Paradigms are like glue

Bateman; John Innes website

Uh oh.  An iconic study of sexual selection bites the dust. Well, with a bit of help. Researchers at UCLA attempted to replicate a 1948 study by English geneticist Angus John Bateman, but without success.  The original study, said to be second only to Darwin's Origin of Species in importance to the study of sexual selection, suggested an evolutionary advantage for males who are promiscuous and females who are choosy in selecting mates.  Indeed, one of the foundational paradigms of the field is named after Bateman.  "Bateman's principle" is (quoting Wikipedia) "the theory that females almost always invest more energy in producing offspring than males invest, and therefore in most species females are a limiting resource over which the other sex will compete." As the work was the basis of much subsequent research in the field, the failure to replicate is big.

A June 26 story in ScienceDaily cites the lead author of the new study, published June 11 in the online edition of PNAS:

"Bateman's 1948 study is the most-cited experimental paper in sexual selection today because of its conclusions about how the number of mates influences fitness in males and females," said Patricia Adair Gowaty, a distinguished professor of ecology and evolutionary biology at UCLA. "Yet despite its important status, the experiment has never been repeated with the methods that Bateman himself originally used, until now.

"Our team repeated Bateman's experiment and found that what some accepted as bedrock may actually be quicksand. It is possible that Bateman's paper should never have been published."

In the original experiment, Bateman isolated selected fruit flies in jars, 5 males, 5 females, or 3 of each, and allowed them to mate freely.  He then determined the number of offspring of each mating, which he was able to do because he chose adults with distinct mutations, such as narrow eyes or curly wings or thick bristles, that appeared in the offspring and were markers for parentage.  From Mendelian expectations, and assuming that the mutations did not affect survival, one-quarter of the offspring should be double-mutants, carrying a mutation from each parent, half would be single mutants, and one-quarter would be mutation-free. But the proportions of offspring in each class found by the replication done by Gowaty et al. "departed strongly from Mendelian expectations." 

Fly with curly wing mutation

Gowaty says that while this identification technique was clever, it turns out it also was responsible for the study's fatal flaw.  For his identification purposes, the offspring that Bateman counted had to be double-mutants but these flies turn out to be less viable.  Thus, Bateman "overestimated subjects with zero mates, underestimated subjects with one or more mates, and produced systematically biased estimates of offspring number by sex."  Indeed, he assigned more offspring to fathers than mothers, which is why he concluded that male fruit flies produce many more viable offspring when they have multiple mates but that females produce the same number of children that survive to adulthood whether with one mate or many.

In their repetition -- and possibly in Bateman's original study -- the data failed to match a fundamental assumption of genetic parentage assignments. Specifically, the markers used to identify individual subjects were influencing the parameters being measured (the number of mates and the number of offspring). When offspring die from inherited marker mutations, the results become biased, indicating that the method is unable to reliably address the relationship between the number of mates and the number of offspring, said Gowaty. Nonetheless, Bateman's figures are featured in numerous biology textbooks, and the paper has been cited in nearly 2,000 other scientific studies.

This is not the first time Bateman's study has been questioned -- in fact so many species violate the principle that many evolutionary biologists have suggested that it shouldn't in fact be considered valid.  But, as Gowaty points out, "Bateman's results were believed so wholeheartedly that the paper characterized what is and isn't worth investigating in the biology of female behavior."  And, she says, "Paradigms are like glue, they constrain what you can see.  It's like being stuck in sludge -- it's hard to lift your foot out and take a step in a new direction."

The term 'paradigm shift' was first proposed by Thomas Kuhn in his influential The Structure of Scientific Revolutions, first published in 1962.  He argued that most of the time we are doing 'normal science', designing studies and interpreting results in terms of an accepted theoretical framework.  We cling to that theory the way a religious zealot clings to a sacred text, resisting any breech of the accepted truth.  Difficult results are explained or rationalized away, until a truly better idea comes along.

In the paper, Gowaty et al. conclude

The paradigmatic power of the world-view captured in Bateman’s conclusions and the phrase “Bateman’ Principles” may dazzle readers, obscuring from view methodological weaknesses and reasonable alternative hypotheses...

Here we don't see a new theory but just a refutation of an iconic proof of the existing theory, but the clinging drive is the same. This is of course true in any field, but it may be even more true of beliefs we (including scientists, who we idealize as being objective) hold about ourselves -- our inherent nature and our behavior, and their evolutionary origins and value.  What is, or what we choose to see, must be good or 'natural' because it evolved.  It is why so many stories, including fossil accounts (see Holly's award-winnning post), lead to resistance or even resentment when they are questioned by those the majority typically denigrate as nay-sayers.

We as scientists like to believe we interpret our data free from ideology.  That is rarely if ever true.