Evolution: It's Not Just A Single Theory, It's A Lot Of Theories

The Darwinian would say, "Gene dissemination: Successful competition in the hunt allows the fox to survive and breed, maximizing its individual representation of genes in the population. That's how natural selection works." The ecological biologist would say, "Sure, but first and foremost, the fox is eating the quail to stay alive. It's a cross-species energy transaction between two individual members of a tightly integrated ecosystem. As a result, the fox survives and manages to pass genes on to future generations. Natural selection is a side effect, fallout from the essential process of seeking energy to stay alive." A systematist, mainly concerned with phylogeny (evolutionary genealogy), would say the fox is eating the quail because that behavior represents an ancient evolutionary adaptation. A cladisticist (a more narrowly construed systematist) might beg the question by insisting that eating was irrelevant here, but the teeth revealed significant canid relationships. A developmental biologist would care most about how the two creatures developed into a fox and a quail in the first place. And the newcomer, the molecular biologist, would ask for blood samples from both the fox and the quail and say, "I'll run these over to the lab for sequencing and get back to you." "Evolution is not a monolithic field," says paleontologist Niles Eldredge, curator of the American Museum of Natural History (AMNH) in New York. "There is no department of evolution, evolutionary biology, nor even one single definition of evolution acceptable to all in the United States. Each field is conducted in its own right, but there are always people in each field who are into evolution." Eldredge first encountered the fox-and-quail scenario as a graduate student in 1966, while reading the recently issued Adaptation and Natural Selection (Princeton University Press, 1966). That work was written by George Williams, who drew the traditional Darwinian conclusion about the involvement of the fox's quail-eating behavior in natural selection. In his book, Williams was intent on defending Darwinian selection from recent criticism. But in 1972, paleontologist Stephen Jay Gould and Eldredge published a controversial chapter in Models in Paleobiology, entitled "Punctuated Equilibria, an Alternative to Phyletic Gradualism" (edited by T.J.M. Schopf; San Franciso, Freeman, Cooper & Co., pages 82-115), challenging the strict Darwinian model of "progress" in evolution--steady, gradual change that would be found in the fossil record if more of the past's life forms had been preserved. Looking at the trilobites and land snails that Eldredge and Gould, respectively, had studied during their thesis work, the paper found not steady change over time, but stasis interrupted by relatively brief flurries of evolution. This paper cast Gould and Eldredge as big-picture men, macroevolutionary in their views and devoted followers of the fossil record. Even so, it was Eldredge who initiated the drive to bring a $2 million molecular laboratory to AMNH. There are enormous differences in the scientific background and training of a paleontologist and a molecular biologist. Yet many molecular biologists now view the patterns of the chemical ordering of DNA and RNA as types of molecular fossils. The major scientific work of collections-based natural history museums like the Smithsonian Institution, the British Museum, or AMNH, since their very founding, has been systematics--tracing and documenting the history of life. These days, part of it is done in the molecular biology lab, mostly with fresh blood samples, but also using, at times, aged specimens of skin, hair, bone--even the rare fly preserved in amber when it's possible to recover relatively uncontaminated specimens. New species are discovered and described; relationships are discovered between known species. The pattern shows which animals are more similar. The director of the AMNH lab, Ward Wheeler, takes an extreme deconstructionist view of evolution. He believes that much comparative molecular lab work needs no reference to evolution, and he is awaiting the publication in Cladistics of his paper questioning whether the word "species" has any real validity. "Things commonly called different species do interbreed, as in the case of coyotes, wolves, and dogs," says Wheeler. He argues that good scientific theory requires testable premises, and the notion of species as separate, integral units has "almost never been tested." According to Jan Geliebter, a molecular biologist at Rockefeller University, the greatest contribution genetic studies have made to evolutionary theory is to show that "morphology is only skin-deep," while molecular methods are quantitative. Geliebter gives Motoo Kimura, a population geneticist at Japan's National Institute of Genetics, credit for this insight. "Science in its purest form is something you can measure. If you have three different genes, two with only two molecules different and the other with 40 different, you know which two are more closely related. The gene tells you what you're looking at in black and white," says Geliebter. In his "neutral evolution theory," Kimura found that many different allelic forms of genes are selectively neutral--varying forms apparently conferring no selective advantage. In a paper for a 1990 symposium in Kyoto on the evolution of life, Kimura wrote, "The great majority of evolutionary changes at the molecular level (such as DNA base substitutions and amino acid replacements) are the result of such neutral evolution, rather than the result of Darwinian adaptive evolution." To Geliebter, the major question in evolutionary biology is the origin of self-replication: "Catalytic activity in RNA is the most satisfying place to look for answers. This reverse transcriptase, like DNA, can act like a template, but unlike DNA, it can also act as a catalyst. This is one of the most promising areas in terms of research potential." Rudolf Raff, a developmental biologist at Indiana University, is interested in macroevolutionary questions, problems of how life became so diverse. Another application of molecular biology, with the latest discoveries in homeobox genes, is "one of the science stories in evolution that's important now," says Raff. Homeobox genes, long recognized as key "switches" in embryonic development, are important to evolutionary theory because they encode for the same 60-amino-acid sequence that endures through much of the history of life, from fruit flies to humans. It also appears that these genes may help to determine body plan. For the first time in developmental biology, "what" and "how" --pattern and process--in evolution have met at a new crossroads. "What's happening now is probably as important in its way as the joining of evolution with genetics was 50 years ago," says Raff. "Evolution has traditionally tied into population genetics and natural selection; not much of it was tied into development--the actual action of genes and the development of genes." George Williams, the evolutionary biologist who wrote the aforementioned 1966 book defending Darwinian evolution, predicts that while the past focus of evolutionary theory has been on the mechanisms of change, it is nonchange, or stasis, that will increasingly attract the attention of serious study from the molecular level on up. Homeobox genes, with their nonchanging DNA sequences, are just one example of how molecular biology will contribute to the understanding of stasis. "There's a big distance between fact and theory, especially with respect to stasis," says Williams. "If you go in the woods and study real organisms in the field, if you look for it, you see how rapidly evolution is going on. The shift in gene frequency has been documented over and over. "Most people think there wasn't enough time for all the change that takes place in the fossil record, but if you look at the arithmetic, there's too much time--too much stasis. Go out and look at the English sparrow. It experiences genetic changes on an order of 5 percent per century. At that rate, just during the Pleistocene, you could turn sparrow bones into ostrich bones about 200 times and back again." Gould says he looks forward both to a greater synthesis of molecular and developmental biology and beyond microevolution for higher- level answers. Because the microevolutionary picture involves looking at the generational churning of genetic information in local populations, it is difficult to match this information up with the classical approach of examining gross patterns of extinction--the comings and goings of species and entire ecosystems. "On the macroevolutionary level, we're looking at patterns. The evolution of the genome is so different from that," he says. Mick Wycoff is a freelance medical and science writer based in Ridgewood, N.J.

Evolution: It's Not Just A Single Theory, It's A Lot Of Theories

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