The transformation of domestic plants and animals from age-old selective breeding to todays genetic engineering has had an enormous impact on all living things on the planet.
One of the most remarkable features in our domesticated
races is that we see in them adaptation, not indeed to the animals
or plants own good, but to mans use or fancy.
Sitting high atop a hill with an unbroken view to the west, Seattles Harborview
Hospital enjoys a commanding vista of the vast city sprawling along the shores
of the inland waterway known as Puget Sound. On clear days the rugged Olympic
Mountains float above the far western horizon, seemingly trying to snare
the setting sun with their jagged rocky tops, while to the south Mt. Rainier
looms like some upstart Baskin Robbins flavor of the month. Perhaps no other
public hospital in the world enjoys such a magnificent setting.
Such bucolic pleasures are largely lost on Harborviews clientele, however. Most who come here are beyond caring about such things, for those passing
through these doors usually do so only as a last resort. And they do not
come alone. The transients, drug addicts, gunshot victims, and uninsured
who make up a large percentage of Harborviews patients often bring with
them exotic new strains of microbes, organisms that are assuredly only
Beginning in the middle part of the twentieth century, scientists and doctors waged a campaign of eradication against bacterial illnesses, using the then newly developed antibiotic drugs. The result was a mass extinction of bacteria a concentrated interval of death resulting in the loss of uncountable individual microbes, and for all intents and purposes, the extinction of whole species. Smallpox, rabies, typhoid, rubella, cholera: the ancient microbial scourges of humankind were wiped out. The bacteria causing these ancient plagues were faced with two alternatives: evolve or die. Most died. But a few evolved forms resistant to antibiotic drugs. Fifty years after their invention, these miracle drugs have unleashed a diversity of new drug-resistant species that would never have evolved but for the influence of humanity. These are surely but the beginning of a host of new microbial species.
And so too with the biosphere, except that the antibiotic is us. As a result of our antibiotic influence, and the current pulse of extinction it has engendered, many currently living species will die. Some, however, will survive and thrive, becoming the rootstock of a new biota. Some have already done so, for one of the precepts of this book is that significant portions of the recovery fauna that follows any mass extinction are already with us, and dominating terrestrial habitats, in the form of domesticated plants and animals. Evolution obviously continues, but much of it is now directed for human purposes, or occurs as a by-product of human activities.
Charles Darwin began his On the Origin of Species with a chapter on domestication.
Before introducing any other data or argument, he pointed to the many varieties
of domesticated animals and plants as one of the clearest proofs of the
existence of organic evolution in this case, the evolution of new types
and plants bred to serve as food or as companions to humanity.
As with most of his conclusions, Darwin was right about this point. But we can take it one step further. Domesticated animals and plants are the dominant members of what can be called the recovery fauna accruing, directly or indirectly,
from the extinction of megamammals (and that extinctions impetus toward the development of agriculture). That many of these animals are taking the functional place of extinct or endangered megamammal species is no accident. The cows, pigs, sheep, horses, and other familiar domesticated animals now covering the worlds grasslands rapidly replaced the many species of extinct or endangered large wild grazers. The stimulus to that evolutionary changeover has, of course, been the stern hand of humanity.
Although most animal species can be tamed, or to some extent habituated
to the presence of humans if raised by them from a young age, domestication
goes far beyond this simple behavior modification. Domesticating a species
requires not only a concerted effort over extended periods of time, but certain
pre-existing features of the species in question. In the past this effort
was made only for reasons such as enhanced food yield, transportation, or
protection from predators. Domesticated animals are the evolutionary results
of human-induced unnatural selection.
Very few large mammals have been domesticated. Biologist Jared Diamond of UCLA showed that of the 150 species of terrestrial, noncarnivorous mammals larger than about 30 kilograms, only 14 have been domesticated. All but one of these originated in the Eurasian region; the only New World exception to this rule is the llama. All of the domesticated species are derived from wild species with similar characteristics: all seem to show rapid growth to maturity, an ability to breed in captivity, little tendency to panic when startled, an amenable and tractable disposition, and a social structure and hierarchy that permits domestication. All of these characteristics were further selected for by a brutal form of natural selection: those individuals that showed the favored characters were allowed to breed; those that did not were killed.
