Hawaiian Evolution 1: Starting From Scratch

There are a number of places on this planet where the signal of evolution is readily apparent to anyone who cares to look. Most of those places are islands. It's no coincidence that Darwin made the Galapagos famous, or that Wallace did his most important work in the Malay Archipelago. As helpful as those places were to the discovery of evolution, they pale in comparison with the Hawaiian Islands, and I'm not saying that because I work there. We've got examples of evolution out here that will knock your socks off.

More than ninety percent of everything that is native to the above sea level portions of the Hawaiian chain is endemic - found only here. Some of these plants and animals are relatively ordinary, aside from their limited distributions. Others are pretty damn weird. We've got spiders out here that have abandoned web-weaving. Instead, they spear their prey, using sharp projections from their forelimbs. We've got flies that give birth to a single, live larvae. There are caterpillars that pretend to be sticks until some unfortunate insect comes along to get ambushed and eaten, and there are caterpillars that actively hunt snails. All that just scratches the surface. Hawaii has seen lots of exuberant evolution, and there's a reason for that.

There are two main kinds of island in the world: those that have always been islands, and those that used to be connected to mainland areas. This might not seem all that important at first glance, but it actually turns out to have a big effect on the evolutionary patterns that are seen on them - so much so, in fact, that it is sometimes possible to figure out whether an island used to be part of a mainland area just by looking at its native flora and fauna.

Islands that have a continental connection at some point in their past start out their current island incarnation with a fair selection of the living things found in similar areas on the parental continent. They may not keep the full set - islands often lack the resources needed to support all of the species found in similar areas on the continent. They may remain separated from the continent for long enough for new species to evolve. Both may happen, or, neither, but whatever does happen, the signature of the old continental connection will remain. This type of island is sometimes known as a "remnant" or a "secondary" island - New Guinea is a prime example of this.

Other islands start out as islands. These islands do not begin with any sort of pre-existing biota. Instead, they start out as bare rock, and gain inhabitants slowly, as things migrate out over time - often by literally drifting in on the wind or currents. They start out with a much more impoverished flora and fauna, but it is just that humble beginning that presents the opportunity for the early arrivals to explore evolutionary pathways that they would not have otherwise had the chance to. This type of island is referred to as a "primary" or "Darwinian" island.

The Hawaiian Islands are, if you hadn't already guessed, primary islands. Each island in the chain started off as an undersea volcano, with absolutely no above-sea-level life (or, for that matter, land). That's part of the reason that these islands have been a playground for evolution for far longer than for tourists. The other part of the equation is the isolation.

The Hawaiian Islands are located almost precisely in the middle of nowhere. There is, quite literally, no other archipelago that is so far from the nearest continent. The closest part of North America is currently about 2500 miles from Honolulu; the nearest approach to Asia is about 3500. The extremity of the isolation has two separate effects. The first is that the probability of something reaching Hawaii for the first time is coming from Asia isn't all that different from the probability that it's come from North America, so the biota here has a very mixed origin - unlike, for example, the Galapagos, which are also Darwinian islands, but are so close to the coast of South America that their endemic life has a very clear South American flavor. The second effect that the huge distances separating Hawaii and the continents has is that it makes it very hard for things to find their way out to the islands. New arrivals are rare, which gives the ones that were there before much more opportunity to evolve in piece. It also means that there are some common ecological niches on the continents that are either unfilled or filled in strange ways in Hawaii, because the continental occupants were unable to make it here on their own.

The geological history of the islands has also had quite an effect on the way things have evolved here. That's going to be the topic for the next post in the series, which will appear, with luck, on Monday morning.