[Taxacom] Biogeographers' Exam Q 8

John Grehan calabar.john at gmail.com
Thu Apr 6 15:21:38 CDT 2017

Biogeographers’ Annual Exam Question 8

What is the historical relationship between Mexican tectonics and the
principal clades of the *Fuchsia *group?

“Four of the groups in the Onagraceae repeat a distinctive, standard
pattern in which endemics from Mexico (above all, western Mexico) and Central
America have worldwide sistergroups (Heads, 2009c). Dispersal theory
accounts for the pattern by inferring centers of origin in Mexico and Central
America. Instead, in vicariance theory, the region represents a zone of
episodic differentiation in ancestors that were already widespread through
the Americas or around the globe. Episodic tectonics was a feature of the
central boundary in Mexico, which separates the western Guerrero terrane
from the eastern Craton. Alternating phases of compression/accretion and
transtension/rifting occurred along this belt through the Jurassic and
Cretaceous (Centeno-Garcia et al., 2008: Fig. 3).

Subsequent studies have made a slight modification (from Levin et al.,
2004): they have proposed a new monophyletic clade in which* Hauya* of
Mexico and Central America is sister to *Circaea* +* Fuchsia* (Ford &
Gottleib, 2007; cf. Berry  et al. 2004). If Mexico is a zone of repeated
fracturing in widespread ancestors, rather than a center of origin, this
would explain the break there between *Hauya* and its widespread sister
group (whether this is the rest of the Onagraceae except Ludwigia [as in
Levin et al., 2004], or just Fuchsia + Circaea (Figure 10.1).

A series of repeated fracturing in Mexico would also explain the next break
there, between the southern *Fuchsia* (trees, shrubs, and lianes) and the
northern *Circaea *(herbs) (Figure 10.2) (fossil of* Circaea* and *Fuchsia*
are known, but only from within the extant range of their respective genus,
Xie et al., 2009). The geographic boundary between Fuchsia and Circaea does
not represent a simple, ecological boundary between a tropical group in
southern Mexico and a temperature group in northern Mexico, as the southern
component, Fuchsia, extends to Tierra del Fuego. Thus, it is more likely
that the break had a tectonic origin, and it coincides with the
Mojave-Sonora megashear. This belt of strike-slip deformation crosses
northern Mexico from the Pacific to the Atlantic coasts and underwent ~800
km of left-lateral displacement in the Late Cretaceous (Campos-Enriquez et
al., 2011). Its possible effects on the regional biogeography have been
discussed by Souza et al. (2006). The megashear is part of the Basin and
Range province of southwestern North America. In this area, extension has
produced distinctive topography, with northwest trending mountain ranges
and valley bounded by normal faults, and metamorphic core complexes."

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