Undeniable (17 page)

Dobzhansky's influence was so profound that we often forget about it. He tied the phenomenon of gene mutation—the natural mistakes that happen in making copies of elaborate or complex molecules—to the happy accidents that become, in Darwin's term, the favored descendants. If the mutation was of value to the offspring in reproducing, that mutation will get passed on and on.

Before this synthesis, the means by which a species became separate or a new species emerged was not exactly clear. Investigators had presumed, quite reasonably, that every individual of a given species had pretty much the same genes. What made one different from another was thought to be connected to genes, even though the variation among individuals within a species is not very large. The reason you and your sister have different colored hair was understood to be genetic but a small fraction of one's inheritance. With the twentieth-century “modern synthesis,” it became clear that every feature of an individual is expressed in her, his, or its genes. We take this idea for granted now. It gave scientists an understanding of what it takes to become a species. The genes mutate enough through enough generations, and you get individuals that can no longer reproduce with each other; they've become separate or a new species.

The most important insight from the modern synthesis is that it enables scientists to understand how populations split into different species. It identifies one of the key mechanisms behind life's diversity. Let's dig deeper by thinking about a population of beetles. Believe me, there is no shortage of different beetle populations; there are 350,000 known beetle species, and probably plenty more left to be discovered. Let's say they live in a forest. One year in the mountains above our forest, there is a significantly higher snowfall than there has been in decades. As the snow melts, a larger than normal flow of water cascades down a major hillside forming a new river right through the beetle's natural valley habitat. The beetles, who happen to make their living in the forest litter or duff below the trees on the forest floor, are now separated into two communities, the left-bank beetles and the right-bank beetles. They no longer communicate and conduct beetle transactions, including egg fertilizing and egg laying.

With imperfect copying of the genes in the two populations, generation after generation, eventually the lefters and the righters have genes that are too different, and the two groups can no longer reproduce with each other. The two populations, in our simplified but not unreasonable example, became distinct species. Meanwhile, other natural selective forces are at work as well. The lefters may live on a portion of the river where this same out-of-the-ordinary snow event flooded the banks and killed off a large swath of trees by drowning, essentially suffocating, their roots. Certain individuals in the left-bank population may have chanced upon a slightly different jaw or mandible that makes them ever so slightly better at chewing the dead wood of the suffocated trees. They will be better nourished. Their eggs may hatch under better conditions, such as freshly chewed-out galleries that keep the eggs at a more productive temperature than their fellow tribesman (tribeetles?). So, their eggs do a little better, and more left-bank beetles are born with better-suited mandibles and gallery-chewing capability. Meanwhile, the right-bankers carry on as they did before the flood. Their environment hardly changed at all, and yet the left and right populations diverge.

There are two more consequences to this sort of turn of events having to do with populations of organisms becoming isolated or cut off. In our example of the beetles and the catastrophic river-forming flood, it is quite likely that the two populations will not be of the same size. In my imagination, I see the left-bank beetle community being much smaller. Their territory was overrun by the melting snow flood. The right-bankers, literally on the other hand, were on the inside lane of the river bend and very few of them were drowned in the great snowmelt, river-forming flood. They have almost as many individuals as before. When it comes to insects, the numbers can get huge. If you've ever been in Minnesota in July, the flies … well, they are uncountably numerous and ornery. If there were only 10 million, a Minnesotan canoeist's life would be a lot less troublesome. With that in mind, let's say we have 10 million beetles on the right bank all carrying essentially the same genes as they had before the flood.

On the left bank, we have only ten thousand beetles who made it through our imagined cataclysm. Beetles being the way they are, they reproduce like crazy, and the ones with the better-suited-to-chewing-dead-tree jaw do especially well. On top of that, their predators, which may have included other species of insects like praying mantises and birds such as wrens, would not have responded to the novel landscape. They may not be as good at finding beetles in the muddy zone full of dead tree trunks, which would enable the new style of beetle on the left bank to reproduce with even more success. For this thought experiment, let's say the populations eventually stabilize. The predator and prey relationships settle out to stable numbers of each. Even if the left-bank population reaches the same size as the right-bank population, their genes on the left bank will have a great deal less variety, because they are all descendants of a much, much smaller population. This is called a genetic bottleneck, because all of the beetle ancestors passed through a narrowing of their genes that is analogous to the narrow neck of a bottle.

Here's the wonderful thing about bottlenecks. We can measure the genes in different populations and infer a great deal about who begat whom in the plant and animal kingdoms. From those inferences, we can draw conclusions regarding the natural history of whole continents and ecosystems. With the tools of modern gene sequencing (including some amazing chemical reagents and extraordinary machines), we can sample the DNA from the blood of the different beetles and determine which population has more genetic diversity. The population that survived the flood would have less diversity than the population that was largely unaffected. This is what we would expect. But it's important to keep in mind that it was possible to understand the outlines of this process long before anyone knew about DNA and sequencing.

A powerful, real-world example of a genetic bottleneck took place in the Galapagos Islands. A young rambling Charles Darwin noticed, among other things, that the island birds, specifically the finches, were all very similar. Nevertheless, they also had slight but distinctive differences in their beaks. Darwin realized the implications of what he was seeing.

Let's say you are a happy finch flying around on the mainland of what is now Ecuador, and an enormous storm blows through, a cyclone a few hundred kilometers across. You and several members of your community, your flock, get caught up in the high winds, while you are idly tweeting about varieties of nuts. You all get blown out to sea. Many of you become too tired to fly and disappear beneath the tumultuous waves. But a few of your comrades and you end up on an island. There are plenty of nuts, because apparently some nut tree seeds had been blown here during similar storms many years earlier. The nuts are good enough to eat. You and certain of your community members have beaks that other birds often chided you about. You and a few others have a bit of a hook on the ends of your beaks. These hooks work great for knocking nuts open. You form a new community and reproduce for many years. Your descendants several generations hence fly about and alight on rocks and branches discussing nuts and what the neighbor finches are up to.

