Introduction

The evolution of domesticated forms of plants involved the selection of traits that were suited to the human rather than the wild environment. The types of traits that are selected have been similar across different species plants giving rise to the concept of the domestication syndrome. Some controversy still persists about the nature of selection of such traits and the degree of human consciousness involved, although the majority of the field accepts that most traits were probably subject to unconscious selection. It has become apparent in recent years that understanding the nature of the plurality of processes underlying the domestication syndrome is key to understanding the origins of domestication.

Definition

The domestication syndrome can be defined as the characteristic collection of phenotypic traits associated with the genetic change to a domesticated form of an organism from a wild progenitor form. The term “adaptation syndrome” as applied to traits automatically selected through a harvesting regime of cereals was first coined by Harlan (Harlan et al. 1973), and the phrase “domestication syndrome” was first coined by Hammer (Hammer 1984) a decade later. Typical syndrome traits include the loss of shattering (the ability for a plant to abscise and drop its seeds), changes in seed size, loss of photoperiod sensitivity, and changes in plant physiology and architecture. The concept was developed very much with cereals in mind but has been extended to other plant types as well, usually with traits that had already been recognized in cereals. For instance, tree species have often undergone a thinning of the pericarp under domestication, and pulse and tuber-forming species have reduced toxin content.

Key Issues/Current Debates/Future Directions/Examples

Understanding how the domestication syndrome traits arose has become key to understanding the origins of domestication itself. There are several reasons for this. Firstly, archaeological evidence in recent years has demonstrated a staggered appearance of traits (Fuller 2007) rather than the previous perception of an effectively synchronous rapid arrival of traits. In the case of cereals and pulses, seed size increases are seen some time before the loss of shattering for instance. This has served to emphasize the plurality of processes involved; different traits are selected by different particular pressures. Secondly, in parallel, in the genomic era of biology, the genetic mechanisms underlying traits are steadily being unraveled (Fuller & Allaby 2009). In some cases, traits are more or less monogenically controlled, such is the loss of shattering, and in other cases, there is polygenic control, such as in the case of seed size, often with pleiotropic effects of genes – genes being involved in more than one function in the plant. In particular, the network of genetic interactions within which a causative genetic locus sits is being better understood, such as the loss of an important component of the circadian clock (the molecular clock that measures day length that most plants and animals require) to achieve insensitivity to day length and so grow in new latitudes (Jones et al. 2008). In short, there is a growing appreciation of the interdependence of the genetic causes of the domestication syndrome, particularly with background genome function.

The drawn-out nature of the arrival of domestication syndrome traits has reopened the debate regarding the role of human agency. In particular, efforts are underway to explore the different human behaviors that give rise to the fixation of various traits (Fuller et al. 2010). Models are therefore being discussed which actively try to explore the coevolution of humans, both in terms of behavior and in physicality, and plants together. Naturally, this reopens the door to the debate about consciousness of selection, since it has been argued that while some less obvious traits may be unconsciously selected for, more obvious traits may be part of a different, conscious selection (Abbo et al. 2010). Either way, it is clear that the rise of the domestication syndrome involves both the evolution of plants and humans. This latter has been brought into sharp relief by recent studies on the increase in human alpha-amylase copy number believed to be an adaptation to an increased starch-based diet introduced by agriculture (Perry et al. 2007).

The problems of balancing the plurality and commonality of the causes of the rise of the domestication syndrome have been further highlighted by attempts to measure the strength of selection of very different traits directly from the archaeological record (Purugganan & Fuller 2011). We understand much more about the genetic basis for traits such as seed size (a complex polygenic-controlled trait) and loss of shattering (a simple monogenic trait) and have models about the different selection pressures that most likely drove their fixation (sowing and harvesting practices, respectively), which serves to underline the plurality of the system. However, surprising signs of commonality are also evident in that both traits appear to have been subject to very similar strengths of selection that are very low compared to what may have been previously supposed about the strength of artificial selection and in fact in the range found in instances of “normal” natural selection.

The future challenge is one of integration of quite complex but interdependent factors, outlined in brief here. It remains to be seen whether the domestication syndrome includes pluralities that can be truly separated out and applied in parallel across plant species or whether there are pluralities that have dependence on each other that can only move together.

Cross-References

Agricultural Practice: Transformation Through Time

Agriculture: Definition and Overview

Archaeobotany of Agricultural Intensification

Archaeobotany of Early Agriculture: Macrobotany

Archaeobotany of Early Agriculture: Microbotanical Analysis

Bananas: Origins and Development

Barley: Origins and Development

Breadfruit: Origins and Development

Childe, Vere Gordon (Political and Social Archaeology)

Childe, Vere Gordon (Theory)

Complex Hunter-Gatherers

Darwin, Charles R.

DNA Interpretation Constraints in Archaeology

Grapes: Origins and Development

Hunter-Gatherer Subsistence Variation and Intensification

Hunter-Gatherers, Archaeology of

Kuk Swamp: Agriculture and Domestication

Maize: Origins and Development

Manioc: Origins and Development

Millets: Origins and Development

Near East (including Anatolia): Origins and Development of Agriculture

Northern Asia: Origins and Development of Agriculture

Pigeon Pea: Origins and Development

Plant Domestication and Cultivation in Archaeology

Potato: Origins and Development

Rice: Origins and Development

Sweet Potato: Origins and Development

Wheats: Origins and Development

Yams: Origins and Development