Sam Rosenbaum
Biogeography
Literary review 6
4/12/11
Johnson, Kevin P. and Weckstein, Jason D. 2011. The Central American land
bridge as an engine of diversification in New World doves. Journal of
Biogeography. (J. Biogeogr.) (2011) 38
Johnson is from University of Illinois, Champaign, IL and Weckstein is from the
Field Museum of Natural History in Chicago, IL.
The Research Question
What is the potential role of the formation of the Central American Land bridge in
creating diversity through repeated rare dispersal events followed by isolation?
Background knowledge leading up to study
About 3.5 Ma the land bridge between Central and North America formed. This
made it possible for organisms to disperse between these regions. Even though
birds are able to fly over water, as the land bridge began to close, the frequency
of dispersal increased. A species can disperse from one continent to another,
become isolated and speciate, then disperse back to the original continent. We
focus on mid-sized New World doves. The three genera, Geotrygon, Leptotila,
and Zenaida, have multiple species in North and South America with a
concentration of their diversity around the land bridge.
Methods Used
We used a molecular phylogeny based on four gene regions for the doves.
Distinct diversification events were marked and dated using a Bayesian relaxed
clock analysis (a technique in molecular evolution that uses fossil constraints and
rates of molecular change to deduce the time in geologic history when two
species or other taxa diverged) and internal calibration points for endemic island
taxa with known island ages.
Results
The phylogeny produced by the data indicated nine different dispersal driven
divergence events between North and South America. There have also been five
in the recent past that have not become different taxa.
The Weaknesses of the Study
Since the molecular clock analysis is not a time based scale, it is difficult to exact
a time from it. This is built into the system Bayesian relaxed clock analysis yet is
still something that has to be taken into account.
What I learned from the study
I learned that after the land bridge was formed, there was a large increase in
dispersal of organisms as well as a large increase in diversity that is show in the
phylogeny.