Then another enormous storm blows through this island. Major storms are quite common in that part of the world. Despite the experience of the ancestors, or perhaps even because of it, these great, great, many times great, grand-birds also get blown out to sea to the west, this time many drown, but a few alight on a new next-to-the-west island. A new population gets a new start, and so on. With each successive catastrophic storm event and landing on each successive island in the Galapagos Archipelago, we have a population or community that came to grow and multiply from a set of genes that inherently has less and less diversity.

Darwin came on the scene long before anyone knew about genetic diversity. A great many researchers since his time have incorporated the new ideas into his theory. The gene sets of the finches are what you'd expect if this were what went down (or flew up) in this part of the Pacific Ocean. Along that line, there are iguanas in the Galapagos region that are reminiscent of those on land, but quite different. They can swim, for one thing. Jungle iguanas have no such skill. These lizards didn't get to the islands by flying, but storms are occasionally powerful enough to get a few of them out there by other means. Certainly a great many logs or fallen trees end up in the ocean along the heavily forested coast. If you're a lizard clinging to a tree, what are you going to do if it ends up in the sea? You hang on. If you end up on an island, you do what you can to make a living and participate in—we presume hot, albeit deliberate—lizard sex with another drifting tree-mate that made it out there by the same means. We can measure their genes and compare them with the genes of the lizards back on land and get just the result that evolutionary theory predicts: Less diversity from east to west along the archipelago.

We call these sorts of novel populations that take hold and make a living in areas that are new to them “founders.” They found a new community, just as human founders start companies or institutions. Only these animal founders fought for their lives and especially the lives of their descendants.

In general, founders arise from, and give rise to, genetic bottlenecks. Because there are fewer individuals that make it to a new area to found a new community, there will just be less diversity in the genes of that smaller founding population than we would find in the native tribe or community whence they came. An oft-cited example of this is the Afrikaners in South Africa. They came from Holland, and apparently carried a gene that makes a human susceptible to Huntington's Disease, a degenerative brain disorder. People who suffer from it exhibit jerky movements especially in their faces and shoulders, and ultimately suffer from a type of dementia. There are a great many more Huntington's patients in the Cape of Good Hope region of South Africa than in the rest of the world's population. The cities of Cape Town and Johannesburg were founded by a relatively few Hollanders, some of whom carried this troubling gene. The genes of the Afrikaner population in southern South Africa passed through a genetic bottleneck, when their communities were founded. Another famous example is the prevalence of hemophilia in the inbred British royal family.

The phenomena of bottlenecks and founders have led scientists to speculate on the role of contingency or happenstance in evolution. Scientists have long debated the importance of new environments in biological and especially genetic diversity. I like to put it this way: Do we need catastrophes big and small, including the devastating mass extinctions, to make new species? The beetles in our earlier thought experiment would be an example. It's a fascinating question, because it takes us back to the deeper question: Where did we all come from? Put yet another way: Would we all be here if all hell hadn't broken loose here on Earth a few times?

Now that geologists know what they're looking for and have designed the instruments to go looking, they have discovered dozens of meteoric impact craters around Earth. The dinosaur-menacing rock that struck Mexico 66 million years ago was hardly the only one. You have to figure that if you're a living thing, like our beetles in a forest, and a giant white-hot rock lands nearby and sets everything on fire, there's going to be trouble. You, and every beetle in your community, may get burned to Coleopteran crisps. Or perhaps, you have the good fortune to be on the edge of the inferno. You and a few comrades live through it, and before you in the aftermath is a whole new landscape to explore with very few living things upon it. Furthermore, you may have very few competitors for some time, perhaps decades. Even if you don't live to explore and eat the shoots of the new plants that grow in the newly exposed nutrient-rich soil, perhaps the eggs you managed to lay that first season do survive, and your offspring have the run of the new landscape for years. A huge successful population gets established, one with much less diversity, but still carrying your genes, and so on and on and on.

And as we've seen, asteroid impacts are not the only catastrophic challenges life has faced. Enormous flood-basalt volcanic eruptions, similar to the ones in Siberia and India, have poisoned the globe with dust and noxious gases at least fifteen times during known geologic history. Global cold spells may have led to periods when the entire planet was encased in a casket of ice. The motions of the continents have repeatedly triggered drastic changes in climate, ocean circulation, and ocean chemistry. There were probably other grave insults we don't even know about yet. And there were certainly countless smaller but still influential environmental crises (droughts, floods, etc.) that wiped out populations and created the right conditions for new founders to emerge.

The question for evolutionary biologists such as Dobzhansky and, more recently, the influential American researcher Stephen Jay Gould, was or is how many such events do we need to account for the diversity we see in the world today? How much contingency do living things need to get mutations sufficient to account for all of us Earthlings?

Some evolutionary scientists argue that we'd all have four limbs and a jaw whether there were catastrophes or not; that would be convergent evolution. They claim that life on Earth would look roughly the way it does today no matter how many happy accidents occurred, or didn't occur. Some argue that a planet just has to have catastrophes in order to provide new places, new so-called ecological niches, to create diversity. They point to the mass extinction events in the fossil record and argue that without those worldwide resets, we wouldn't see the animals, plants, and microbes we see today. You can take it down from worldwide to local and argue the same thing. Without big changes in the environment or the predator-prey relationships, we would not have diversity.