Charles Darwin’s theory of natural selection is central to Kolbert’s study of the Sixth Extinction. Darwin posits that species are in a constant process of adapting to environmental changes based on competition for a finite number of resources (such as food, water, and shelter). The “fit” species (which can access the world’s resources and have offspring) survive, while “unfit” species die out. Generally, the “fit” species succeed at accessing resources because of their ability to adapt to their changing environments by bearing offspring with genetic qualities that are better suited to new conditions. For example, Darwin posits that the existence of wolves and other predators made good hearing a valuable survival trait for rabbits—hence, rabbits developed their characteristic long ears over the course of generations.
“Generations” is a key word here; for the most part, the Darwinian process of natural selection presupposes that a species has a lot of time—perhaps multiple generations—in which to bear offspring that might survive environmental change. Darwin couldn’t have predicted the magnitude and speed of the environmental changes facing the planet in the 21st century, which means that, for many species, there isn’t enough time to bear offspring with new, useful genetic traits. Kolbert demonstrates that the Sixth Extinction is, in effect, an extreme example of Darwin’s theory: a new and dramatic circumstance to which a significant portion of the world’s species cannot adapt, essentially condemning them to extinction. This is apparent, for example, in the rapid acidification of ocean water (caused largely by humans burning fossil fuels) that has caused creatures, such as barnacles and clams, to have thinner and more fragile exteriors. As the oceans are acidifying at a faster rate than barnacles and clams can reproduce, these species may go extinct before they can adapt to the changes to their habitats. Seen in Darwinian terms, life in the age of the Sixth Extinction is a “race against the clock”: species must reproduce quickly in the hope that some of their offspring will be able to survive the changing environment.
Ultimately, Kolbert argues that the Sixth Extinction is both highly Darwinian and beyond the scope of Darwin’s thinking. The process by which species are dying out reflects the basic rules of natural selection. However, Kolbert stresses that Darwin believed extinction to be a gradual process, not a sudden catastrophe that could wipe out the planet’s biodiversity. The difference between extinction as Kolbert understands it, and extinction as Darwin understood it, perhaps explains the tonal difference between Kolbert and Darwin’s writings. In On The Origin of Species, Darwin adopted an impartial, descriptive tone while describing the process of natural selection, reflecting his belief that natural selection was an inevitable part of life on Earth. Kolbert, on the other hand, seems to believe that the Sixth Extinction could be partly averted if humans were to change their behavior now. Due to this, she adopts a more forceful, insistent tone, and encourages her readers to find ways to preserve the world’s endangered species.
Natural Selection and Mass-extinction ThemeTracker
Natural Selection and Mass-extinction Quotes in The Sixth Extinction
The process continues, in fits and starts, for thousands of years, until the species, no longer so new, has spread to practically every corner of the globe. At this point, several things happen more or less at once that allow Homo sapiens, as it has come to call itself, to reproduce at an unprecedented rate. In a single century the population doubles; then it doubles again, and then again. Vast forests are razed.
The history of life thus consists of "long periods of boredom interrupted occasionally by panic."
By the middle of the nineteenth century, many of [Georges Cuvier’s] ideas had been discredited. But the most recent discoveries have tended to support those very theories of his that were most thoroughly vilified, with the result that Cuvier's essentially tragic vision of earth history has come to seem prophetic.
… if there were four extinct species, Cuvier declared there must be others. The proposal was a daring one to make given the available evidence. On the basis of a few scattered bones, Cuvier had conceived of a whole new way of looking at life. Species died out. This was not an isolated but a widespread phenomenon.
Darwin's familiarity with human-caused extinction is also clear from On the Origin of Species. In one of the many passages in which he heaps scorn on the catastrophists, he observes that animals inevitably become rare before they become extinct, "we know this has been the progress of events with those animals which have been exterminated, either locally or wholly, through man's agency." It's a brief allusion and in its brevity, suggestive. Darwin assumes that his readers are familiar with such "events" and already habituated to them. He himself seems to find nothing remarkable or troubling about this.
But how, then, to make sense of cases like the great auk or the Charles Island tortoise or, to continue the list, the dodo or the Steller's sea cow? These animals had obviously not been done in by a rival species gradually evolving some competitive advantage. They had all been killed off by the same species, and all quite suddenly—in the case of the great auk and the Charles Island tortoise over the course of Darwin's own lifetime. Either there had to be a separate category for human-caused extinction, in which case people really did deserve their "special status" as a creature outside of nature, or space in the natural order had to be made for cataclysm, in which case, Cuvier— distressingly—was right.
