Extinction (Sort of) Explained

Standard textbooks on evolutionary biology and paleontology

hardly mention extinction. Much is said about the origin of

species and the evolution of species once they are formed, but

discussions of extinction are usually limited to casual references

and the enigma of the great mass extinctions. On causes of extinction

we are apt to read, "Species become extinct when population

sizes drop to zero," or "Species die out if they are unable to

adapt to changing conditions." These statements are true, of

course, but are virtually devoid of content.

-- David Raup, 1991

Everybody knows what extinction is. The dictionary defines it as "the act

of extinguishing, or, the fact of becoming extinguished or extinct." (Extinguish

is in turn defined as "to put out a fire, a light, or to bring an end to.")

More to our point, Thain and Hickman's Penguin Dictionary of Biology

(1996) defines extinction as "Termination of a genealogical lineage. Used

most frequently in the context of a species, but applicable also to populations

and to taxa higher than species." Thus the fundamental precept of

extinction is self-evident: a species (or population, genus, or family) is extinct

when its last member has died.

Yet extinction theory is greatly complicated by a number of factors,

among them the inability of biologists and paleontologists to agree on exactly

what a species is. "Alarmingly," noted Purvis, Jones, and Mace (2000),

"there are over 20 species concepts now in common use." For living animals,

we recognize as separate species those that are morphologically

similar but cannot interbreed. The same criterion obviously cannot be applied

to fossils, so paleontologists have to make use of anatomical differences

and similarities -- when there is enough fossil material to make a

determination.

In recent times, however, with the introduction of DNA analysis,

what was long believed to be a single species can now be fragmented into two or more. Killer whales, the most widely distributed of all cetaceans,

found from southern polar waters to the Arctic and many places in between,

were once thought to be a single worldwide species -- Orcinus orca.

New observations and analyses have shown that there may be an Antarctic

species that is quite different from its northern counterparts in coloration

and behavior; it has been provisionally named O. glacialis. Where

there were once believed to be six species of balaenopterid whales (blue,

fin, sei, Bryde's, and two species of minke), two more were added as of

2003 (see pp. 254–55). The two distinct species of elephant -- African and

Asian -- have been subdivided into three with the recent addition of the

genetically distinct Bornean elephant; there may actually be as many as

six different subspecies. The gorilla may indeed be not one (Gorilla gorilla)

but two closely related species. In a New Scientist article dated November

22, 2003, Bob Holmes and Jeff Hecht wrote, "A new trend is to delineate

species as evolutionarily separate lineages, including separated populations

that are evolving in divergent ways. This has already happened for

albatrosses. There are 13 recognized species, but the IUCN* lists 21 threatened

lineages." On the other hand, it has recently been shown that the animal

known as the red wolf, which was once awarded the species name of

Canis rufus, is not a separate species at all, but a hybrid of the gray wolf

and the coyote.

Even if we are able to plug in an acceptable definition of a species,

however, identifying the moment that it became extinct is much more

problematic. For example, rumored sightings of such animals as the ivorybilled

woodpecker and the Tasmanian tiger continue to circulate, and

while these animals are generally considered extinct, it is impossible to

state unequivocally that a few stragglers may not be found in their often inaccessible

habitats. The 1938 discovery of the coelacanth, thought to have

been extinct for 75 million years, is the paradigmatic case of a rediscovered

"lost" animal, and the unexpected appearance of the previously unknown

megamouth shark in Hawaiian waters in 1975 indicates how difficult it is to

make categorical statements about existence or nonexistence. If it is so dif-

ficult with modern animals, imagine how hard it is to decide from fossil evidence

alone that a particular species became extinct at a particular moment. There are no more trilobites, pterosaurs, or ichthyosaurs, but

when did the last one die?

Extinction is one of the most powerful forces on earth, and one of the

most enigmatic. It affects every species that has ever lived, and has eliminated

most of them. In time, it will eliminate us too. Despite its tremendous

importance, however, nobody is quite sure what it is or how it works.

We know that there have been countless numbers of living things that

have walked, run, crawled, flown, swam, or just remained stationary over

the past 3 billion years, and that the great majority of them are gone; but

beyond that, we know very little. The fossil record not only supports the

all-encompassing theory of evolution, demonstrating conclusively that life

changes over time, but it is also our primary evidence for extinction. Because

so many creatures are no longer viable, extinction can be clearly read

in the fossil record, although the actual evidence of evolution -- "change

over time" -- is only infrequently revealed.

Until the nineteenth century, almost everybody—scientists included -- accepted the traditional Christian view that the Bible was to be taken literally,

and that God had made the sun, the moon, the earth, and the oceans.

He also made all the mammals, birds, alligators, snakes, fishes, and insects,

but his crowning achievement was "to make man in our own image, after

our likeness; and let them have dominion over the fish of the sea, and over

the fowl of the air, and over cattle, and over every creeping thing that

creepeth upon the earth." Even Aristotle believed that the animals had

been divinely arranged in a ladder, with humans confidently perched on

the top rung, the epitome of life.