Of Mice and Sand - Evolution in Action

This is the year of the great Darwinfest and, inevitably, sand makes its
contribution. The immediate reference would be to the “sand walk” at Down House
in the Kent countryside, Darwin’s “thinking path” that he walked every day. A
visit to Down House (appropriately under consideration as a World Heritage site)
is, for many of us, an intellectually spiritual experience. Walk the sand walk,
and the sense of place is resonant. What is arguably the greatest idea ever was
formed here. It is the place where our understanding of ourselves and our place,
and the place of every living thing in the natural world, was born.

The process of natural selection, illuminated today so brilliantly and so
subtly by genetics, is a natural wonder, the place where the scenes of life’s
history, as revealed through geology, develop into the dramas of today’s
biosphere.  Sand, being the ubiquitous and hyperactive player in the games of
our planet’s past has, of course, always influenced evolution. Granular
materials, whether in the desert or the surf zone, are not the easiest places to
live, and the challenges have driven endless adaptations. But what I want to
focus on here is an example of evolution in “real time,” changes that are
happening at a pace that we can comprehend on ourtimescale.

The coastline of Florida as we see it today is young. Quite apart from the
fact that the coast today is not what it was yesterday, the major features,
however they are constantly changing, were sculpted a few thousand years ago as
sea levels rose after the last Ice Age. The barrier islands of the Gulf Coast
and the sweeping beach and dune systems of the Atlantic side are works in
progress. As are their populations of mice. Peromyscus is the most widespread
mammal in North America, a species that has adapted to and enjoys a wide range
of habitats, including the mainland of Florida, its Atlantic beaches, and its
recently formed barrier islands off the Gulf Coast. But the mice that live on
the light sand
beaches and among the dunes have distinctly lighter fur than
their relatives who inhabit the dark soils of the mainland, and  Hopi E.
Hoekstra, John L. Loeb Associate Professor of the Natural Sciences in Harvard’s
Faculty of Arts and Sciences, together with her colleagues at the University of
California, San Diego, and Leipzig in Germany, have demonstrated that this is
evolution at a dramatic pace. A paper in this month’s Molecular Biology and
Evolution (http://mbe.oxfordjournals.org/cgi/content/abstract/26/1/35?etoc)
reports the latest results from a project that has been going for some time. Not
only does their work reveal some of the complexity of genetic switches,
nucleotides, and amino acid mutations that choreograph evolutionary change, but
it importantly sheds new light on the thorny and much-debated topic of
convergent evolution, showing that, like so many things, it’s not as simple as
we might like to think. Now, when I say “we,” I am not really referring to
myself - as a geologist, the time it takes me to be out of my depth in the field
of molecular biology can be measured on the nanoscale, so I shall return to the
influence of sand.

Hoekstra’s project has studied beach mice in diverse habitats, among them
Santa Rosa Island, one of the longest barrier islands in North America, and the
site of a Civil War battle. The mice forage on the white sands and among the
dunes, eating mainly sea oats, and trying to avoid predators from the skies -
owls, herons, and hawks. And, nature being red in tooth and claw, mice with
lighter-coloured coats are less visible and more likely to survive. As natural
selection, it’s as simple as that (putting aside the complexity of the genetic
mechanisms that actually allow it to happen). Completely distinctive subspecies
have evolved and it’s all happened in just a few thousand years - as Hoekstra
says, “It’s a large effect mutation. And what it says is that adaptation does
not always occur gradually, but may happen in these relatively large jumps.” The
barrier island beach mice are genetically more similar to (but still different
from) their Atlantic Coast relatives hundreds of kilometers away than they are
to their nearby mainland cousins.

Unfortunately, avian predators are not the only threats that the beach mice
have to deal with. Our lunatic obsession with beach front property development
is destroying their habitats, and, barrier islands being where they are,
hurricane damage is frequent and extensive; Hurricane Ivan excavated a
completely new breach in Santa Rosa Island as seen in the image below - a great
example for fans of storm-generated overwash sand deposits.

The beach mice are disappearing rapidly - since Hoekstra’s project began, one
subspecies has become extinct and six of the seven remaining are endangered.

There are other examples of sand habitats actively driving evolutionary
change. The gloriously named bleached earless lizardlives among the
stark white gypsum dunes of the White Sands National Monument, again formed only
a few thousand years ago. The lizard is thus named because it isbleached - compared to its darker relatives beyond the dunes, it has lost
its colour as a protective adaptation (it has been shown that this adaptation is
not driven by temperature regulation). Spiders, scorpions, toads, and mammals
living in the dazzling dunes are all paler than their brethren beyond the sand.

[see Hopi Hoekstra’s site: http://www.oeb.harvard.edu/faculty/hoekstra/Links/ProjectsPage.html.
Beach mouse photo from

http://www.oeb.harvard.edu/faculty/hoekstra/Links/ProjectsPage.html,
credit J.B. Miller, Santa Rosa hurricane damage photo from

http://coastal.er.usgs.gov/hurricanes/ivan/photos/florida.html.
For more on evolution at White Sands, see http://www.nps.gov/whsa/naturescience/white-animals.htm;
lizard photo from that site.] SIGNATURE

Comments

0ut0f0rder (2010-08-18):

Evolution- Biology- Rate of Change
There are moments throughout time where evolution pushes forward and different species are capable of evolving at a faster rate than usual. However, this accelerated rate of change takes anywhere from 10,000 years to a million years to occur. With the acceleration of technology in the past 50 years it may become impossible for human evolution to keep up with the pace of technology. What do we do when evolution can’t keep up with the rate of change in technology that humans are now forced to confront?

Sandglass (2010-08-18):

A very good question! I am reminded of Albert Einstein’s observation that “The significant problems we face will not be solved by the same level of thinking that created them.” I will have to read your book…

Ricky White (2011-03-21):

Awesome info! I have been searching for something such as this for quite a while now. Bless you!

Originally published at: https://throughthesandglass.typepad.com/through_the_sandglass/2009/01/of-mice-and-sand-evolution-in-action.html