Undeniable (16 page)

The issue of missing links has been kept alive mostly by people who believed (and even now believe) that Earth is no more than ten thousand years old and that humans are unique, with no ancestral connection to any of the billions of living things that came before us. By insisting, quite falsely, that there was no known transitional form between apes and humans, they introduced doubt into an enormous number of minds.

Even after the so-called missing links were found, though, some of the big mysteries that troubled Darwin lingered on. If anything, filling in the fossil record made them even more troubling. First, new species seem to show up pretty fast in the geologic record. Darwin pondered this problem when he wrote: “… Why then is not every geological formation and every stratum full of such intermediate links…?” Second, once a species is established, it and its descendants often hang around, or hang upward into the rock strata, for a long time. The trilobites alone survived in various incarnations for more than 250 million years. Somehow, evolutionary change seemed to happen very fast, but also very slowly.

Making sense of that paradox required blending the latest knowledge about modern ecosystems with the study of creatures that lived long, long ago. It wasn't easy to get everyone talking. As my colleague and friend Don Prothero wrote, “Meanwhile, systematists (biologists, who study the naming and relationships of organisms) were busy describing new species, but few thought of the evolutionary implications of their work. There was simply no common thread among them, and there appeared to be no way to show that Darwinian natural selection was compatible with genetics, paleontology, and systematics.”

This challenge was tackled brilliantly in 1972 by two young (but now very well-known) evolutionary biologists: Niles Eldredge and Stephen Jay Gould. They did compelling analysis of a tremendous number of fossils and came to realize that, although we have a great many fossils that show us big lines of descent, there is a surprising absence of fossils that would tie certain of these lineages to other lineages. It still wasn't obvious exactly how dinosaurs became what we think of as modern birds, even once the overall course of that evolution was quite clear. Similarly, it wasn't obvious how fish ended up walking on land, or how land animals went the other way and ended up swimming around as air breathing fluke-thwapping whales and smiling dolphins. Some of life's biggest transitions seem to have happened so rapidly that they disappeared between the grooves (or digital bits) of the fossil record. That's what Eldredge and Gould set out to explain with a spectacular new extension of Darwin's ideas.

You may have heard the phrase they coined for this phenomenon: “punctuated equilibrium.” I met Stephen Jay Gould at a small group dinner, and I can bear witness that he had a helluva vocabulary. Along with his command of English, he seemed to be nearly fluent in Latin. At any rate, punctuated equilibrium (“punk eeck” among slang-talking paleontologists) has caught on as a description of the mechanism that produces species. Someone like me might have called it “cutoff change,” or “isolation speciation,” or “genetic island formation.” For me, it might be: “I's all good, less the creek don't rise.” Which might be expressed
Nybonically
with better grammar: “If the creek rises, a population may get isolated on a genetic island” (whimsical
Nybonic
expressions have or pertain to the characteristics of the way in which Bill Nye makes up words and phrases).

Whatever name you use, the important thing to know is that the answer to Darwin's puzzles lies in the size of populations—specifically in small, isolated populations. Darwin had pictured one whole species giving way to another. That's how it went for Darwin's famous finches on each of the Galapagos Islands, for example. Once you let go of that old uniformitarian way of looking at things, the situation becomes a whole lot clearer. When a small group of organisms gets isolated (in an isolated patch of forest, on the other side of a rising creek, etc.), some individuals are more prone to form new species. In a small group, any mutation is a much bigger part of the mix, and a successful mutation is immediately a much bigger deal.

Since the landmark 1972 paper by Gould and Eldredge, many studies have been done both with real populations and with mathematical simulations. The results explain why evolution appears both fast and slow: It
is
both fast and slow. Large populations tend to stay genetically about the same. Paleontologists say populations tend toward stasis. Small ones can diverge quickly into new species. Now that you've read this explanation, I hope your response is something like: “Well, obviously…” This could also be expressed here in the early twenty-first century with a single syllable dripping with sarcasm: “'Cha…” It seems to have derived from: “Well, yeah…” Keep in mind though, it was not obvious to many people in the preceding one hundred years or so.