Interestingly, neurologist Terry Deacon has noted a further characteristic: all domesticated animals appear to have undergone a loss of intelligence compared with their wild ancestors. This observation makes one wonder whether humans, compared with their ancestors, have also been domesticated, and undergone a similar reduction in intelligence.
The first domesticated species may well have been the dog. All modern dog species seem to be derived from the Asiatic wolf. Although the first anatomically
Foremost among the survivors of the next millennium will be those species that humans have had a hand in developing: domesticated plants and animals.
modern dogs may date back as far as 12,000 years ago, bones of canids recognizable
as dogs first appear around human campsites between 7000 and 6000 B.C.
Even though wolves can still interbreed with dogs, implying that true biological
speciation has not occurred, behavioral differences make such interbreeding
rare; thus the modern dog varieties as we known them are functionally distinct
from wolves. The domestication of the dog also led to distinct anatomical
changes. Compared with wolves, dogs have smaller skulls, shorter jaws,
smaller teeth, and pronounced variation in coat color. Dogs also appear
to be less intelligent than wolves. Most dog varieties now recognizable
were produced in the eighteenth and nineteenth centuries; prior to that,
dogs generally were used for hunting (hounds) or tending flocks (sheepdogs).
The bones of domesticated livestock first appear in the fossil record slightly later than those of dogs. Sheep and goats came first; the earliest evidence for their domestication, dated at around 8000 B.C., comes from various sites in southwestern Asia (the areas now composed of Israel, Iran, Jordan, and Syria). Cattle were derived from an entirely extinct species of wild cowlike creatures. Domesticated pigs also date back to about 8000 B.C. Four thousand years later, the domesticated horse was developed from wild horses in Eastern Europe. (The ancestor of the domestic horse, called Przewalskis horse, still exists, in small numbers, in reserves in Poland.) Donkeys, water buffaloes, and llamas were domesticated at about the same time, while chickens and camels were not brought into the menagerie until about 2500 B.C. A wide variety of smaller pets have been domesticated as well: house cats, guinea pigs, rabbits, white rats, hamsters, and various birds. All are the result of human effort.
In almost every case, the transformation of a wild species into a domestic species involves substantial physical and behavioral modification. It has long been thought that this process occurred in stages, beginning with taming and progressing to gradual genetic transformation. How such great genetic change was forced in such a short period of time has always been a puzzle. New research by geneticists may have provided an answer. There appears to be a master gene controlling a complex of genes that in turn affect tameness, reaction to stress, coat color, facial morphology, and social interactions. By changing a single gene rather than an entire complex, domestication could proceed relatively quickly.
While it may be argued that the near coincidence of the beginning of agriculture
and the end of the Age of Megamammals is just that coincidence a strong
argument also can be made for cause and effect. The extinction of many of
the larger animals upon which humans depended for food, coming as it did
with a major climate change (affecting plant and small animal resources also
used for food) may not have been mere chance.
While cereals such as wild wheat and barley were harvested as much as 12,000 years ago, it seems that the first domestication of plants took place about 10,000 years ago, at the time when the last mammoths, mastodons, and many other larger animal species were dying out in North America and had just disappeared in Europe and Asia. This was the time when food-gathering peoples began to collect the seeds of wild plants and replant them in the ground. The domestication process appears to have involved the natural hybridization of several wild species, followed by selection by humans for desired characteristics. Thus domestication of plants, like that of animals, involved the genetic modification of the wild species through a very rough form of natural selection: those plants with usable traits were kept; those without were killed. Since the trend in plant modification has been toward an increase in the size of the edible or usable parts, most plant species have lost the ability to disperse widely, and protective mechanisms such as thorns have generally been lost as well.
The number of domesticated plant species is relatively quite small. There are more than two hundred thousand species of angiosperms, or flowering plants, yet only ten of these provide the vast majority of human food. Among these ten are grasses and cereals such as wheat, rice, and maize, which are all characterized by seeds rich in starch and protein. Cereals are planted on 70% of the worlds cropland and produce about 50% of the calories consumed by humanity. Other plants in the top ten include sugarcane, yams, potatoes, bananas, soybeans, and manioc. Worldwide, about three thousand species of plants are used as human food, but only about two hundred of these have become domesticated.
