If all three populations had the exact
same mutation, we might be justified in
concluding that the mutation occurred
only once in one of the populations and
spread to the others through migration and
interbreeding. However, the observation that
each population has a different mutation
for albinism demonstrates that it evolved
independently in these three populations,
which makes sense given the distance
The expedition team seins for surface tetras in a river near the caves.
In all three of these populations, the cave
fish cannot make the pigment melanin,
which is the black pigment normally found
in some of their skin cells and in parts of
the eyes. Further investigation showed that
the inability to produce melanin is caused
by mutations in the exact same gene in each
of the populations, but that the mutations
are different in each population.
In one population a portion of the gene
is missing, so it can’t function properly
because some of the required genetic
information just isn’t there. In the second
population a different portion of the gene
is missing, giving the same result—loss of
function. In the third population, the full
gene sequence is there, but the switch that
turns the gene on seems to be inoperative.
Horst Wilkens, a German scientist
whose work has taught us much of what
we know about blind cave tetras, started
experimenting with making hybrids between
individuals from different cave populations.
He observed that in some of the crosses, the
hybrids had larger vestigial eyes and more
pigmentation than their parents. He also
observed that this effect was greater when
the caves were far apart from one another.
We extended Wilkens’ work by making the
first collection of Molino fish for scientific
studies. The importance of this population
is that it is the most isolated from any of
the originally discovered populations and
shows the greatest effect when mated with
other cave fish. The reason it took so long
to be introduced to scientific analysis is the
220-foot-deep pit that has to be negotiated to
get to this treasure.
The loss of sight and coloration is not a disadvantage for cave-dwelling individuals, as their environments are completely devoid of light.