Friday, December 9, 2016

The Domestication Syndrome

Humanity’s transition to agriculture in the last 10,000 years could not have occurred without the domestication phenomenon. Domestication of animals and plants was associated with the transformative revolution in agriculture which led to significant human population gains. Background for this discussion is our recent post of 11/26/16:

Fundamental changes in animal and plant life were accomplished by a process of deliberate human selection over time. Therefore, scientists know both the cause and effect of domestication, but this leaves many important questions unanswered. It has been reported that 148 animal species were candidates for domestication but only 14 have actually been successfully domesticated. Some animals, for example, zebras, tigers, and rhinoceros do not make good candidates for domestication, but modern cattle, chickens, and horses and a few other species are the result of an ancient domestication process.

Indians of the New World domesticated over 300 food plants hundreds or thousands of years before explorers from the Old World discovered New World lands. Old World residents soon learned New World secrets of plant domestication. Today four billion people consume primarily a plant-based diet while two billion rely mostly on meat-based nutrition. Statistics tell us the extent and importance of domestication: Human biomass (mass of all humans on earth) is roughly half the biomass of domesticated livestock. The biomass of wild mammals is now less than 10% of domesticated livestock. Twelve species of domesticated plants provide an 80% share of Earth’s crops. The phenomenon of domestication fueling the world’s agricultural revolution is vitally important as we consider the welfare of the world’s teeming billions.

Many modern studies have affirmed the genetic basis of domestication. Looking at the chromosomes of living organisms in our cells’ nuclei, we see that genes are superimposed upon the chromosome structure, and that alleles are superimposed on genes. Different alleles from two parents provide diversity of traits. As humans observed the products of reproduction, they selected the traits they found most desirable for future generations. The original domesticators of prehistory were completely unaware of the biological wonders taking place at the level of unseen atoms and molecules in the living animals and plants surrounding them. The causes and effects were apparent to the domesticators, but even in our day geneticists still do not know how molecular entities at the level of cells actually produce their novel finished product—new tissues, new morphology, and new behaviors.

We illustrate with a mundane example from a diverse field. The sequence of events resulting in the construction and occupancy of a new residential dwelling may be described reasonably well by the layman who knows little about home construction. The prospective new owner must conceptualize, plan, communicate with a builder, design, and finance his new project, to outline a few steps in the event sequence. A skilled contractor must purchase material, assemble the concrete, wood, and steel and follow the building blueprint in an organized manner at the appropriate location. Then, and only then, is the structure ready for production. The new owners are free to observe and understand what is happening at the level of production. Our family observed the production process for building three different new homes in the past 45 years. Not having been a skilled master of building construction, I was able, however, to observe the production process and describe the building and installation process reasonably well after visits to the active construction site. Skilled workers used bodily and electrical energy applied to drills, hammers, saws, and a variety of other tools to assemble our home.

Returning to the cause and effect chain of domestication by which our planet’s animals and plants have been altered to promote human population growth ever since the agricultural revolution, we introduce the term domestication syndrome (DS) to the discussion. Domestication syndrome is a “group of traits” observed to occur together in domesticated animals and plants. At the risk of oversimplification, we mention a few examples of the “group of traits.” In animals, domestication results in tameness, docility, coat color changes, smaller jaws, floppy ears, tooth size reduction, cranio-facial morphology, and changes in body chemistry. In plants, domesticates produce seeds on plants easier to harvest, cultivate and sow. These plants provide uniform seed germination and ripening. In terms of other physical traits, domesticated plants have improved taste, fruit size, color, and shape.

We provide several examples to prove the point concerning the domestication syndrome. Modern dog breeds provide the best example. All dog breeds are technically the same species and can interbreed. The uniqueness of dozens of individual canine breeds is an extreme example of selection resulting in the domestication syndrome described above, a suite of characteristics common in some degree to all domesticated animals. Dog breeders exercise wisdom concerning the prevalence of low genetic diversity in dog breeds. Phenotypic traits of dogs are governed by a very small number of genes. Sometimes unfortunate defects result from over-manipulating the limited genome of dogs. In contrast, human genetic traits are exceedingly diverse and result in hundreds of characteristics that create wonderful diversity in humanity.

With respect to plant life, we illustrate by envisioning a beautiful, ready to harvest field of corn from the US Midwest during October. At the beginning of the human agricultural revolution no plant crop resembled modern domesticated corn even remotely. The Creator intelligently designed living things to provide the potential for modern harvests in both the animal and plant world. Modern agriculturalists must recognize the perils of domestication along with its benefits. This topic is the subject of much study by biologists.

Finally, we return to the causes and effects of the molecular processes inherent in the production of a domesticated animal or plant. Causes? Scientists have determined that the selection process is the cause of domestication. Effects? Likewise, scientists have discovered the effect of selection: the domestication syndrome described above. Our questions relate to just how thousands of molecules scurry about to assemble new proteins and produce new organs in the bodies of the domesticated animals or plants, or to accomplish changes in their behavior. How are these changes accomplished?

Many hypothesized evolutionary processes, including the processes of domestication, are explained with naturalistic statements that, “Causes explain processes.” I propose that citing causes does not explain the processes involved in domestication. Our Creator designed intelligent strategies for accomplishing domestication of animals and plants. The processes by which domestication is accomplished are spectacular, but not always explainable merely by citing a cause.