New Directions in Evolutionary Theory

views updated

New Directions in Evolutionary Theory

Overview

Darwinian evolution, that is, evolution through the accumulation of almost undetectable changes over millions of years, has been the mainstay of evolutionary theory since its formulation by Charles Darwin (1809-1882) and Alfred Wallace (1823-1913) in the mid-nineteenth century. In the last half of the twentieth century, this view of evolution was further developed to accommodate the realization that natural selection can operate in alternate ways, that evolution does not always operate with geologic slowness, and that the fittest do not always survive. These revisions to evolutionary theory have had a profound impact on how we view the natural world and our place in it, as they serve to emphasize that there is no evolutionary "reason" for us to exist. Instead, it is becoming more obvious that humans, like any other species, are the result of a number of factors, including random chance and contingency.

Background

Evolutionary theory was born in the Galapagos Islands and the Indonesian Archipelago, the intellectual child of Darwin and Wallace. Both men, working independently, reached the conclusion that species must be able to change, or evolve over time, giving rise to new species and that natural selection, or "survival of the fittest," was the primary factor in determining which traits became the basis for new species. Although Darwin and Wallace acknowledged that natural selection was not likely to be the sole motive force for evolution, and that evolution did not necessarily proceed in a gradual and smooth manner, they did express a preference for gradual, smooth transitions between species. This gradualist ("Darwinian") viewpoint held sway from 1859, the year that Darwin published his landmark work On the Origin of Species, until 1972, when Niles Eldredge and Stephen Jay Gould (1941- ) suggested that evolution consisted of long periods of evolutionary stasis punctuated with bursts of extremely rapid evolutionary change and speciation. This view of evolutionary theory was called "punctuated equilibrium" and it rapidly gained adherents.

At the same time, genetic research performed by Motoo Kimura (1924- ) suggested that not all genetic changes (mutations) resulted in a change in a creature's fitness to survive. Many mutations were neither good nor bad but were, instead, neutral in their effects. This gave scientists the equivalent of a clock, based on neutral mutations, that could be used to date the amount of genetic change accumulated in different organisms; a clock that could be used to determine when two lineages diverged in the geologic past.

In addition to Kimura's work, Gould and others pointed out that natural selection need not always choose the organisms best suited for survival. Modern species may, instead, simply be those that were lucky enough to survive catastrophic events. For example, an asteroid striking the Amazon would likely kill every organism living in the region of the impact. Not considering the climatic effects this event would have, Amazonian organisms would become extinct, not because they were less fit to survive, but because they happened to have evolved in an area that was struck by the asteroid. After the dust settled the region would be full of ecological niches with no native organisms to fill them. This would invite rapid speciation by the first organisms to arrive, and the fossil record would show the sudden appearance of a multitude of new species. By conventional, Darwinian logic, these new organisms would be found in the fossil record and would be assumed to have been better adapted. Using the tools of punctuated equilibrium and contingency, a researcher may now suggest otherwise. In reality, both mechanisms affect evolution, but contingency and punctuated equilibrium are proving to be far more significant than previously thought.

Finally, there is no reason to suppose that evolution leads inevitably to intelligence. In other words, humans are not the natural endpoint of evolution. Bacteria have dominated life since its first appearance and continue to do so today. Gould posits that the apparent "direction" of evolution toward increasingly complex organisms simply records the fact that evolution works randomly, towards both increasing simplicity and increasing complexity. However, there is a "wall," below which simpler life cannot exist. Therefore, random change will tend to be towards the direction of complexity simply because that is the only direction to go. Yet, he also points out that complex organisms tend to simplify with time as frequently as they become more complex.

To summarize, evolutionary theories have changed dramatically since the 1970s. While paleontologists and evolutionary biologists affirm the important role that natural selection plays in evolution, most also agree that other factors, including luck, have had equally important roles in determining the shape of modern life on earth. Put together into what is called the Modern Synthesis, these concepts have had an important impact on the way we view life and evolution in general, as well as our place in the world.

Impact

From a strictly scientific standpoint, viewing evolution in terms of punctuated equilibrium has opened the field up tremendously. In many cases, scientists struggled to explain some changes in species in terms of perceived adaptive benefit, often constructing hypotheses that seemed far-fetched at best. Understanding that factors other than strict Darwinian fitness can govern evolutionary selection has coincided with the realization that strict uniformitarianism is also an oversimplification. Uniformitarianism states that evolutionary forces have worked with the same intensity and the same frequency since life emerged. Punctuated equilibrium reflects the fact that geologic change takes the form of episodic catastrophe as well as slow, steady accumulation of minuscule effects. In both cases, there is a uniformity of physical laws and processes, some processes just happen to occur randomly and lead to very rapid change as opposed to the majority of processes that operate almost invisibly over millions of years. In this way, these two fields of study have tended to reinforce each other, each leading to a fuller and more dynamic view of the earth's history.

