To the Friends of Mesquite

DrPaulREarl pearl at CCR.DSI.UANL.MX
Fri Apr 24 14:43:03 EST 1998

Dear Colleague:

Section Algorobia is equal to Prosopis, the genus, and it has one or a
few species. First, it is recognized that the mesquite cline in North,
Central & South America is an interbreeding continuum, despite its
remarkable variation in foliar sizes and shapes.  Second, the plant
systematist might follow only sexual isolation in defining a species.
Then, interspecific hybrids do not exist. Then, perhaps, new species
come from new genes by mutation not migration. New morphology can come
from hybridization. Hybrid morphology might be the reference of J. P.
Lotsy in Evolution by Means of Hybridization (1916), and the mistaken
taking of morphological differences by taxonomists as species criteria
was the basis of his attack on their species concept (Genetica, 1931).
But also see Winge (Sven. Bot. Tidskr., 1932).
	Are we referring to tradition ? Examination in the field simply cannot
stand up to what is later found out in the laboratory. The laboratory
provides adjustments and corrections, rather than contradictions.
Contradicting is merely humanistic. We certainly must have the field
data and specimens first. In Prosopis, we have finally reached the point
where the cline is the species, and this is credited to Saidman &
Vilardi (Theor. Appl. Genet., 1987) of the Juan Hunziker cytogenetic
school of Buenos Aires. After looking at isozoyme results on 9 Argentine
mesquite species, they believed that in essence the taxonomy of Burkart
(J. Arnold Abor., 1976) is overranked. 
	Let us look at the approach of Timofieff-Ressovsky (Mutations and
geographical variation, in Julian Huxley's The New Systematics,  pp.
91-92, 1940). "Under biological isolation (of a species) we understand
the impossiblilty or non-occurrence of normal hybridization under
natural conditions." In the very end, new genes come from mutation, not
migration, yet this does depend on a person's species definition. For
Verne Grant, a species is "...a community of crossfertilizing
individuals linked together by bonds of mating and isolated
reproductively from other species by barriers to mating." (The plant
species in theory and practice, in Ernest Mayr, The Species Problem: pp.
39-80, 1957).
 	In Mexico, desertic Prosopis microphylla von Humbold & Bonpland, 1825
of Michoacan hybridizes with temperate P. bonplanda Earl & Lux, 1991
(Publ. Biol. FCB/UANL Mex.) of Coahuila. P. velutina Wooton, 1898, a
microphyllan derivative, hybridizes with P. glandulosa variety torreyana
Benson, 1941 (Am. J. Bot.) and with P. g. var. glandulosa Benson, 1941 (
my unpublished field observations).  It follows that all North American
algorobian mesquites belong to 1 species.
	Turner (Evol., 1974) suggested that in animals and plants genes can
evolve at 2 rates: 1) those determining most morphological characters
evolve quickly in response to selection and random flucuations, and 2)
others are found unresponsive to most ecological changes as most found
in isozyme surveys. Then, the task can be finding the genes that control
foliar shapes and sizes in mesquites, partly because it is the visual
variability that causes the plant taxonomist  to make a most natural and
traditional mistake in interpreting species. You can start with
Sharrock& Quail (Genes Dev. 1989).
     	If you don't have any problems to solve, please try this one.
Best, Paul R. Earl, Facultad de Ciencias Biologicas,  UANL, Monterrey,
NL, Mexico

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