Darwin's successors inherited the "much slow extermination” problem. The uniformitarian view precluded sudden or sweeping change of any kind. But the more that was learned about the fossil record, the more difficult it was to maintain that an entire age spanning tens of millions of years, had somehow or other gone missing. This growing tension led to a series of increasingly tortured explanations. Perhaps there had been some sort of “crisis,” at the close of the Cretaceous but it had to have been a very slow crisis. Maybe the losses at the end of the period did constitute a "mass extinction."
Ammonites produced very tiny egg, only a few hundredths of an inch across. The resulting hatchling, or ammonitellae had no means of locomotion; they just floated near the surface of the water, drifting along with the current. Nautiluses, for their part lay very large eggs among the largest of all invertebrates, nearly an inch in diameter.
Ocean acidification increases the cost of calcification by reducing the number of carbonate ions available to begin with. To extend the construction metaphor, imagine trying to build a house while someone keeps stealing your bricks.
Thousands—perhaps millions—of species have evolved to rely on coral reefs, either directly for protection or food, or indirectly, to prey on those species that come seeking protection or food. This coevolutionary venture has been under way for many geologic epochs. Researchers now believe it won't last out the Anthropocene.
There are various ways to calculate migration rates: for instance, by the number of trees or, alternatively, by their mass. Feeley grouped the trees by genus. Very roughly speaking, he found that global warming was driving the average genus up the mountain at a rate of eight feet per year. But he also found the average masked a surprising range of response. Like cliques of kids at recess, different trees were behaving in wildly different ways.
How many species overall will be capable of moving fast enough remains an open question, though, as Silman pointed out to me, in the coming decades we are probably going to learn the answer, whether we want to or not.
You could be studying a chain of islands or a rainforest or a nearby state park, and you'd find that the number of species varies according to the same insistent equation: S = cA squared.
Smaller areas harbor smaller populations, and smaller populations are more vulnerable to chance. To use an extreme example, an island might be home to a single breeding pair of birds of species X. One year, the pair's nest is blown out of a tree in a hurricane. The following year, all the chicks turn out to be males, and the year after that, the nest is raided by a snake. Species X is now headed toward local extinction. If the island is home to two breeding pairs, the odds that both will suffer such a string of fatal bad luck is lower, and if it's home to twenty pairs, it's a great deal lower. But low odds in the long run can still be deadly.
Long-term relationships between pathogens and their hosts are often characterized in military terms; the two are locked in an "evolutionary arms race," in which, to survive, each must prevent the other from getting too far ahead. When an entirely new pathogen shows up it's like bringing a gun to a knife fight. Never having encountered the fungus (or virus or bacterium) before, the new host has no defenses against it.
If we look even farther ahead than Elton did—millions of years farther—the biological world will, in all likelihood, become more complex again. Assuming that eventually travel and global commerce cease, the New Pangaea will, figuratively speaking, begin to break up. The continents will again separate, and islands will be re-isolated. And as this happens, new species will evolve and radiate from the invasives that have been dispersed around the world. Hawaii perhaps will get giant rats and Australia giant bunnies.
If, on the other hand, people were to blame—and it seems increasingly likely that they were—then the import is almost more disturbing. It would mean that the current extinction event began all the way back in the middle of the last ice age. It would mean that man was a killer—to use the term of art an "overkiller"—pretty much right from the start.
It’s only fully modern humans who start this thing of venturing out on the ocean where you don't see land. Part of that is technology, of course; you have to have ships to do it. But there is also, I like to think or say, some madness there. You know? How many people must have sailed out and vanished on the Pacific before you found Easter Island? I mean, it's ridiculous. And why do you do that? Is it for the glory? For immortality? For curiosity? And now we go to Mars. We never stop.
The Neanderthals lived in Europe for more than a hundred thousand years and during that period they had no more impact on their surroundings than any other large vertebrate. There is every reason to believe that if humans had not arrived on the scene, the Neanderthals would be there still, along with the wild horses and the woolly rhinos. With the capacity to represent the world in signs and symbols comes the capacity to change it, which, as it happens is also the capacity to destroy it.
Certainly humans can be destructive and shortsighted; they can also be forward-thinking and altruistic. Time and time again, people have demonstrated that … they’re willing to make sacrifices on those creatures’ behalf.
Among the many lessons that emerge from the geologic record, perhaps the most sobering is that in life, as in mutual funds, past performance is no guarantee of future results.