Finicky mating behavior appears to be the driving force behind the speedy speciation of the Laupala cricket, the scientists wrote in the Jan. 27 issue of Nature magazine.
Females in the Laupala genus detect tiny differences in the pulse rates of male courtship songs, which differ from one Laupala species to the next. They refuse to mate with males of other species, thus promoting the formation of new species.
The scientists say their findings shed light on the role of individual choices in the evolution of species and the growth of biodiversity.
"Animals with nervous systems and brains have preferences and can make choices," says Tamra Mendelson, an evolutionary biologist at Lehigh. "Changes in these preferences and choices appear to drive speciation.
"That raises the question: Can something seemingly so individual as a choice have macro-evolutionary consequences in terms of increasing biodiversity? If so, this affects how life on the planet looks. The more species you have, the more complex the ecology is going to be."
In down-to-earth terms:
"What turns a female cricket on? Why does she prefer one pulse rate over another? Whatever the reason, it's very important that she exercise this preference in order to keep the species distinct."
Mendelson spent three and one-half years studying the Laupala cricket with Kerry Shaw, an evolutionary biologist at the University of Maryland.
The thumbnail-sized Laupala spawns new species at the rate of 4.17 every 1 million years, or more than 10 times faster than the average speciation rate for invertebrates. This rapid evolution is contributing to an "explosion" of new cricket species, especially on Hawaii, the largest and youngest island in the Hawaiian archipelago, say the two scientists. Some 38 different species of the cricket now inhabit the island chain.
Among all animals, say Mendelson and Shaw, only the African cichlid fish spawns new species more quickly.
While early biologists based their estimates of speciation rates on morphological, or structural characteristics, Mendelson says, scientists today are more likely to use genetic data to come up with estimates.
Closely related species of Laupala have no clear morphological differences, Mendelson says. They are similar in appearance, they have similar diets, and they live in similar habitats. Members of closely related species possess no physiological differences that would prevent them from interbreeding.
But closely related species are distinguished by subtle differences in the pulse rates of male crickets' simple courtship songs, a secondary sexual trait that plays a large role in mate attraction.
Among all species of Laupala, pulse rates of male courtship songs range from .5 to 4.2 pulses per second. Female crickets can detect these differences, says Mendelson, and they tend to hop towards the pulse rate of their own species and to reject songs sung at a different tempo.
The Laupala mating ritual lasts up to eight hours and consists of eight to 15 transfers of spermatophores between male and female, says Mendelson. The final spermatophore, or capsule, to be transferred contains the actual sperm cells that impregnate the female.
As a species begins to split into two separate species, says Mendelson, "the songs appear to be the first characteristic that changes."
During the early part of the speciation process, says Mendelson, crickets of the two emerging lines interbreed. After a point, however, members of the two distinct species no longer mate with each other.
This discovery led Mendelson to name one of her graduate-school papers after the pop song "Will You Still Love Me Tomorrow?" The Nature article was given the more conservative title of "Rapid Speciation in an Arthropod."
"Over time," says Mendelson, "lineages - species that have split - cease to recognize each other as mates. Although the ancestors recognized each other as mates, the descendants no longer do because of evolutionary change. In the case of Laupala, members of the two species are physiologically capable of mating. But they appear to lose the desire to interbreed before they lose the capacity to interbreed."
Mendelson and Shaw used a technique called amplified fragment-length polymorphisms to establish the evolutionary tree, or phylogeny, of the Laupala. They based their estimates of the cricket's rate of speciation on the Laupala's phylogeny and on the age of the islands in the Hawaiian archipelago. The youngest island, Hawaii, was formed less than 500,000 years ago.