The genetic engineering our ancestors used to introduce new characters into their agricultural crops and domestic animals was crude but effective: save the favored
varieties and let them breed; kill off the others. But in the twentieth century
a new type of genetic engineering appeared one that alters the genomes
themselves. This new way of introducing novelty is sweeping the agricultural
regions of the Earth, and will surely have unintended consequences. It
may be that the transgenic revolution will bring novelty into the biotic
world in ways almost unimaginable and not all of them desirable. It appears,
for example, to be on the verge of creating superweeds.
Modern genetic technology allows the transfer of genetic material from one species to another. This new genetic information is permanently integrated into the genome of the second species, conferring new traits upon it. For all intents and purposes, a new type of organism is let loose into the biosphere each time this is done. The organism thus transformed is called a transgenic plant, animal, or microbe. These transgenic creatures have not arisen through the natural processes of evolution, but they are among the most portentous developments for the future of evolution on this planet.
Transgenic organisms are possible because of the existence of certain genes capable of jumping from one chromosome to another. The first discovery of jumping genes was made by American geneticist Barbara McClintock in the 1940s. McClintock was studying the genetics of maize (corn), and observed that certain genes, such as those responsible for seed color, were capable of moving from one chromosome to another. The significance of this discovery was largely overlooked until the 1970s, when it was independently rediscovered by other researchers examining the ways in which certain bacteria develop resistance to antibiotics. The genes, or sections of DNA, coding for these specific characters in bacteria do not actually jump; instead, they produce copies of themselves, which are inserted at other points either on chromosomes or in genetic code-carrying organelles called plasmids.
The discovery of these jumping genes, technically called transposons, unleashed a torrent of research in the 1980s and into the 1990s. These peculiar strings of DNA have the ability to repeatedly cut and paste themselves into different parts of an organisms genetic code. What made them famous and may eventually make them infamous is that the transposons of one organism can be used to paste new genetic information into the DNA of entirely unrelated organisms.
Much of the research using transposons was conducted on fruit flies. The fruit fly Drosophila is one of the stalwarts of experimental genetics, since it breeds
quickly and its genetic code is well known. In the early 1980s Gerald Rubin
and Alan Spradling of the Carnegie Institute discovered a transposon in
Drosophila that could be used to incorporate new genetic information into
these flies. They succeeded in changing the genetic code of the transposon
and re-inserting it into the fly. With this operation, they had succeeded
in creating a fly with a new genetic code that could be passed on to a
succeeding generation. They had created a transgenic species one entirely
new to the world, a species with a genetic code created not by nature,
but by science.
These early experiments altered very little in the new fly. The majority of its genetic code was the same as that of the unaltered species. But certain characters, such as color or eye type, could be changed. Further work showed that certain fruit fly transposons were not only useful in changing the genetic code in fruit flies, but could be placed into entirely unrelated species. A method had been developed that allowed true genetic engineering of insects.
The goals of genetic alteration of insects are laudable. Insects create havoc in human society in two ways: they serve as vectors of diseases (e.g., malaria, yellow fever, and some types of sleeping sickness), and they consume a large proportion of human crops. Genetic engineering is attempting to mitigate both of these problems. Nevertheless, the results of this work have been slow. In certain disease-spreading mosquitoes, geneticists have so far been able to change eye color, but they have yet to alter the structures involved in spreading disease-causing microbes. To further aid this process, geneticists have infected the target insects with viruses that act like transposons. The virus, once in the body of the insect host, can alter the way a disease is passed on. Some crop pests, such as the Mediterranean fruit fly and the screwworm, have been successfully targeted using transgenic or other genetic techniques (e.g., producing sterile members of a species that spread among the viable members of the population).
While transgenic techniques are just coming into use in controlling insect pests, such tools are already used widely in crop plants. Genetic engineering has succeeded in adding new genes to the DNA of various crops, whereas conventional breeding only adds variants to an already existing genetic complex. So, while domestication has enhanced the valued characteristics of many plant species, transgenic research has added entirely new characteristics, such as greater tolerance of heat and drought, greater resistance to insect predation and disease, and greater yield.
