Water beetles mate themselves to an evolutionary standstill
Female adaptations to male harassment result in a barrier to speciation. Nick Carne reports.
A species of diving beetle is having mating problems, and that’s of interest to more than just diving beetles.
Grapoderus zonatus has come to something of an evolutionary standstill due to sexual conflicts between males and females. The outcome challenges the theory that such conflict is a driver of speciation.
“Usually females evolve ways to escape the mating harassment from males and this could initiate the evolution of new species,” explains Lars Iversen from Arizona State University in the US.
“Here, we document an alternative outcome: that sexual conflict instead prevents populations from diverging from each other and becoming new species.”
It’s not a new idea, but Iversen and colleagues from Sweden and Denmark say their recent research provides the first empirical evidence.
They studied the behaviour of the beetles in 29 lakes across Sweden and published their findings in the journal Proceedings of the Royal Society B.
Diving beetles were a good choice because, as the researchers note, they are “one of the few known cases in which the co-evolution of male and female traits has been shown to be sexually antagonistic”. There is certainly no courtship period.
Here’s what’s evolved over the years.
In many diving beetles, males have suction cups on their front legs to attach to the back of females during mating. However, females may be harmed under high mating pressure that can last for hours, and some have developed rougher backs that are harder to attach to.
What the recent research discovered was that males have evolved suction cups to match the back structure, whether smooth or granulated, of the females. Thus, within populations of this species, two pairs of male and female mating traits have developed.
When granulated females become numerically dominant, the mating pressure from the matched males is so strong that females with a smooth back get an advantage. The opposite is also true. Thus, the outcome is a situation with no consistent long-term advantage for any single female type.
Instead, the researchers say, populations move towards a state where both smooth and granulated females are equally abundant, minimising the mating pressure on a specific female type.
Hence, the diving beetles are kept in an evolutionary limbo and the two type of females are maintained by the ongoing and intense mating harassment from the males.
"This study will be an important baseline for developing a better understanding of the evolutionary outcome of sexual conflict in natural populations" says Erik Svensson, from Sweden’s Lund University.
"The story is more complicated than we previously thought. We now know that sexual conflict can prevent population divergence and halt speciation."