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Bateman's Sexual Selection: Another Darwinian Pillar Falls


It was a classic experiment supporting Darwin's theory of sexual selection. It generated a catchy phrase -- Bateman's Principle -- and launched a paradigm. But this year, a replication of Bateman's original 1948 experiment showed his methods were so biased and flawed that none of his conclusions are valid.

Angus John Bateman, working on fruit flies in 1948, published his findings which became known as Bateman's Principle: the idea that males tend to be promiscuous (because sperm is cheap) while females tend to be choosy (because eggs are expensive). This principle, so impressive with its math, jargon, and presumed application of the scientific method, seemed to support Darwin's theory of sexual selection.

It took on a life of its own, especially after R. L. Trivers in 1972 and S. J. Arnold in 1994 brought attention to it. According to Gowaty, Kim and Anderson, who decided to test it, citations soared and "Bateman's Principle" took on paradigmatic status. They note that "legions of graduate students" have read the paper since it was published.

The Wikipedia entry on Bateman's Principle acknowledges that many biologists have found exceptions to it; some even doubt its status as a scientific principle. But now, for the first time, Gowaty et al. have replicated the experiment using Bateman's own methods -- but with the added advantage of 64 years of advances in genetics.

In short, they found that the experiment was useless. Bateman failed to take into account biases inherent in his methods, failed to measure factors that discounted his conclusions, and left a mess of data that is perfectly hopeless for making predictions about fitness due to sexual selection.

Here we show that inviability of double-mutant offspring biased inferences of mate number and number of offspring on which rest inferences of sex differences in fitness variances. Bateman's method overestimated subjects with zero mates, underestimated subjects with one or more mates, and produced systematically biased estimates of offspring number by sex. Bateman's methodology mismeasured fitness variances that are the key variables of sexual selection. [emphasis added]
Their open-access paper, "No evidence of sexual selection in a repetition of Bateman's classic study of Drosophila melanogaster," is available at PNAS for those who want the details.

Of overarching concern is how a 64-year-old classic that influenced "legions of graduate students" (and uncounted legions of undergrads, too) was fatally flawed yet went unchallenged all those years. Thomas Kuhn was right: paradigms govern a scientific program until anomalies accumulate that require a new paradigm.

In this case, though, Gowaty et al. offered no new paradigm. They did not specifically falsify the 1948 paper; as charitably as they could, they suggested ways to test Bateman's principle (or "Bateman's hypothesis") with better-designed experiments. Even, so, they recognized, it would require snooping on the little fruit flies' sexual activity, not just measuring viable hatchlings, because "absence of offspring is not necessarily absence of mating." Their last paragraph was more far-reaching and less charitable:

We are left wondering why earlier readers failed to spot the inferential problems with Bateman's original study. The main implication we take from the present study is one earlier critics made: The paradigmatic power of the world-view captured in Bateman's conclusions and the phrase "Bateman's Principles" may dazzle readers, obscuring from view methodological weaknesses and reasonable alternative hypotheses....

Many ID supporters have undoubtedly felt the same incredulity at how so many scientists fail to spot the inferential problems in Darwinism. They have shared the same bewilderment at the paradigmatic power of world-view behind other Darwinian notions and catch-phrases that, while dazzling to readers, obscure their view of alternative hypotheses like intelligent design. The more Darwin pillars that crumble, though, the more the view may improve.

Photo credit: wallygrom/Flickr.