from female preferences have favored
sword evolution and variation among
species, other factors, such as predation,
also play a part. For example, swordtail
predator Astyanax mexicanus, the Mexican
tetra, also exhibits preferences for swords.
That is to say that a variety of behavioral
and environmental forces give rise to the
variation observed in this and other traits
in wild swordtails.
Some of the most interesting and extreme
variation observed in nature, including in
swordtails, occurs when two species cross
and create hybrids—much like the crossing
of species in hobbyist aquariums. The
causes and consequences of hybridization
in the wild are a central focus of researchers.
Hybridization between the lowland X.
birchmanni and the highland swordtail
(X. malinche) is a source of abundant
morphological variation in streams of the
Rio Panuco drainage along the eastern slope
of the Sierra Madre Oriental of Mexico.
As these morphologically distinct species
cross, their offspring inherit genes from
both species and exhibit new combinations
of morphological traits (e.g., sword length,
vertical barring, caudal patterns, and dorsal
fin size). Both genetic and morphological
variation are the basis for studying a wide
variety of evolutionary mechanisms that
generate and maintain such variation and
are a continual source of excitement as
new and different-looking fish are found
during each trip to the field because each
subsequent breeding season in the wild
produces new combinations of genes that
create new phenotypes not previously seen.
Apantla is a population with abundant
variation in male morphology (Figure
1). The mature males pictured here were
collected in 2008 from a tributary to the
Rio Claro, all from the same locality near
Apantla, Hidalgo. It is a small stream with
clear, shallow water around 350 meters
( 1,150 feet) in elevation. X. malinche are
found at relatively lower elevations in the
Rio Claro, with pure populations at least
as low as 650 meters ( 2,130 feet). The
fish in the photo come from a population
composed of both pure X. malinche and
hybrid fish. The variation that exists is
apparent in that some of these adult males
lack swords and vary in body size and
vertical bar patterns. However, as the
genetic background of these fish is more
like X. malinche, the overall morphology
of these fish is more similar to X. malinche
as evidenced by the frequent caudal blotch
pattern, slightly smaller dorsal fins, and
streamlined bodies lacking the nuchal
hump observed in X. birchmanni.
The Rio Calnali.
Exhibiting even more extreme variation
are males from Acuapa, Hidalgo (Figure 2).
The mature males pictured here were all
collected from a single locality on the Rio
Huazalingo in 2008. This locality is more of
an intermediate stream with deeper pools
at an elevation of 500 meters ( 1,640 feet).
The variation observed here among just six
males collected at the same time is more
extreme than above.