The genetic engineering techniques used in agriculture are widespread and
sophisticated. Genetic engineers can move genes from virtually any biological
source into crop species. Genes have been added to engineered crops from
organisms as diverse as chickens, hamsters, fireflies, and fish, as well
as from a slew of plant and microbial species. The new transgenic plants
are genuinely novel organisms, some of them containing the genetic codes
of plants, animals, and microbes in a single species.
The addition of new genes to various plant species has yielded spectacular dividends in terms of crop yield. But this new technology also poses substantial risks and clearly will affect the future of evolution on this planet. The creation of new plant types could affect the biosphere in various ways. The most important of these is the possibility that newly inserted genes will jump to other, nonengineered species (such as weeds) or move out of the agricultural fields altogether into wild native plant populations. It is this potential intermixing of new genes with those of already established plant species beyond the boundaries envisioned or designed by agricultural scientists that could have the most interesting and potentially biosphere-altering effects. Under rare circumstances, if new traits of transgenic species escape into the wild, weeds adaptively superior to native plants could be created. Since most of the traits being transferred into transgenic crops, such as hardiness, resistance to pests, and growth rate, increase their fitness relative to the original species, there is great potential for transgenics to become, or to help produce, new weedy species.
There are several avenues by which the genes of transgenic crops can become established in the wild. First and simplest, the transgenic crop itself can escape and become a weed. Second, the transgenic crop can release pollen into the wild, which can be incorporated into a wild relative of the original transgenic host. The incorporation of the new genes into the wild plant creates a new transgenic weed.
Weeds have many definitions, which are often colored by human values. In agriculture, they are plants that occur in the wrong place at the wrong time (some plants are weeds in certain situations and favored crops in others lawn grass, for example). A more human-specific definition of a weed is any plant that is objectionable to or interferes with the activities or welfare of humans. Nevertheless, weedy species have a number of characteristics:
Their seeds germinate in many environments
Their seeds remain viable for long periods of time
They grow rapidly
Their pollen is usually carried by nonspecialized pollinators or by the windThey produce large numbers of seeds
They produce seeds under a wide range of environmental circumstancesThey usually show vigorous vegetative reproduction or regeneration from fragments
They often compete either by choking out other plants or by producing toxic chemicals deleterious to other plants
From this list, we can see that the characters of weeds are also highly
desirable in crop plants. One goal of transgenic technology, therefore, has
been to confer characteristics of weeds on crop species. Transgenes spliced
into crops may alter traits such as seed germination ability, seed dormancy,
or tolerance of either biotic or abiotic factors such as pests, drought,
heat, or disease, creating a more persistent or resistant species in the
process. Such new traits may enhance the new crops ability to invade other
habitats. Genes affecting seedling growth rates, root growth rates, and drought
tolerance are currently being developed.
Jane Rissler and Margaret Mellon of the Union of Concerned Scientists have studied the ecological risks of transgenic crops in thorough detail. One of their most important concerns relates to the transformation of nonweedy crop species into weeds through genetic engineering. They note that a widely held notion is that changing a non-weed into a weed involves the conversion of many genes, not just the two or three currently used in transgenic crops. Changing corn from a crop to a weed, for example, would involve a number of genetic changes, since corn is one of the most human-dependent (and thus intolerant) plant species on Earth. Other crops, however, already possess many weedy traits, and thus one to three new traits could indeed create a new weedy species. Examples include alfalfa and other forages, barley, lettuce, rice, blackberries, radishes, raspberries, and sunflowers.
Transgenic species may produce secondary effects as well. The invasion of transgenic plants into new habitats affects not only the invaded plant populations, but the entire ecosystem, including the suite of animals living within that ecosystem. Perhaps more dangerous than the conversion of crop plants into weeds is the escape and transference of new genes into already existing weeds, making them superweeds. The transfer of disease resistance or pest resistance to established
weedy species may change a familiar weed into an even more formidable pest.
