Nearly a decade ago, Darwin's thinking on sexual selection was considered by numerous scientists to be overdue for revision. At the 169th annual meeting of the American Association for the Advancement of Science, researchers took "dead aim at one of Charles Darwin's pet evolutionary theories - the theory of sexual selection, which says that males should compete among themselves for access to mates, or compete for the favours of choosy females." Two leading voices were those of Joan Roughgarden ("There are too many exceptions for the theory to hold") and Patricia Adair Gowaty, who said that the theory may still hold, but researchers have been accepting it without good evidence ("What's wrong is our failure to test the theory adequately"). It has taken a long time for these critiques to mature, but we are now in a position to revisit this issue because some crucial experiments have been repeated. To start, let's remind ourselves of Darwin's much-cited words:
"The female, on the other hand, with the rarest exceptions, is less eager than the male. [. . .] she is coy, and may often be seen endeavouring for a long time to escape from the male. Every observer of the habits of animals will be able to call to mind instances of this kind."
Charles Darwin, The Descent of Man (1871) Part Two - Sexual Selection, Chapter VIII - Principles of Sexual Selection, page 273.
A new sexual selection study replicates an iconic 1948 study and finds it flawed. The graphic shows that children of fruit fly parents with different mutations have an equal chance of inheriting just the mother's mutation, just the father's mutation, both mutations or neither mutation. (Credit: Kim DeRose, source here)
The concept of sexual selection became accepted by Darwinists, although the evidential base was circumstantial. A big impact was made by Angus John Bateman in 1948 drawing on his experimental work with fruit flies. He concluded that for many species, there is a combination of "undiscriminating eagerness in the males" and a "discriminating passivity in the females". He considered that in derived monogamous species (such as man) "this sex difference might be expected to persist as a relic". This analysis of sexual selection became the new orthodoxy, dignified by the term Bateman's principle, as indicated by Knight (2002):
"The explanation, Bateman argued, is that sperm are small and cost next to nothing to produce - so the wider a male can spread them, the better off he will be. A female, on the other hand, produces many fewer eggs, and invests a relatively large amount of energy in each one. All she really needs is one good male to fertilize them to reach her maximum reproductive output. It all seemed to make sense, and Bateman's principle soon became one of the grounding truisms of behavioural biology."
Moving to the year 2007, Brian Snyder and Patricia Gowaty published their concerns about this classic study, documenting numerous flaws in his methodology. They concluded that Bateman's results were unreliable and his conclusions were questionable. However, it still remained to put some rigour into developing an evidential base. They wrote: "We call for repetitions of Bateman's study using modern statistical and molecular methods".
It has nevertheless taken several more years to do the work and publish the findings. This has now been done. The results are of far-reaching significance because Bateman's conclusions have dominated discussions of sexual selection - in academia as well as in education.
"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." (Source here)
For those wanting the details of the experiments and the flawed methodology of Bateman, the best option is to use the links provided. Of greatest interest here is what can be learned about science from the new findings. Undoubtedly we can note the self-correcting nature of science - although 64 years is a long time to wait. Of even more relevance to us today is the power that some concepts have to grip the minds of students and scholars alike and gain a dominant place in their thinking. The evidence for dominance is found in the many biology textbooks that passed down Bateman's work to successive cohorts of students, and in the fact that Bateman's paper has been cited in nearly 2000 other scientific studies. Here is Gowaty again:
"Here was a classic paper that has been read by legions of graduate students, any one of whom is competent enough to see this error," Gowaty said. "Bateman's results were believed so wholeheartedly that the paper characterized what is and isn't worth investigating in the biology of female behavior." Repeating key studies is a tenet of science, which is why Bateman's methodology should have been retried as soon as it became important in the 1970s, she said. Those who blindly accept that females are choosy while males are promiscuous might be missing a big piece of the puzzle. "Our worldviews constrain our imaginations," Gowaty said. "For some people, Bateman's result was so comforting that it wasn't worth challenging. I think people just accepted it." (Source here)
Here is the important point: while some worldviews allow people to open their minds to evidence, other worldviews act to close up the imagination and discussion. This case reinforces the importance of bringing an awareness of philosophy into education at an early stage and embedding it in the curriculum (as is discussed here). This new work will have the effect of clearing the decks, sweeping away the old paradigm and allowing a new conceptual framework to emerge. But what is needed is a willingness to follow the evidence wherever it leads. Many of us fear that the old Darwinian paradigm will rise up again and try to reassert its dominance. Let us not forget that on the issue of sexual selection, the old paradigm has been discredited.
Yet shaking the bedrock of the Bateman paradigm may help the field examine new perspectives.
"Paradigms are like glue, they constrain what you can see," she said. "It's like being stuck in sludge - it's hard to lift your foot out and take a step in a new direction." (Source here)
No evidence of sexual selection in a repetition of Bateman's classic study of Drosophila melanogaster
Patricia Adair Gowaty, Yong-Kyu Kim, and Wyatt W. Anderson
Proceedings of the National Academy of Sciences, Online June 11, 2012 | doi:10.1073/pnas.1207851109 [open access]
Abstract: We are unique in reporting a repetition of Bateman [Bateman AJ (1948) Heredity(Edinb) 2:349-368] using his methods of parentage assignment, which linked sex differences in variance of reproductive success and variance in number of mates in small populations of Drosophila melanogaster. Using offspring phenotypes, we inferred who mated with whom and assigned offspring to parents. Like Bateman, we cultured adults expressing dramatic phenotypes, so that each adult was heterozygous-dominant at its unique marker locus but had only wild-type alleles at all other subjects' marker loci. Assuming no viability effects of parental markers on offspring, the frequencies of parental phenotypes in offspring follow Mendelian expectations: one-quarter will be double-mutants who inherit the dominant gene from each parent, the offspring from which Bateman counted the number of mates per breeder; half of the offspring must be single mutants inheriting the dominant gene of one parent and the wild-type allele of the other parent; and one-quarter would inherit neither of their parent's marker mutations. 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.
Snyder, B.F and Gowaty, P.A. A Reappraisal Of Bateman's Classic Study Of Intrasexual Selection, Evolution, Volume 61, Issue 11, pages 2457-2468, November 2007 | DOI: 10.1111/j.1558-5646.2007.00212.x
Bateman, A. J. Intra-sexual selection in Drosophila. Heredity, 1948, 2: 349-368.
Knight, J. Sexual stereotypes, Nature, Vol 415, 17 January 2002, 254-256.
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