Dr. Rosemary Gillespie has been on a roll of late. A little more than a year ago, the Berkeley biology prof and director of the University of California’s Essig Museum of Entomology published a paper in the prestigious Science magazine on the mechanisms of evolutionary diversification in Hawaiian spiders. The article attracted attention beyond the usual small scientific circles because it may point to trends that happen across the natural world, well beyond the arachnids.
“Evolution is coming up with the same product over and over again,” Gillespie told the audience at the Smithsonian Tropical Research Institute’s (STRI’s) Tuesday afternoon lecture series on February 15.
The professor is here in Panama for a short-term fellowship with STRI, which is a high-powered and very international little outpost of American academia, whose Tupper Auditorium on any given Tuesday afternoon surely brings more PhDs together than can be found in any other room in Panama. Those lectures attract lay people and scientists of diverse specialties and thus the really good ones are both stimulating to sharp and well educated minds and comprehensible to non-specialists. This was one of those, as might have been expected from a scientist who maintains a spider web page for kids as part of the Smithsonian's JASON educational program.
Gillespie, who used to teach at the University of Hawaii, has spent years studying the spiders of that American state as well as in the Marquesas and the Society Islands. At the start of her talk she noted that in the geological and evolutionary sense there are two sorts of islands, the fragments of terra firma that have drifted away from mainlands and those spots in the ocean that have risen from the sea where there was no land before, for example as the result of volcanism. Noting in passing that fragment islands are frequently home to evolutionary relics, she concentrated her presentation on the islands that form from nothing, which of necessity acquire their biological populations by dispersal from other places. On those places you get a lot of adaptive radiation --- species moving into vacant ecological niches and adapting to them --- and neo-endemism as opposed to fragment islands’ frequent paleo-endemism.
The classic example of adaptive radiation and evolution in the Hawaiian Islands has been documented by ornithologists studying the various species of honeycreepers there. These small birds with curved beaks designed to extract nectar from certain flowers are, she said, “the Hawaiian equivalent of Darwin’s finches.”
Moreover, they are an example of nature coming up with similar adaptive answers from different genetic lineages. In the Americas, including in Panama, “honeycreepers” are members of the subfamily Dacninae, while Hawaiian honeycreepers are members of the not very closely related Drepanididae family. It’s just that the two groups of birds developed the same sorts of morphologies to fit into analogous niches in widely separated places.
The problem with using Hawaiian honeycreepers in a search for knowledge about evolution in general is that the various members of that bird family are rare, endangered or extinct. However, Gillespie said that a number of other Hawaiian species are good for studying the radiation of original populations and the ways they adapt, in a scientific search for repeatability and predictability in evolution.
Studying spiders at high elevations --- at lower altitudes urban and agricultural sprawl, the effects of such exotic invasive species as ants and other factors pose problems for researchers --- using old techniques of morphological classification and newer ones of DNA analysis, and overlaying her discoveries on what’s known of Hawaii’s geologic history, the professor has looked at one clade of the several dozen species of Tetragnatha spiders and figured out how they have spread across and evolved in the archipelago.
Hawaii is a volcanic chain formed at the intersection of gradually moving tectonic plates. The volcanism pretty much stays in one area, with the islands it forms drifting away toward the northwest. The oldest of the larger islands, Kauai, is about 5.l million years old. Its younger siblings, in descending order of age, are the islands of Oahu, Molokai, Lanai, Maui and Hawaii, the latter well under one million years of age.
The various Tetragnatha species either build flimsy webs over water or have abandoned web construction entirely. The spiny legged ones upon which Gillespie’s paper in Science and lecture to STRI concentrated have given up on webs, but like all spiders they do produce silk, and they are known to climb to high, wind-swept spots, shoot out strands of silk and float away on the breeze. The all derive from a common ancesctor found on Kauai some five million years ago. Often changing along the way, they have repeatedly spread from the older islands to the newer ones as they emerged. “They’ve undergone a spectacular radiation,” Gillespie noted.
Once arrived on a new island, these arachnids may occupy similar niches to those they left behind at the places from whence they came, essentially spreading the range of a particular species. But then it’s common for the spiders to move into new niches on the islands they have more recently colonized, evolving into new species.
By a morphological analysis, four general types of spiny legged Tetragnatha spiders have arisen on the Hawaiian Islands. The green type live on leaves, the maroon ones are mostly found on mosses, the small brown ones make twigs their home and the large brown ones spend their lives blending in against backgrounds of tree bark. But DNA tests show that very often a spider with the same niche and morphology as a relative on another island will actually be more closely related to a spider on the same island with a different appearance and habitat. Thus, for example, the maroon types on Oahu are more closely related to the green types on that island than to the maroon Tetragnatha on Maui.
Hopping down the islands from west to east, it appears that time and again Hawaiian spiders have moved on, colonized new niches and diversified --- or not diversified. But that’s not the most tantalizing part of the picture. The greater variety found around one volcano on the eastern end of Maui holds the honor of being “where the action is.”
Dr. Gillespie believes that evolution tends to create more adaptations than nature will sustain, overshooting the equilibrium among diversification, immigration and eliminating lineages until a new equilibrium is reached. She's charted it out to show a curve in whch the number of species emerging overshoots nature's reserves to sustain them, and die-offs ensue. (Understand, however, that this entomologist is not indifferent to extinctions: she lists conservation of arthropods as one of her areas of activity on her website, and is documenting the effects that invasions of exotic species have had on the indigenous arthropods.)
Eastern Maui is the place where the spiny legged Tetragnatha have splintered into seven species, while in the older islands there are at most three or four species apiece but Gillespie thinks there used to be more. The younger island of Hawaii also has but four species, all of them descended from migrants from Maui. Looking at lineages over time, Gillespie expects only a few of eastern Maui’s species to survive, and the rise of several new Tetragnatha species on the big island of Hawaii, followed by a big fall for some of them.
It’s a matter of nature forcing different creatures to come up with the same answers to analogous problems, then picking winners and losers among those who have adapted. If that pattern can be shown to exist throughout nature, it may in turn give insights into such diverse phenomena as the relationship between modern humanity and the australopithecine hominids, the similar shapes of mammalian cetaceans and many fish species and what’s likely to happen to species that may be forced out of their niches by global warming.
