Textbooks often don't discuss extinction - the death of all members of a species - in any detail. No surprise there, it's a frustrating and depressing topic.
Frustrating because museums would bid billions to bring back a live tyrannosaur. And depressing because good answers are often not available. So discussion can lurch dangerously into the realm of folklore.
When that happens, folklore wins hands down over fact. The extinction of the entire superorder of dinosaurs [1] which marked the close of the Cretaceous era - perhaps mainly due to an asteroid hit - has become a pop culture icon that now supports a variety of views and causes.
Pop culture need not - and does not - address the real history of life. For example, the extinction of all species of trilobite, the signature fossil of the Cambrian era, goes largely unnoticed simply because trilobites never became a pop culture icon.
In any event, when discussing extinctions, competent and honest scientists can reach different conclusions. [2] But understanding the history of life requires that we grapple with what we do know about natural* extinctions, as well as about the actual patterns of evolution (short periods when life forms undergo signfiicant change) and of stasis (vast periods in which nothing much happens to a life form).
The overwhelming majority of species that have ever existed on Earth are extinct [3 - see update note here as well], so we must assume that extinction is a normal event. One of the best treatments of extinction, easily grasped by interested readers, is Extinction: Bad Genes or Bad Luck? (New York: W.W. Norton & Company, 1991) by University of Chicago paleontologist David Raup. Raup makes a useful distinction between local and global extinctions.
Local extinctions: They said goodbye, and never told us what was wrong ...
Local or "background" extinctions may occur when a given species does not have the resources that enable it to adapt to a changing environment, and cannot solve the problem by migrating to a more suitable environment. This is especially likely if the species exists in low numbers. These types of extinctions are described in The Design of Life (as part of a discussion on speciation), as a loss of the ability to adapt to the environment:
If a population is unable to expand into new environments or adapt to changing conditions, it is likely to remain small and may even go extinct. Small population size is a danger to any species. When an organism mates, it contributes a sperm or an egg to the offspring. The gametes contain only half of the organism's genes. Thus, when mating occurs, the partners contribute only half of their total complement of genes (excluding sex-linked alleles). By having a large number of offspring, organisms ensure that most of their genes will be expressed through mating (although only half of their total genetic complement is contributed to each offspring, it is a different half each time.)
Consequently, the larger the number of offspring, the greater the number of gene combinations and the greater the percentage of the gene pool preserved. It follows that a low reproductive rate will increase the probability that genetic information will be lost. Such loss of information reduces the polymorphism of the population. If it continues,. the species' ability to adapt to changing environments will be lost and the species itself may become extinct. Rare or endangered species often become extinct due to a loss of genetic diversity and an attendant reduction in polymorphism. (pp. 35-36)
Consider, for example, an island plant species with only a few living members, adapted to a tropical environment. No living member has resources - at any stage of its life cycle - for surviving sub-zero temperatures. A series of cold snaps on the island can extinguish all members of that species.
According to Raup, the average animal or plant species lasts about four million years [4].
Thus, only about one species in four million dies a natural death each year. If forty million species live today, only ten will go extinct in an average year. Species extinction without human influence is rare. ....
- David M. Raup, Extinction: Bad Genes or Bad Luck? (New York: W.W. Norton & Company, 1991), p. 108.
One frustrating consequence for the biologist is that unless the life form is being extinguished by human activities, the extinction may not be detected and may not lend itself to systematic study.
Global extinctions: They ALL said goodbye - just like that!
Rapid environment changes affecting large areas of the planet, possibly triggered by catastrophes, may cause global extinctions, that is, extinctions of a large number of species all at once. As Lee Siegel reports at Space.com, paleontologists recognize five major, generally accepted extinction events, in which large numbers of species went extinct over a short period of time (as opposed to a species or two going extinct each year due to local causes). For example,
The Permian-Triassic catastrophe was Earths worst mass extinction, killing 95 percent of all species, 53 percent of marine families, 84 percent of marine genera and an estimated 70 percent of land species such as plants, insects and vertebrate animals.
After each extinction, the surviving species find their niches in a new ecology that may be greatly changed. However, only about 5% of extinctions occur in global catastrophes. [5]
Why did they just leave like that?
Do we know why most extinct species died out? According to David Raup, we do not:
"The disturbing reality is that for none of the thousands of well-documented extinctions in the geologic past do we have a solid explanation of why the extinction occurred. We have many proposals in specific cases, of course: ... These are all plausible scenarios, but no matter how plausible, they cannot be shown to be true beyond reasonable doubt. Equally plausible alternative scenarios can be invented with ease, and none has predictive power in the sense that it can show a priori that a given species or anatomical type was destined to go extinct."
David M. Raup, Extinction: Bad Genes or Bad Luck? (New York: W.W. Norton & Company, 1991), p. 17.
So, in general, we know that a species goes extinct when it cannot continue to adapt to the demands of its environment. But that doesn't mean we usually know exactly how or why it can't adapt (hence all the "death of the dinosaur" folklore). We also do not usually or necessarily have the tools to predict the future chances of a current species (absent human intervention).
Do some extinctions target specific types of animals? Or bad genes?
