Males are more promiscuous and females more choosy in selecting mates so suggest study on an iconic sexual selection by UCLA biologists.
In 1948, English geneticist Angus John Bateman published a study showing that male fruit flies gain an evolutionary advantage from having multiple mates, while their female counterparts do not.
Bateman's conclusions have informed and influenced an entire sub-field of evolutionary biology for decades.
But UCLA biologists, who are the first to repeat the historic experiment using the same methods as the original, say the classic study may, in fact, be wrong.
"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," Gowaty said.
The original experiment on Drosophila melanogaster, also known as the common fruit fly, was performed by creating multiple, isolated populations with either five males and five females or three of each gender in a jar. The insects mated freely in the experimental populations, and Bateman examined the children that made it to adulthood. To count the number of adult offspring engendered by each of his original insect subjects, Bateman needed a reliable way to match parents with children.
Nowadays, modern geneticists would use molecular evidence to determine the genetic parentage of each child, but DNA analysis was not available in the 1940s. Instead, Bateman chose his initial specimens carefully, selecting D. melanogaster flies that each had a unique, visible mutation that could be transferred from parent to child, Gowaty said.
The mutations were extreme. Some of the flies had curly wings, others thick bristles, and still others had eyes reduced in size to narrow slits. The outward differences in each breeding subject allowed Bateman to work backward to determine the parentage of some of the fly progeny and to document each mating pair among the original insects. A child with curly wings and thick bristles, for example, could only have come from one possible pairing.
Yet Bateman's method, which was cutting-edge for its time, had a "fatal flaw," according to Gowaty.
Imagine the child of a curly-winged mother and an eyeless father. The child has an equal chance of having both mutations, only the father's mutation, only the mother's mutation or no mutation at all. In order to know who mated with whom, Bateman used only the children with two mutations, because these were the only ones for which he could specifically identify both the mother and father.
But by counting only the children with two mutations, Bateman probably got a skewed sample, Gowaty said. In repeating Bateman's experiment, she and her colleagues found that the flies with two severe mutations are less likely to survive into adulthood.
Flies use their wings not only to hover but also to sing during courtship, which is why curly wings present a huge disadvantage. Specimens with deformed eyes might have an even tougher time surviving. The 25 percent of children born with both mutations were even more likely to die before being counted by Bateman or Gowaty.
Gowaty found that the fraction of double-mutant offspring was significantly below the expected 25 percent, which means Bateman would have been unable to accurately quantify the number of mates for each adult subject. Further, his methodology resulted in more offspring being assigned to fathers than mothers, something that is impossible when each child must have both a father and a mother, Gowaty said.
Bateman concluded that male fruit flies produce many more viable offspring when they have multiple mates but that females produce the same number of adult children whether they have one mate or many. But Gowaty and her colleagues, by performing the same experiment, found that the data were decidedly inconclusive.
Gowaty's study was recently published in the online edition of Proceedings of the National Academy of Sciences and is scheduled for publication in an upcoming print edition.