Battles between the sexes have been much more important in creating species than scientists have realized, say Swedish researchers.
Lineages of insects rife with sex conflict have split into at least four times as many species compared with related lineages with less conflict, report Göran Arnqvist and his colleagues at the University of Umeå in Sweden. In the Sept. 12 Proceedings of the National Academy of Sciences, they describe their first-of-a-kind comparison of 25 pairs of lineages.
For high-conflict cases, Arnqvist’s team picked lineages in which female insects mate more than once. There, for example, “it’s in the male’s interest to crank up short-term egg production, even at the expense of long-term female fitness,” Arnqvist explains. In one case, male fruit flies dope their ejaculate with chemicals that rev up female reproduction. Researchers compared the high-conflict lineages with related groups in which females typically mate only once.
In one dramatic set of lineage pairs, Arnqvist’s team found 300 species of Chironomus midges whose females mate with multiple males during their week or two of adulthood.
Hatched underwater, these females grow wings as adults, which they use when seeking mates. However, females of the related four species of singly mating Pontomiya are wingless at maturity.
“Males skim along the water, and when they hit a female, they grab her and mate,” Arnqvist says. Within hours, she lays eggs and dies.
Arnqvist compared species richness within lineage pairs by calculating the ratio of the multiply mating species to singly mating species. He considers the resulting mean ratio of 4 to 1 conservative because it ignores extinctions.
A pioneer in mating-conflict studies, Daniel J. Howard of New Mexico State University in Las Cruces, agrees that sexual conflict can drive species formation. Yet he says Arnqvist’s results are less than compelling. The high-conflict species outnumbered the low-conflict ones in only 14 of 25 lineage pairs and roughly equaled them in 5. Says Howard: “We could use more comparisons.”
However, William Rice of the University of California, Santa Barbara saw a “strong pattern” linking conflict to speciation when he reanalyzed Arnqvist’s numbers. These sexual duels could help explain why sex organs of insect species often change quickly. “The gonads are evolving at truly warp speed,” he says.
John Alcock, who studies insect mating at Arizona State University in Tempe, emphasizes “the extent to which the old ideas about reproduction as a cooperative enterprise have been utterly replaced, and rightly so.” Scientists today look at reproduction “as a battle of males versus males and males versus females,” he says. This view, he adds, “may now even alter our understanding of that most fundamental of evolutionary problems, the origin of species.”
