Geography Reference
In-Depth Information
Cenocrons are sets of taxa that share the same biogeographic history, con-
stituting identifiable subsets within a biotic component by their common biot-
ic origin and evolutionary history from a diachronic perspective. After biotic
components are established, time slicing, intraspecific phylogeography, and
molecular clocks can help establish when the cenocrons assembled in the
identified components, incorporating a time perspective in the study of biotic
evolution.
Events of biogeographic convergence produce reticulated area histories
that decrease the chances of establishing area relationships through con-
gruence. The solution to problems posed by instances of biogeographic
convergence is time slicing (Upchurch and Hunn 2002). Whereas assess-
ments of faunal similarity usually are undertaken with faunas of successive
geological ages, cladistic biogeography has used only data on organism
relationships and spatial distributions on a single time plane (usually the
present). Time slicing may reconcile the use of time and a synchronic ap-
proach. Ideally, it is possible to use a synchronic approach for each time
slice identified.
Intraspecific phylogeography studies the principles and processes gov-
erning the geographic distribution of genealogical lineages, especially those
within and between closely related species, based on molecular data (Avise
et al. 1987). Once the population genetic structure has been assessed
based on mitochondrial DNA (mtDNA), it is possible to obtain a network or
cladogram of haplotypes, which allows us to analyze historical patterns and
the processes that shaped them (e.g., dispersal, vicariance, range expan-
sion,andcolonization), sometimesunderastatistical framework.Thisknow-
ledge can suggest when recent cenocrons incorporated into a biotic com-
ponent.
Cladograms based on molecular data may be used as raw data in
cladistic biogeography and intraspecific phylogeography. In addition, the as-
sumption that the rate of molecular evolution is approximately constant over
timeforproteinsinalllineagesallowstheinferenceofaclock-likeaccumula-
tion of molecular changes (Zuckerland and Pauling 1962), where the “ticks”
of the clock, which correspond to mutations, do not occur at regular intervals
but rather at random points in time (Gillespie 1991). This time is measured
in arbitrary units and then calibrated in millions of years by reference to the
fossil record or geological data (Magallón 2004; Sanderson 1998), giving
minimum estimates of the age of a clade, which in turn may help elucidate
the relative minimum ages of the cenocron to which it belongs.
