Geography Reference
In-Depth Information
• Calibration using age of strata where endemic taxa occur: A common
mistake is to assume that a geologist's suggestion that there is no
evidence of land in a region and particular period means that there is
evidence of no land (e.g., when calculating the age of an island and
setting a maximum age limit for a taxon inhabiting it, ignoring the pos-
sibilityofotherformerislandsintheareaorolderstrataburiedbeneath
the present topography).
• Calibration using paleogeographic events: Most of the standard cor-
relations between distributions and paleogeographic events are highly
simplistic. For example, the rise of the Isthmus of Panama, calculated
to be 3 mya, which caused vicariance between Pacific and Atlantic
marinetaxaandalsomarksthebeginningoftheGreatAmericanBiotic
Interchange, is often used as the basis of clock calibrations. However,
many authors have pointed out that several Pacific-Atlantic pairs of
taxa may have diverged well before the final rise of the isthmus.
Methods
There are several molecular dating methods, grouped into three
main classes: methods that use a molecular clock and one global rate of
substitution, methods that correct for rate heterogeneity, and methods that
try to incorporate rate heterogeneity. Each method has it own algorithm. For
a revision, see Rutschmann (2006).
Figure 7.9
Relationship of lineage divergence, origin of a synapomorphy, and oc-
currence of the oldest fossil with the synapomorphy, according to Magallón (2004).
There is a temporal gap between the divergence of a taxon and its sister taxon (t
0
),
the origin of the synapomorphy (t
1
), and the occurrence of the oldest fossil bearing
such synapomorphy (t
2
).
