# Evolution in Action: White Animals at White Sands

# A Summary of the Rosenblum & Kain Report December 2004

At the end of the last ice age, the area which is now White Sands was
covered by a large lake. As the climate changed and the lake dried,
gypsum which had eroded into the Tularosa Basin created the white sand
dunes that we see today. The gypsum dunes of White Sands are home to a
distinctive community of animals, many of which have a dramatic white
coloration. We find insects, spiders, scorpions, toads, lizards, and
mammals with lighter coloration than their neighbors on the surrounding
dark desert soils. The gypsum dunes are geologically young, so we know
that animals with light coloration have adapted to the white environment
recently. White Sands is therefore a striking example of rapid
*convergent evolution*.

The process of living things changing over time is called *evolution*,
and White Sands National Monument is an ideal place to learn about how
living things evolve in response to new conditions. White Sands animals
evolved rapidly in response to the gypsum environment, and this gives
biologists a window into the processes of *adaptation* and *speciation*.
Scientists can use species at White Sands to understand how organisms
become better suited to their environment (*adaptation*), and ultimately
how new types of organisms evolve (*speciation*). White sands is a
dramatic showcase for evolution in action, and evolutionary biologists
have studied animals at White Sands for over 50 years. What have we
learned about white species at White Sands?

*Natural selection* is the primary force that drives local adaptation.
Imagine a population of dark colored lizards colonizing the Tularosa
Basin and finding themselves in light gypsum habitat. In this lizard
population there may have been some *variation* in color, some
individuals were slightly darker and some slightly lighter. Suppose that
the lighter individuals survived better in this new environment and had
more offspring than the darker lizards. If the same thing happened in
subsequent generations, over time, the entire population could
eventually become lighter and lighter.

Natural selection can only lead to adaptation if variation is
*inherited*. The scenario above assumes that color variation is passed
down from parents to their offspring. However, it is possible that light
coloration could be influenced by the environment instead (this is
called *phenotypic plasticity*). For example, color could depend on an
animal's temperature or on pigments acquired from an animal's food.
Research has shown that for most animals at White Sands, light
coloration is inherited and not acquired from the environment. For one
lizard species, the little striped whiptail, scientists have even
discovered the particular gene that is responsible for light coloration.

But why would a light lizard survive better than a dark lizard in an
environment of white sand? There are two main hypotheses to explain
natural selection on animal coloration at White Sands. Light animals may
be well camouflaged on the sand and therefore avoid predators better
than dark animals. Or light animals may be able to stay cooler on the
exposed sand dunes by reflecting more of the intense desert sun. These
two hypotheses could both be important at the same time. However,
research at White Sands suggests that predation, not temperature
regulation, is the driving factor in the evolution of light animals at
White Sands.

The number of species with light coloration at White Sands is
impressive. However, there are also some animals that live at White
Sands which do not have light forms. Why would some species evolve light
forms while others remain dark? One possibility is something scientists
call *constraint* - perhaps the genetic variation for light coloration
just did not exist in these species. Another option is that different
species experience *different selection pressures*. For example, it may
be less important for an animal which spends a lot of time hidden under
vegetation or which has other defenses against predation to be light in
color. There could also be conflicting reasons to be dark or light and
the benefits of being dark \"won out\". For example, for an ectothermic
("cold-blooded") animal, a lighter individual might be better
camouflaged but a darker individual might be able to warm up faster and
collect more food. A final alternative is termed *gene flow* - species
that have long distance dispersal may not be able to adapt to local
conditions since they are always in contact (exchanging genes) with
surrounding dark populations.

White Sands is an exciting place to study evolution because the
formation is geologically young and many of the white forms are not yet
distinct species. But are they \"species in the making\"? Can we observe
speciation in action? More research is needed on this question, but
*mate-choice* experiments have been conducted with the Bleached Earless
Lizard. Scientists discovered that light lizards actually prefer to mate
with other light lizards instead of dark lizards. This kind of
mate-choice may be an early step toward speciation.

At this time, none of the White Sands animals are considered new
species. However, a number of subspecies are restricted to the dune
field and therefore could be considered *endemic*. The number of endemic
subspecies or species depends on how species and subspecies are defined,
an area of great debate in evolutionary biology. Most of the taxonomic
work on dune-field animals was done many years ago, and application of
modern taxonomic techniques could help to clarify the status of endemic
forms at White Sands.

Research on the unique forms at White Sands can also help us learn how
to best conserve our biological resources. Scientists have recently been
finding that *ecotones* (areas like White Sands where there is a
dramatic change in the environment) may be particularly important for
the evolution of biological diversity. Conserving ecotones may allow us
to protect species that already exist while allowing new species to
evolve. Ultimately, White Sands can be both a classroom and a natural
laboratory for the study of evolution in action.