Evolution, Hawaiian Style
Hawaii is a world-class showcase of evolutionary process. In fact, in many ways it surpasses the examples from the Galapagos Islands. Hawaii’s extreme isolation coupled with its phenomenal array of life zones allowed for the small pool of genetic information that arrived here to evolve in spectacular fashion. The first Hawaiian island that emerged from the ocean was barren lava rock. By the time the first Polynesians arrived here, they found an island chain forested to the coast rich in species. Thousands of new species derived from the few colonizers that made it here. Mutations and adaptations allowed species to inhabit different niches than their ancestors. This is called adaptive radiation and is nowhere better developed than in the Hawaiian Islands.
The Hawaiian tarweeds are considered one of the finest examples in the plant world of adaptive radiation. From a single genetic colonizer they adapted to inhabit numerous ecosystems on the islands. Shown are four different Hawaiian tarweeds; three species of Dubautia, and the Mauna Kea Silversword, a federally listed endangered species.
The Hawaiian Drosophila are a family of pomace flies that are believed to be derived from one colonization. Entomologists estimate that there are a thousand species of Drosophila in Hawaii. They have been called “the world’s supreme example of evolutionary process.” At right is a flightless fly and a Picture-wing Drosophila in the grasp of a carnivorous caterpillar.
A Fly that can’t fly? Carnivorous Caterpillars? Another recurring characteristic of Hawaiian evolution is dramatic changes or adaptive shifts. Flight is a tool for escape. If nothing is chasing you, there’s no need to fly. With the lack of predators in Hawaii, many species of flying insects and birds evolved to become flightless. In Hawaii many ecological niches were vacant. Nature abhors a vacuum. Everywhere else in the world caterpillars eat plants. Filling a niche, a group of moths have made an evolutionary adaptive shift and have become “wait and capture” type predators of small insects, like mantids.
One of the most astounding examples of adaptive shifts in Hawaii occurs underground. Despite its young geologic age, over fifty species of cave-adapted creatures, or, troglobytes, evolved here. With no light to see or sun to hide from, troglobytes evolve to lose their sight. Often the eyes disappear completely. Without the sun they also lose their pigmentation. At right is a Hawaiian blind cave cricket, first discovered in the early 1970’s. It has lost both its vision and it’s pigmentation. One of our favorite evolutionary wonders is a tiny eight-legged creature. The Happy-Face Spider has colorful markings on its abdomen that bears an uncanny resemblance to a smiling human face. It’s found on the underside of specific plants in some Hawaiian rainforests. It is one of the few spiders known to take care of its young.
Before humans arrived in Hawaii there were no large animals to eat plants. Mother Nature is very efficient. Defenses weren’t needed and over time were lost. For example, over one hundred different mints evolved here. The chemical that gives mints their mintiness is a defense—animals don’t like it. All of the Hawaiian mints lost the chemical they’re Mint-less mints. Along with mint-less mints, we have briar-less greenbriars, nettle-less nettles, sumac-less sumac, thorn-less raspberries, and spine-less hollys.
Hawaii also has the best example of adaptive radiation in the bird world. The Hawaiian Honeycreepers, Drepanidinae, evolved from one finch like bird into an astonishing array of species. The original colonizer was probably a seedeater. Over millennia the birds adapted and coevolved with plants for food sources. Of particular interest are the nectar feeders, such as Iiwi or the extinct Akialoa, whose bills fit perfectly into long tubular flowers. Along with the nectar gatherers, the honeycreepers evolved into specialized feeders such as crossbills, creepers, warblers, a parrotbill, grosbeaks, and one bird, the Akiapolaau, whose beak is both a woodpecker and a pry bar.
The painting to the right by Douglas Pratt shows representative beaks of the endemic Hawaiian Honeycreepers. Unfortunately these endemic Drepanids are also one of the most endangered families of birds on earth. Over a third of the original species are now extinct while another third are endangered.
CREDITS – Jack Jeffrey: Lava Dubautia, Alpine Dubautia, Endangered Silversword, Rainforest Dubautia; Bill Mull: Picture-winged Drosophila, Carnivorous Caterpillar, Cave Cricket, Happy Face Spider, Mint-less Mint, Briar-less Greenbriar; Douglas Pratt: Hawaiian Honeycreepers painting