Yes, sometimes. Raup says, for example,
The Ice Age extinction was selective in that it affected mammals far more than other organisms and in that extinction rates were far higher among large mammals than small. The cutoff for distinguishing large and small mammals is conventionally set at 100 pounds (44 kg) adult body weight. - David M. Raup, Extinction: Bad Genes or Bad Luck? (New York: W.W. Norton & Company, 1991), p. 91.
Could extinctions be caused by bad genes?
Raup thinks that a reasonable possibility. More than 6000 Cambrian species of trilobite, amounting to three quarters of all fossils found from 570–510 million years ago, have been catalogued But by the end of the Paleozoic era, 325 million years later, all were gone.
He uses a mathematical model and concludes,
“ ... that the trilobites were indeed doing something wrong (or that the other groups were doing something better). One vote for bad genes. This analysis does not tell us, of course, what the trilobites did wrong or what the other animals did better. But it is a start.
- David M. Raup, Extinction: Bad Genes or Bad Luck? (New York: W.W. Norton & Company, 1991), p. 102–3.
Well, that may be a start. It at least makes the whole thing discussable. Were they bad genes, or genes that became bad in a given context?
Are there implications for evolution as a whole from the history of extinction?
Raup raises one very interesting implication:
“Quite apart from the obvious ones for conservation biology, the history of the rain forests is pregnant with broader evolutionary implications. Suppose, for example, that we accept the proposition that rain forest area was reduced by 84 percent in the very recent past, as is indicated by Figure 7-2. Many extinctions would have occurred, some through the species-area effect and some because of the loss of species naturally confined to areas that lost forest. Remember that [137] many tropical species, particularly insects, are found only in one small area, perhaps a single tree.”
If we accept the foregoing, we are faced with the problem of explaining how a substantial portion of the tremendous diversity of the present rain forests evolved in less than fifty thousand years. This seems an incredible speciation rate and raises more questions than it answers.
The thin factual basis of the recent history of rain forests can be remedied, but only through intensive research programs that work with climatic history and paleontological evidence. Until this is done, the history of rain forests, and its evolutionary implications, will remain enigmatic.”
[Note: Raup is NOT talking about current deforestation but about Pleistocene dry periods, which are not reasonably supposed to be caused by human intervention.] -
Raup ends his 1991 book on a curious note, attributing to Darwin's theory of evolution powers he does not actually discuss in his own book - yet he then reserves a key judgment:
Is extinction through bad luck a challenge to Darwin’s natural selection? No. Natural selection remains the only viable, naturalistic explanation we have for sophisticated adaptations like eyes and wings. We would not be here without natural selection. Extinction by bad luck merely adds another element to the evolutionary process, operating at the level species, families, and classes, rather than the level of local breeding populations of single species. Thus, Darwinism is alive and well, but, I submit, it cannot have operated by itself to produce the diversity of life today.
- David M. Raup, Extinction: Bad Genes or Bad Luck? (New York: W.W. Norton & Company, 1991), p. 192.
Interesting! What precisely does he think "Darwin's natural selection" cannot have done? Compare this with what he says in the quotation just above, about the Pleistocene rain forests.
In his day, Raup was taking a huge risk by even suggesting that Darwinism might not be true, so he wisely merely provides facts that dispute it - and then covers his tracks with a resounding promotion of Darwinism in the areas of study that he does not address in any detail in his book.
Sometimes, that is the only way to get key information across.
*Note: In this article, we are not here discussing extinctions caused by human mismanagement of an environment, but rather the loss of all members of a species due to natural causes. Human-caused extinctions are, of course, a pressing issue. Wildlife biologists may spend their careers assessing population numbers. However, the story of population numbers (and population management) can often be more complex than it first appears. Here is an article from Canada's Maclean's Magazine which illustrates the uncertainties in determining, for example, whether the number of Canada's polar bears is actually declining (as often claimed) and how best to manage their numbers in any event. Can species be brought back from extinction? Yes, Canada's bison, for example, have made a healthy comeback. See also the interesting puzzle of the beefalo.
[1] Some argue that the dinosaur did not really become extinct because birds are surviving dinosaurs. That view is well supported but also ably contested. See also a summary of arguments on both sides.
[2] “Many paleontologists have strong views on the issue of the duration of mass extinctions. Competent and honest scientists look at the same data and reach opposite conclusions.” - David M. Raup, Extinction: Bad Genes or Bad Luck? (New York: W.W. Norton & Company, 1991), p. 79.
[3] David M. Raup, Extinction: Bad Genes or Bad Luck? (New York: W.W. Norton & Company, 1991), p. 3. He calls it a "truly lousy" survival record. Update note February 10, 2008: However, that another paleontologist has written to us to say: ... the estimates of extinct species have never been rigorously calculated. They depend on a number of assumptions, including the assumption that past ecosystems have similar degrees of completeness as modern ones. Current paleontological data do not bear out high estimates. Morevoer, most studies (begun with the seminal work of Sepkoski, and also his colleague David Raup) have shown increasing diversity towards the present, even when corrected for the higher probability of discovering more complete ecosystems in more recent strata, which would have less chance of destruction by geological and erosional processes. So the estimates for extinct species' diversity make risky assumptions, and have consistently been underperformed by the actual data.
[4] David M. Raup, Extinction: Bad Genes or Bad Luck? (New York: W.W. Norton & Company, 1991), p. 6.
[5] David M. Raup, Extinction: Bad Genes or Bad Luck? (New York: W.W. Norton & Company, 1991), p. 118.