Theory vs. Hypothesis in Science

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Commentator Ruth Levy Guyer explains the difference between theory and hypothesis in science. And she muses on why the theory of evolution... and Darwin himself... have engendered so much resistance over the ages.

JENNIFER LUDDEN, host:

Commentator Ruth Levy Guyer has also been thinking about Darwin.

RUTH LEVY GUYER:

Ten thousand years ago, a receding glacier carved out the lake in Maine where, last summer, I immersed myself in the extraordinary ideas of Charles Darwin. The fields surrounding the lake are studded with huge boulders. Majestic piles of glacial moraine line the nearby coast. The first time I saw these remnants of the ice age, I finally understood how slow-moving, melting glaciers could sculpt, shape, create and destroy Earth's land forms.

This place, so illustrative of the Earth's evolution, seemed a fitting setting for pondering the evolution of organisms, Darwin's subject. Darwin was a keen observer, an attentive reader, a master synthesizer and a great visionary. His theory of evolution by natural selection explains how, through chance mutations, competition, struggle and physical isolation, species develop and die. Darwin wrote in his autobiography, `I have heard it said that the success of a work abroad is the best test of its enduring value. Judged by this standard, my name ought to last for a few years.' Good grief. It's a century and a half since Darwin published his theory, and if any name and ideas are pervasive and influential in our thinking and culture, they are his. What's surprising, though, is that the 19th-century objections to Darwin's theory are also still with us.

Darwin seems to me the luckless messenger, blamed for delivering the unpopular news. What he observed unnerves people. Nature is raw, survival is precarious; the dinosaurs' fate could be ours. He gets lambasted for reporting what's clear in the fossil record: that all species are not contemporaneous. And while Darwin's legacy is a theory, not a hypothesis, some do not understand that the two are not equivalent in science the way they are in common parlance. A theory explains a phenomenon, accounts for all available data, is supported by a huge body of evidence. Hypotheses are just guesses that need testing. Theories have heft, validity, gravity. Poor Darwin. He's also taken heat for discussing the origin of life, but he never did that. His mystery of mysteries was the origin of species.

Now the French chef, Julia Child, once weighed in on the origin of life, yet no one got mad at her. A video shown for years at the Smithsonian Institution in Washington featured Julia Child preparing `the primordial soup.' She re-enacted a classic scientific experiment, first carried out in the 1950s by scientists testing a hypothesis. Rudimentary building blocks of life could form spontaneously in the prebiotic world. Julia Child stood over her hot stove, tossed chloride, sulfates and `a scientific pinch' of potassium bromide into a roiling pot of water. She passed the soup's steam through a gaseous atmosphere and `Voila!' Out came some fundamental organic compounds.

My theory and hypothesis is that Darwin's theory has remained a flash point precisely because it's not about soup, gravity, relativity, ice ages or the cosmos, but about us. We humans have justly been accused of speciesism. Our bias against other species, our preference for our own, is everywhere, starting with the thing that most clearly separates us from them--language. We coin phrases like `She's wise as an owl,' `He's happy as a clam,' `I'll be a monkey's uncle.' We accept these expressions freely, but only metaphorically.

LUDDEN: Ruth Levy Guyer has evolved from immunologist to bioethicist. She teaches at Johns Hopkins University and Haverford College.

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