The significance is profound in the context of all that came before in evolutionary thinking, and with the distraction of creationists trying to teach science students kooky ideas about the natural history of Earth even today. Punctuated equilibrium explains why we are missing a great many transitional forms in the collections of fossils kept in institutions around the world. Let's say we come across a string of islands (the Galapagos) that form an archipelago in the eastern Pacific Ocean off the coast of what is now Ecuador. If the weather conditions are violent enough, animals from the mainland can find themselves blown or washed onto these islands. (I talk more about that elsewhere in the book.) These islands are close enough to the mainland to allow organisms to get blown or washed there, but they're too far away for the isolated groups to have a significant number of encounters with the old tribe or flock once they've landed. Their populations are isolated. Evolutionary biologists often use the term allopatric, from the Greek for “other fatherland,” or “other homeland.”

Compared with the birds back on the mainland, the finches on these islands are members of a pretty small tribe, or flock. If one of them happens to be hatched with a beak that's just a little better than his neighbor's beak for cracking nuts, the bird with the superior beak has a better chance of getting square meals. The significant thing here is that his better-beak genes are a larger fraction of the flock. His better-beaked babies' genes will become a bigger bucket in that island's gene pool. As I say, it seems obvious once you know the answer. But where we really see it is in mathematical models. Here (or there) you can speed up evolution with electronic computer simulations. The effect of punctuated equilibrium jumps right out. It's the reason we just don't see many of the transitional fossils. There are inherently fewer of them, and the changes happen quickly. Very few of those intermediate organisms get preserved for us to find millions of years later.

If a small population gets just a slight advantage, that small population can become big. Since the population we're talking about is isolated, it can become just different enough from its ancestral tribe or flock or school for its individuals not to be able to successfully mate with the old gang. Those individuals are of a new separate species. When we look at the fossils of the new separate big bunch, we don't see the intermediate linking individuals, because there were so few of them. Once you understand genetic island formation or punctuated equilibrium, it would be weird if things were any other way. The missing nature of missing links is actually further proof of evolution. It's just what we expect to find out there in nature. If the fossil record were perfect—now
that
would be a mystery.

By the way, while I'm writing here about the incompleteness of the fossil record, keep in mind that the incompleteness is becoming less and less incomplete. Every week or so, paleontologists find another amazing animal whose remains were locked in rock. There was a recent discovery of a two-and-a-half-meter-long (8 foot) millipede fossil from the Carboniferous Period, about 300 million years ago. The fossil is well-enough preserved that investigators can see that it was a vegetarian by examining its very long intestinal tract, which is now preserved as solid rock. I've been to Ashfall State Park in Nebraska and seen the seeds preserved in the tummies of two-ton, long-extinct, North American rhinos. We always seek more fossils to learn more about the past, but we sure do have a great deal of information at our disposal these days.

So far this discussion has focused on change (after all, that's where the cool stuff happens), but stasis is a major characteristic of evolution as well. Populations in ecosystems tend to stay in balance. Why wouldn't they? If they stay in the same locale, and have the same amount of sunlight and food resources for years on end, individuals are born and die, while in the bigger picture, things remain about the same. Once in a while, you'll hear people refer to an organism as a “living fossil.” Even I have used the term. While I'm sympathetic to the intent, it's a nonsensical expression. Fossil refers to something that has been dug up. If it's alive, it's not dead.… That aside, I think I know what someone means, when he or she refers to a living fossil. They mean an organism that has been unchanged for a long, long geologic or evolutionary time.

You may have seen or even own a nautilus shell. These are the wonderful shelled sea creatures that move from one chamber to another, building as they grow, in a logarithmic spiral. They have what we might describe as pinhole cameras for eyes. And, they've had them for the last 500 million years. The animals alive today are not fossils. But, they are just like their ancestors, who are. You may have seen pictures of the coelacanth fish. It was thought to be extinct. Only its fossil remains had been found until 1939, when a population was discovered off the southern coast of Africa living as their ancestors had lived for the last 65 million years. By the way, both the nautilus and the coelacanth are endangered species—thanks to humans. We're killing them off for their shells and out of curiosity. We may indeed soon make these living animals into dead fossils, a troubling and permanent condition.

