distribution, unrelated organisms, I. fulva

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