Cane Tucky

The process of evolution by natural selection is taken for granted among scientists to such an extent that when they find an excellent example, they don't necessarily publicize it like they should. A good place to look for such examples occurs with introduced species, of which there are many these days. The challenge presented to native species can be severe. If the natives are able to survive, they may well be able to do so only by changing themselves.

Stephen J. Gould was able to convince a lot of people that the process of change could be a lot faster than previously imagined. His idea, which he called Punctuated Equilibrium, suggested that the customary state of nature was dynamic maintenance of the status quo, particularly in widespread populations. When the environment changes, he suggested, most species have a remarkably flexible genetic reservoir to call upon and adapt very quickly. How quickly? Well it turns out that there is one carefully evaluated change, a rather dramatic change, that has taken place within 70 years.

In 1935, the toxic cane toad was introduced from Hawaii into Queensland, Australia, for the purpose of fighting sugar cane beetles. Unfortunately, as you are probably anticipating, the experiment got out of hand and the species became a major pest in itself. Populations of native predators plummeted wherever the toad showed up, apparently due to poisonings. Snakes, birds, dingoes, monitor lizards and crocodiles were all effected. Presumably, competing species were also impacted by the cane toad invasion.

A couple of Australian scientists decided to document the evolutionary impact of this event. Looking at simple physical measurements in two species of vulnerable snake predators, they were able to identify statistically significant changes over time in body size, which increased, and relative head-to-body size, which decreased. Although they hedged their language very carefully, the scientists concluded that these "gape-limited" snakes were changing in response to the threat. Small snakes that eat large toads were being eliminated selectively from the population. The population means were moving in reaction to that environmental force.

Guess what? Evolution happens! If you have doubts, you can look it up in Proceedings of the National Acadamy of Sciences, Phillips and Shine, 12/7/2004. This result could be confirmed by doing a snake census on matched plots of land, one of each pair within the growing range of the cane toad, and one outside of it.

In all likelihood, this study represents the very small tip of a very large iceberg. We can presume that similar impacts are taking place on every relevant dimension of the snakes' genomes. Certainly their ability to metabolize the toxin is improving. The relative size of liver to body mass is increasing (do snakes have livers?). Their sensory faculties are changing too, probably to make such toads less appealing. Even though we might measure just a few attributes, natural selection does not have neat fences around it. The entire organism is tested by this insult, and the entire population responds in any way it can. Every other creature around them responds as well. Other prey of these snakes, for example, may respond by growing larger, as the cane toads themselves are undoubtedly doing.

In fact, scientists have already determined that the cane toads at the frontier have indeed changed. They are getting faster. Distance serves as a filter to segregate these toads by reproduction rate, speed, directional preference, and who knows what else. If we were to introduce yet another species to eat the cane toads, I imagine that only the fastest-moving toads, able to elude the range of the new predators, would remain. We would have the cane toad equivalent of a greyhound, a new species perhaps, quite unlike the original in Hawaii.

Now, the reason I raised this subject was not to needlessly educate you, but to annoy you with an outrageous assertion. I believe there is an analogous process taking place among humans. In particular, I believe that those in the West who have descended from the Old World Europeans are different from the original stock. Like the toads, maybe they have longer legs, or perhaps nastier tempers. I'm suggesting that Daniel Boone walked to Kentucky and Missouri with changes on board. Distance and difficulty have served as persistent filters, eliminating some characteristics and encouraging others. Certainly, the native peoples have changed in response. If nothing else they have learned to resist the new diseases, to digest new foods and to tolerate alcohol. Some changes must exist, simply because the filter is there, and I expect that, whatever the changes are, they increase in a gradient as you move west, especially toward the interfaces between the imported and native cultures.

OK. Those changes may be temporary, diluted after the introduction of automobiles and travelling salesmen, and they may not be all that significant. But they could be. Evolution happens! Personality tests might give you different results in Glasgow and Laramie. I'm talking genes. We'll get to memes in a later post.

2/21/2006 4:57 PM