The effects of these advances in evolutionary theory have largely been confined to the realm of science and scientists. However, this contingent and dynamic view of life has inescapable philosophical implications. One of the most significant social impacts of this revised thinking involves the continuing debate between those who believe in the evolution of species and those who believe in the literal truth of the Bible. Many biblical literalists (often referred to as religious fundamentalists) view the new scientific debate surrounding evolution, as an indication that the theory is discredited and falling into disfavor among scientists. Drawing some strength from this argument, biblical literalists have managed to reopen arguments concerning the teaching of evolution in American public schools, an issue many presumed was settled once and for all with the famous Scopes trial in 1925. It should be noted, incidentally, that many major religions accept that evolution occurs. Pope John Paul II conceded in a 1996 papal encyclical that "the tehory of evolution [is] more than hypothesis."

Many biblical literalists state that evolution's status as a "theory" also undermines its scientific legitimacy. This, however, is based on a fallacious interpretation of the term "theory." In science, "theory" refers to an idea that is firmly accepted as true by the scientific community, even if some details still remain to be worked out. Thus, we have plate tectonic theory, genetic theory, gravitational theory, and evolutionary theory, to name a few. Science does not doubt that these occur—tectonic plates move, genes exist and transmit information between generations, mass attracts other mass, and species evolve.

Finally, scientific revisions of evolutionary theory have challenged some philosophical views regarding man's place in the universe. For millennia men and women have found solace and strength in the view that humans enjoy a special, privileged relationship with God. Darwin and Wallace challenged this belief, relegating mankind to a place above the ape, but an animal nonetheless, counting beasts in our lineage. Yet, even in this view it was still possible to find consolation in the assumption that evolution's progress led inexorably to humans as the most complex, intelligent, and best-adapted animal yet to appear on Earth. Whether guided by divine intervention or by the laws of the universe, life progressed to form our own "advanced" species.

According to Mark Twain, this was similar to saying that, by analogy, the whole of the Eiffel Tower existed for the purpose of the coat of paint at the very top, and current thinking would not disagree. Instead, we find ourselves lucky to be here, for there was no way to predict that intelligent life would come to pass on Earth at all, let alone life that looked like us. In a sense, we have lost the ability to think of ourselves as the logical outcome of billions of years of evolution, just as we have lost the ability to reason that the laws of nature had to produce a thinking, bipedal creature. From the standpoint of evolution, we are not special. However, this view has proven liberating, too, for accepting that we are the product of chance, contingency, and random process also means that we can revel in our good fortune to be here at all as a species. Also, if we are not created for any specific purpose, neither are we destined to fill any purpose other than what we set for ourselves. Thus, as a species, we have the opportunity and the responsibility to use our existence wisely. These philosophical and ethical implications, in addition to the obvious scientific significance, are among the most profound consequences of the new thinking about evolution.

P. ANDREW KARAM

SCIENCE WARS

Starting in the 1960s there developed an increasing tension between scientists and those who study the philosophy and sociology of science. In 1962 Thomas Kuhn set forth the argument that scientific knowledge—far from being objective—reflected the cultures that produced it. Postmodernists and others involved in cultural studies subsequently began to reassert airy and discarded philosophical viewpoints that properties associated with the physical world were in fluenced by human psychology or cultural perspectives. Scientists maintained that science was not simply another form of cultural criticism and that it remained the most accurate and productive way to understand a real and knowable world. The debate was not new to scientists. During the Scientific Revolution Bishop Berkeley, a contemporary clerical critic of Sir Isaac Newton, once claimed that gravity and matter were "imaginary" constructions of the human mind. Legend has it that Berkeley's assertion was promptly refuted by one Royal Society scientist's stubbing of his toe upon a rock as he pronounced, "I refute it thus!"

The modern debate also often proved so heated and acrimonious that many have dubbed it the "Science Wars." In 1996 Alan Sokal, a respected physicist at New York University, brought the issue to the attention of the general public with his celebrated hoax of the cultural studies journal, Social Text. Sokal deliberately slipped past the editors of an issue devoted to anti-science viewpoints a scholarly sounding but obtuse article titled, "Transgressing the Boundaries: Toward a Transformative Hermeneutics of Quantum Gravity" that repeatedly offered gibberish prose to support sweeping philosophical, cultural, and political conclusions based on bizarre representations of mathematical and physical theories. Sokal perpetrated the hoax to reveal what he maintained was a lack of scholarly rigor on the part of those who viewed science as a cultural expression. The hoax went unnoticed by the editors and reviewers of Social Text until Sokal revealed his deception in the journal Lingua Franca.

Science and Its Times: Understanding the Social Significance of Scientific Discovery