science, health & technology

Adaptation and the Origin of the Species
by Eric Jackson

As we all know, the theory of evolution is that among the many mutations that randomly arise among a given species, most are inconsequential, some are harmful to those afflicted with them, and then there are those that give those who get them an advantage in adapting to a given environment. Those beneficial mutations tend to win nature's competition, the fittest survive, and those mutations express themselves in new species. Right?

Actually, a key component of that popular shorthand understanding of Darwin's famous theory is subject to question.

On February 12 Dr. Dolph Schluter, a University of British Columbia evolutionary biologist, spoke to an unusually large crowd at the Smithsonian Tropical Research Institute's Tupper Auditorium. Darwin's On the Origin of the Species "changed the way we think about everything," Schluter began, but not as much about adaptation as about selection.

Adaptation, he explained, is "a change resulting from natural selection that makes the species more suited" to its environment. But speciation, which might just be "the crowning achievement of the Big Bang," is something else. "It marks boundaries on several levels," on micro- and macro- evolutionary changes (the latter of which the creationists reject, but not necessarily the former --- there are folks who wouldn't want their fundamentalist religion to get in the way of their ideal of a master race!) as well as in the the field of taxonomy, which is undergoing a revolution thanks to the applications of DNA science.

In the "early period," from Darwin's time in the mid-19th century for nearly 100 years, speciation was seen simply as trait divergence, with great importance given to the amount of difference that could be observed with the tools and methods then available.

That idea began to break down with the 1940 publication of a paper on the adaptation of sea lampreys that had made their way not only into fresh water systems, but into habitats where there was relatively little prey to which they could attach and thus they dropped their parasitic ways. From that it was suggested that environment controls evolution. In this "middle period," ideas developed in Dr. Theodosius Dobzhansky's 1937 book Genetics and the Origin of the Species, which saw speciation mainly in terms of the evolution of reproductive isolation. According to Dobzhansky's view, speciation is when the hybridization of evolved forms becomes physically problematic or when half-breeds turn out to be sterile.

More recently, however, there is a genetic approach, which through DNA analysis finds the genes responsible for reproductive isolation. There is also recent work in which the old importance of the adaptability of phenotypic traits is maintained, but the test of speciation is seen in a larger context of evolution and reproductive isolation.

Schluter's work that brought out such a large crowd is about these little fish of little or no apparent food or commercial value, the sticklebacks. A number of stickleback species live in isolated North American lakes that are about 12,000 years old, dating back to the melt at the end of the last ice age. There are also Atlantic and Pacific marine sticklebacks, and species that live in rivers and streams that feed those oceans.

It has been found that there have been repeated unrelated evolutions of freshwater stickleback species from marine sticklebacks. Different freshwater species that have evolved separately from the marine varieties all tend to become smaller than their oceanic ancestors, and to similarly lose plates of body armor that their seagoing cousins have.

So what would a theory that rests on random mutations have to say about this?

And then, when one gets into experimentation, it can be shown that female freshwater sticklebacks will mate and reproduce with freshwater males of different species rather than more closely related marine males, and females of marine species similarly prefer males from similar environments and with similar morphology over those freshwater varieties that are more closely related. "Body size counts more than genetic closeness," Schluter noted.

So what's behind this strange phenomenon of parallel evolution that Schluter called an example of "ecologic speciation?"

He looked to DNA analysis for the answer to this.

It turns out that the alleles (genes responsible for hereditary variations) linked to the loss of size and body armor are ancient.

"Parallel phenotypic evolution involves the same genes over and over again," Schulter explained. It's not a matter of a random mutation of marine species, but of the expression of genetic traits that have long existed among sticklebacks in general, that have become recessive among those in oceanic environments but which soon assert themselves when these species colonize freshwater environments.