Weeds that have developed resistance to herbicides because of escaped or
transferred transgenes are already appearing in some parts of the world.
Agribusiness, which counts transgenic technology as the jewel in its technological crown, is in the business of feeding the worlds humans and making profits in doing so, of course. One of the great fears of those producing transgenic crops is that farmers will simply take the seeds from the first crop and use them ever after, and will not need to buy new seeds from the corporation that produced them in the first place. Like the software industry, which fears the copying of its products above all else, the major biotechnology companies dealing in transgenic crops have been searching for some way to stem the illegal use of their products after the first purchase. The solution is something known as the terminator gene.
The first terminator gene was produced by the large American biotechnology firm Monsanto, and was engineered to protect Monsantos patent rights on several types of transgenic crops. It is a genetic modification that prevents seeds from germinating after the season in which they were sold. In the works are genes that will allow but a single crop and not produce future seeds a bit like the seedless watermelon, but more efficient.
The great fear is that such terminator genes will jump to unmodified varieties of crop plants. If the terminator gene in a tomato plant jumped to other varieties of tomatoes, there a is real potential of plants never producing the crop they were intended to produce.
Our species has learned how to circumvent the normal rules of evolutionary change: we have learned how to build new species. Have we also achieved the ability to alter those rules? Norman Myers of Oxford asks this question in his prescient and disturbing 1998 paper, The Biodiversity Crisis and the Future of Evolution. Myers makes a subtle but important point: humans pose pronounced threats to certain basic processes of evolution such as natural selection, speciation, and origination. Myers has cried wolf before, but the wolf was always there, dining happily on flocks of the worlds species. Is he being alarmist in this case? Although many have warned of a biodiversity crisis, Myers alone warns of an evolution crisis. He bases his conclusion on two perceptions: first, that we have entered a new phase of mass extinction, and second, that the normal recovery
period from mass extinction will not pertain to this one; in fact, the recovery
will be considerably delayed.
Myers cites three aspects of this particular mass extinction that will affect its evolutionary outcome (and which make it different from any mass extinction of the past):
Its onset was extremely fast (compared with those of the past), within a single century or two, and thus there will be scant opportunity for ecosystem reorganization and evolutionary response.
There is currently a higher biodiversity on the planet than at any time in the geologic past, so that if 50% of species are lost, the total number going extinct will be higher than in any mass extinction of the past.During past mass extinctions, plant species have been largely spared, but that may not be the case in the current mass extinction.
The current mass extinction may be unique not only in what it kills, but
in how its recovery proceeds. In past times the tropical regions of the world
have served as storehouses for recovery. Because they have always held the
greatest diversity of species on the planet, they have long served as evolutionary
powerhouses areas that seem to spawn new species and new types of species
at a higher rate than other parts of the world. Paleontologist David Jablonski
of the University of Chicago has shown that innovation can be related to
geography. Innovation in evolution is the appearance of evolutionary novelty,
and the tropical regions seem to be home to more innovation than other regions.
Yet the tropics are now the sites of the highest densities of humans and,
the greatest human population increases. This pattern may curtail the evolution
not only of new species, but also of new types of species.
The current crisis in biodiversity may also substantially reduce the number of new species evolving a large body size. Megamammals need very large habitat areas to survive; it may also be true that they need equally large areas to speciate. With the reduction of wild habitat, and especially free rangeland, virtually everywhere on Earth, there may be no way for large mammals and other vertebrates to produce new species. Therefore, a consequence of human population growth and habitat disturbance may be not only the extinction of large mammals, reptiles, and birds, but the inability of new large species to take their place, simply because the mechanism of speciation for large body sizes has been derailed by environmental fragmentation.
A large and vibrant community of conservationists, scientists, politicians, and laypeople are actively engaged in intensive efforts to preserve biodiversity. One of the most important such efforts is habitat preservation. Yet even the most Herculean of efforts will save only patches of habitat in a sea of agricultural fields and spreading human landscapes. As long as humanity rules, it is doubtful that hundreds of thousands of miles of unfenced, unimpeded native habitat will be available to replace the species already lost since the end of the Ice Age. This fact has led Norman Myers to pose the following questions:
Is it satisfactory to safeguard as much of the planetary stock of species as possible, or should greater attention be paid to safeguarding evolutionary processes at risk? This is an entirely new way of looking at the world not in terms of losing species, but in terms of losing pathways of speciation. Perhaps the motto should be save speciation rather then save species.
