Adapted from Jim Conrad's Naturalist Newsletter of August 10, 2007
issued from Sierra Gorda Biosphere Reserve, QUERÉTARO, MÉXICO
CHAPULTEPEC PARK'S PALE & MOTTLED SQUIRRELS
1600-acre large (647 ha) Chapultepec Park is twice the size of New York's Central Park. It occupies land on which the Aztecs lived as early as 1200 AD. Spanish King Carlos V declared the area a nature zone in 1537. Wikepedia's illustrated page about the park is at http://en.wikipedia.org/wiki/Chapultepec.
Since the meeting I needed to attend lasted only a couple of hours I had most of last Saturday free until my bus back to Jalpan left at 9:30 PM. I spent much of the day wandering all through Chapultepec Park.
Squirrels there are as unafraid of humans and eager for a handout as any I've ever seen. Several times I saw kids freak out when they got to see a squirrel really close up. They're the Red-bellied Squirrels, SCIURUS AUREOGASTER, I told you about in the uplands here, in the July 20th Newsletter.
Instead of being gray squirrels with well defined, brightly rusty underparts like ours, Chapultepec Park's squirrels come in many variations, ranging from the standard design to nearly all-white individuals, and many have blotchy patterns, often with rusty spots above as well as below.
This color variation in a wild animal is worth thinking about.
Darwin pointed out that wild animals tend to look alike but domesticated ones often show variations in color and pattern. For example, wild Rock Doves, the ancestors of city pigeons, usually look alike, but the plumage of city pigeons, considered to be the same species as wild Rock Doves, is very diverse.
The usual explanation for this is that in nature there's such a struggle for survival that evolution has equipped each species with an optimum behavior and appearance for survival. Any deviation from that optimum puts an individual at greater risk than the general population, so genetic information allowing the deviation soon disappears -- when the carriers of the information die.
But when an organism is domesticated, many pressures are removed -- those from predation, for example. Once the pressures are removed, then there's less "culling" of those individuals looking and behaving in nonstandard ways. A wild Rock Dove with a white feather in a wing is more visible to predators than his flock mates but a white feather in a domesticated pigeon means nothing, so the genetic information permitting white feathers and maybe other features even more atypical get passed on.
So, since 1537 when Chapultepec Park's squirrels found themselves in a protected area, genes of the park's squirrel population have had many selective pressures removed, their genes have been at greater liberty to experiment, and today's pale and blotchy squirrels have resulted.
In human populations the pressures encouraging genetic conformity also have been removed. In fact, because of medical advances, you could say that now human evolution has changed direction. People who in the past would have died young with genetically based diseases now pass along their predispositions to future generations. What does this mean for the future of human evolution, and what, if anything, should be done about it?
A controversial and thought-provoking webpage on this very subject, suggesting that "The rate of human descent is perhaps thousands of times faster than the rate of its ascent" can be visited at http://www.onelife.com/evolve/degen.html.