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Olivia Judson is a research fellow in biology at Imperial College, London.

[edit] No Vacancy, No Evolution

Why aren’t there more insects in the sea?

Yesterday, I claimed that a major reason large evolutionary changes often don’t happen is that competition from the creatures around you stops you from changing. In other words, in environments that are already rich in different species, natural selection often prevents large changes. My piece of evidence for this was a claim that, when you take other organisms away — when you reduce competition for food, or space — evolution explodes. Today, I want to examine the truth of these claims more closely.

I want to start with a question: Why aren’t there more insects in the sea? Insects, after all, make up the bulk of all known animal species — most animals are insects — yet hardly any insects live in the sea.

It’s not because of the water. Many insects live in freshwater for at least part of their lives. Think of mayfly nymphs, or caddis fly larvae — these are the ones that often build themselves tubular houses out of stones and gravel. (Caddis fly larvae can make amusing pets. If you keep them in an aquarium, and at appropriate intervals provide them with gravel of different colors, they will construct a house that looks like, say, a French flag.) Or think of water striders, skating along the top of a pond (where, incidentally, they have very rough sex).

[edit] The Empty Niche Theory

Could it be the salt? No. Brine fly larvae live in the Utah’s Great Salt Lake. Some mosquito larvae do fine in salt, too — they can even live in the hypersalty Dead Sea. Some caddis fly larvae live in rock pools along the shores of New Zealand and southeastern Australia; females of one species even lay their eggs inside the body cavities of local starfish. A few bugs (bug is, believe it or not, a technical term for a particular sort of insect) known as coral treaders live in crevices in corals — but mostly in the intertidal zones. Seals and walruses have body lice; the lice can survive months at sea, although they reproduce only when their hosts haul out onto land. In short, insects are perfectly capable of evolving to deal with salt water, but hardly any live in the sea.

I put it to you that the reason so few insects get beyond rock pools or dip their toes beneath the low tide mark is that the niches they would occupy in the ocean proper are taken already. Crabs, lobsters, shrimp, barnacles, water fleas (which are not fleas, but cute little crustaceans) and company have got all niches covered. My prediction — which I hope we won’t be testing — is that if all the crustaceans were wiped out of the oceans, insects would move straight in.

My prediction rests on the fact that it is typically difficult to evolve to occupy a niche that is already full. An invader must be better at exploiting the niche than the current occupant, who has already evolved to make effective use of it. By contrast, when a niche is empty — when seeds are falling to the ground and no one is eating them, say — it doesn’t matter if, at first, an animal is a bit inept at finding and opening the seeds.

[edit] Adaptive radiation

Consistent with this idea is the observation that when new niches open up — perhaps because new islands or lakes or cave systems have formed, or because an asteroid has hit the earth and eradicated millions of species — the first organisms to become established in the new environments evolve quickly and reliably into all sorts of new species. This phenomenon is known as adaptive radiation.

Newly erupted islands are famous for this. Over and over again, archipelagos see explosive bursts of evolutionary change and the rapid appearance of species found nowhere else. New Zealand is full (and was fuller) of an amazing array of unique flightless birds, including parrots, ducks and kiwis. Once, it had the huge moas, too. (Birds that live on islands often lose the ability to fly, for reasons I’ll discuss in another column.) Hawaii has an abundance of unique fruit flies, spiders, silverswords (these are rather lovely plants) and birds. Madagascar has all manner of lemurs — (usually) small primates related to bushbabies. And everyone knows about the Galápagos.

Rapid bursts of evolution can also happen in new lakes — which in many respects are islands too, just islands of water surrounded by land. Indeed, right now, the great lakes of tropical Africa are the backdrop for the fastest known radiation of vertebrates, the cichlid fishes. Lake Victoria, for example, appears to have dried up and then refilled around 14,600 years ago. Since then, perhaps 500 different species of cichlid have evolved within it, with all manner of habits. Lake Victoria has cichlids that eat algae, cichlids that eat other cichlids, cichlids that eat fish eggs — cichlids, in short, that have evolved to eat everything that can be eaten. Some fish live in shallow water; others prefer the deeps. They have evolved a huge diversity of sexual behaviors, too.

(These fish provide another illustration of why interpreting the fossil record is a treacherous business. Many of the species have distinctive coloration and behavior but their skeletons are indistinguishable from one another, so even if specimens of all the species were to fossilize perfectly, a paleontologist of the future would grossly underestimate the extent of evolutionary change that has gone on.)

[edit] Fossils, islands and test tubes

Ideas about adaptive radiation can also be tested in experiments. Many bacteria can whiz through hundreds of generations in a month. This makes it relatively easy to use bacteria to look at radiations. Here’s what you do. You create two sets of environments, one simple, and one complex. The complex environment might have several different places to live, or a variety of sources of carbon. The simple environment has just one habitat or foodstuff. Then, since bacteria reproduce asexually, you take genetically identical individuals, and release them into the two different environments. Sure enough, mutations happen, and the bacteria rapidly evolve to exploit the different niches. After a month, you will find that bacteria from the complicated environment have become genetically diverse. Those from the simple environment, in contrast, remain unevolved.

In short, empty niches are a license for evolutionary change. Once the new niches are full, natural selection acts to stop further change, and the rate of evolutionary change slows. Fossils, islands and test tubes — they all show the same dynamics.

This raises two subjects to wonder about. The first is whether all lineages have the same potential to explode, or whether some are intrinsically more explosive than others. I’m not sure how to look at this — but I’d love to know. (If anyone is placing bets, I’ll bet on the second — call it a hunch.) My second wondering is about the vanished archipelagoes from the deep history of the earth, when the continents we know today were all jammed together in the Southern Hemisphere. Were there islands in the north? What amazing creatures lived there? Probably, we’ll never know.

Olivia Judson, an evolutionary biologist, is the author of Dr. Tatiana's Sex Advice to All Creation: The Definitive Guide to the Evolutionary Biology of Sex, which was made into a three-part television program. Ms. Judson has been a reporter for The Economist and has written for Nature, The Financial Times, The Guardian, The Daily Telegraph and The New York Times, which publishes "The Wild Side" blog.

She is a research fellow in biology at Imperial College London.

Originally published in The New York Times on June 20, 2006. Copyright © Olivia Judson and The New York Times, 2006.

Reference: Dr. Tatiana's Sex Advice to All Creation
Text credit: OJ
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