requires that you pay very close attention to
details when rappelling down into the pit.
Although 20 or even 80 miles might
not mean much to a bird, or even to a
fish in open waters, for cave animals the
distances between the caves are enormous.
Caves that are five or more miles apart are
effectively isolated from one another. The
broad geographic range of this species is
highly unusual for a cave animal.
Adaptation to Darkness
Animals that live in total darkness don’t
need eyes, and they save the resources
needed to maintain eyes if evolution can
suppress their development. Similarly, since
no other animal can see them, pigmentation
is also unnecessary. Both tend to be lost
over the span of many tens of thousands of
years because of evolutionary changes in a
cave population. Typically the loss of eyes
is not complete, and Mexican cave tetras
retain small eye rudiments buried under
skin, scales, and other tissues.
As a fish embryo grows, its eyes start
to develop from brain tissue in the first
day under the influence of many different
genes that perform important functions.
In a normal surface fish, the eye continues
to grow as the embryo develops. Not so in
the cave fish. For the first day or so, its eye
development appears normal, but then its
growth slows down and it never proceeds
beyond the rudimentary stage.
The large geographic separation of the
populations suggests that they evolved into
cave forms independently of each other—
the cave fish of Molino Cave resulted from
a separate invasion of surface fish into the
cave than the invasions that gave rise to
Chica, Tinaja, or Pachón cave fish.
If all the cave fish populations resulted
from a single invasion, we would expect
them to all be more closely related to one
another than to any surface fish, but they are
not. Some are more closely related to surface
fish now present in the area or to other
surface fish found as far away as Belize, than
they are to other cave fish. In spite of their
not being closely related, fish from different
caves pretty much look like one another.
Expedition members assess the drop into Caballo Moro Cave, where they will collect blind
Thus natural selection removes it and its
mutation from the population.
Natural selection is normally the brake
on the spread of harmful mutations, but
mutations that affect eye development are
not removed by natural selection in cave
populations because the cave is dark and
there is no penalty for having smaller
or non-functioning eyes. In fact, a blind
individual has a selective advantage in
not wasting resources on eye development
and maintenance. It’s a similar story
for pigmentation, which has important
functions in a lighted environment but
doesn’t in total darkness.
How It Happens
There are many genes that help form
the eye, and all of them are essential. For
example, some genes are responsible for
making the proteins that comprise the lens
and cornea. Both must be transparent so
the image of what the individual is looking
at can reach the retina in the back of the
eye. If a mutation changes the function of
any of these genes significantly, the lens or
cornea might not be transparent, and the
animal would not be able to see.
Because the blind cave fish populations
are distributed over a broad geographic
range, there is almost no chance for fish
from one cave to mate with fish from
another population. Thus, a new mutation
in one cave population can never spread
directly to another distant cave population.
As an example of the effect of this isolation
on the evolution of the cave fish, let
us consider the evolution of the albino
condition in three well-separated cave
populations: Molino, Pachón, and Japones.
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