Of prime importance is the question of biodisparity the number of body types. There could be many species on Earth, but few body types. Is it enough to save a large number of species if we fail to save biodisparity as well?
Should the evolutionary status quo (the current makeup of the Earths biota) be maintained by preserving precise phenotypes of particular species that will enable evolutionary adaptations to persist, thereby leading to new species? For example, should two elephant species be maintained, or should we keep the option of elephant-like species in the distant future?
Is there some minimum number of individuals necessary not just for the survival of a species, but the survival of the potential for future evolution in that species? Should the slow breeders (the megamammals) be given greater attention than, say, the rapidly breeding insects? Are we in a triage situation?How do we assess the relative importance of endemic taxa as compared with evolutionary fronts such as origination centers and radiation lineages? Myers thinks it far more appropriate to safeguard the potential for origination and radiation than any individual species. Let endemic taxa go.
This last recommendation is heresy by the rules of modern conservation. It has long been argued that endemic centers those regions that contain species found
nowhere else are among the most important places to save. But Myerss point is that endemic centers exist because they have not produced large numbers of successful species. Endemic centers are often living museums of ancient species that do not have much potential for future evolution.
The vast human enterprise has created a new recovery fauna, and will continue
to provide opportunities for new types of species that possess weedy qualities
and have the ability to exploit the new anthropogenic world. Chief among
these will be those species best preadapted for dealing with humanity: flies,
rats, raccoons, house cats, coyotes, fleas, ticks, crows, pigeons, starlings,
English sparrows, and intestinal parasites, among others. These and our domesticated
vassals will dominate the recovery fauna. Among plants, the equivalents will
be the weeds. According to many seers, this group of new flora and fauna
will be with us for an extended period of time a time span measured in
the millions of years. And if humanity continues to exist and thrive (as
I believe it will), this recovery biota may dominate any new age of organisms
A sense of how long the recovery fauna may last was estimated in a disturbing paper published in the prestigious journal Nature in the spring of 2000. The authors, James Kirchner and Ann Weil, posed a question: how quickly does biodiversity rebound after a mass extinction? How long will the world exist at a very low biodiversity? The answer, it turned out, was far longer than anyone had heretofore estimated. By analyzing the fossil record of all recoverable organisms (compiled by the late Jack Sepkoski of the University of Chicago), Kirchner and Weil found that fully 10 million years passed by, on average, before the biodiversity of the world recovered to its pre-extinction values. Even more surprising than this long lag period between extinction and full recovery was their finding that it occurred whether the extinction was small or large. We paleontologists had assumed that the time to recovery would somehow correlate with the magnitude of the extinction that after a small extinction, the biosphere would recover quickly, and that it was only after the greatest of the mass extinctions that a long recovery period was necessary. But to the surprise of us all, Kirchner and Weil found this not to be the case 10 million years was necessary even after the smaller extinctions. They concluded their paper with the following passage:
Our results suggest that there are intrinsic speed limits that regulate recovery from small extinctions as well as large ones. Thus, todays anthropogenic extinctions are likely to have long lasting effects, whether or not they are comparable in scope to the major mass extinctions. Even if Homo sapiens survives several million more years, it is unlikely that any of our species will see biodiversity recover from todays extinctions.
It appears that our return to a new biota will take a long time after the mass extinction is finished. And what might that new fauna and flora look like? Some predictions can be made and such predictions are the subject of the next chapter.
| Biological Futures
|INTRODUCTION||The Chronic Argonauts||1|
The Deep Past: A Tale of Two Extinctions
|TWO||The Near Past: The Beginning of the End of the Age of Megamammals||37|
|THREE||Into the Present||47|
The Near Future: A New World
The First Ten Million Years: The Recovery Fauna
After the Recovery: A New Age?
The Future Evolution of Humans
Scenarios of Human Extinction: Will There Be an After Man?
Deep Time, Far Future