For animals like nautiluses and coelacanths to live unchanged from generation to generation, they have to live in environments that don't change too much over long spans of time. They keep accumulating mutations in their genes, yes, but the environment's stability does not give major changes an advantage. It's not a coincidence that these animals live in the ocean. There, you have a much better chance of swimming around in an unchanging environment; leastways you did, before humans showed up. From time to time, you have probably used sentences that include the phrase “balance of nature.” But when there's a big change—say a volcano erupts in your neighborhood, or an enormous storm blows you out to sea and you land on a happily unpopulated island—populations get isolated. That's when things can happen fast.

I lived in the Pacific Northwest for many years. I still visit often. The smell alone is enchanting, not to mention the alpine vistas and fecund bays and inlets. There was a great deal of controversy concerning a particular bird that, to many, had no great economic value and was not of any particular interest to people who wanted to cut down ancient trees for lumber. The ancestors of these birds lived in the conifer forests, flying around and eating voles and mice. Then humans showed up and started cutting down trees like crazy. Humans wanted the wood to build houses and structures for commerce. The old-growth lumber is the best you can get. The lumber is fine-grained with little wane. It's beautiful to look at, and it's ideal for the construction of strong buildings that are stiff enough to handle high winds, yet compliant enough to flex their way through a powerful earthquake now and then.

Well, if you're one of the spotted owls,
Strix occidentalis caurina
, this is bad news … Humans are coming to cut down your home. If you can manage it, have your offspring, come up with a clear-cut way to make a living in a clear-cut, or you'll go extinct, which is what the Northwest spotted owl may soon be. We are changing environments around the world. We are changing Earth's climate. We are causing an extinction of an astonishing number of species. If history serves as a guide, new species will arise to take their places, but the rising will be on geological timescales. We're the only animal out there that can turn this rapid change around.

Populations of living things tend to remain in equilibrium, but now and then the whole ecosystem runs into an exclamation mark; certain individuals and populations hit a full stop. That can change things in a hurry. The story of evolution is one of equilibrium punctuated with big changes. We have a strong interest in minimizing the full stops and emphasizing life's run-on sentences, for the sake of all the living things that are not humans yet on whom we depend for a healthy world—our world, and the world of our progeny.

 

17

CONTINGENCY, BOTTLENECKING, AND FOUNDING

How do different species originate? It's the original question. When Charles Darwin was piecing together his ideas, nobody had any knowledge of DNA or genes. He was able to deduce the principles of natural selection, but he was almost completely limited to life's external and internal shapes. Today, we can look into the code of life inside every living thing. Now we can get at the mechanism of evolution. We can look at the molecular record that documents the inside story of how new species emerge and drift away from each other over time. A revolution in gene mapping has sparked a revolution in evolutionary theory as well.

In 1973 the Ukrainian-American geneticist Theodosius Dobzhansky wrote a compelling essay, “Nothing in Biology Makes Sense Except in the Light of Evolution.” He is generally credited with starting the discussion or intellectual dialog often called the “new synthesis” of evolution. Dobzhansky incorporated the biochemical details and role of the technical description of a gene: the specific sequence of nucleotides (aka, the genetic code) that comprise a portion of a chromosome. Described this way, a gene is a construction plan that ultimately determines the order of amino acids needed to create a specific protein. Simple enough? Actually it is fantastically complex, and biologists are still learning the details of how it works. However, this molecular point of view is absolutely, completely, in every way consistent with the observations and conclusions that Darwin made: DNA directs the construction of strings of chemicals; those chemicals influence the configuration of the whole organism; that configuration influences how likely it is that the organism will reproduce and keep spreading more copies of the code.