Plant Breeding and Domestication: From Wild to Cultivated
Agriculture and Improvement
We live surrounded by a vast variety of agricultural products. Often unknowingly, we favor some and reject others based on personal preference.
Variety in plant breeding is defined as a group of individuals that are not uniform in various characteristics.
In the past, farmers made their own selections, keeping grains and livestock of their choice. Now, farmers are limited to using what breeders provide.
Humans have continuously selected for profit. Varieties arising from mutations in the wild may persist or not. Agriculture must be understood as a system that either maintains these varieties or doesn’t.
The roles of breeder and farmer were among the last to separate, but they did so to adapt to a changing situation. This led to the emergence of a new type of agriculture in the 18th century (more productive, technological, developed, and polluting) versus subsistence farming.
The breeder must work to simplify the work of the farmer. The breeder may not necessarily be a farmer themselves.
This decoupling allows for greater specialization. Modern farmers can focus on their operations, while breeders create more varieties to choose from.
Plant Breeding as an Activity
Plant breeding has been applied to numerous species, and genetic engineering may open unforeseen possibilities by disregarding barriers between species.
The plant material used in breeding includes both cultivated and wild forms:
- Products of farmer and breeder work: landraces, modern varieties, hybrids, etc.
- Forms from which they derive: wild or spontaneous species, including endangered ones.
- Materials of all kinds, from which useful traits can be extracted and engineered into any biological material.
From Wild to Cultivated: The Domestication Process
The transition from hunter-gatherer life to agriculture may have been driven by various circumstances, although necessity likely prevailed. This transition was only possible because some wild plants and animals changed significantly through constant contact with humans.
Agriculture existed because these changes were passed down through generations, even though humans didn’t understand the reasons.
Simply planting seeds collected from a wild plant places them in a drastically different environment than their natural one (more water, softer soil, protection from pests, more companions of the same species, etc.).
The plant will respond differently than its wild counterparts.
A cultivated plant is thus synonymous with a plant modified for human needs.
- Imagine collecting numerous wild plants and scattering their seeds near a village at the dawn of agriculture. On a specific date, only the plants that sprouted, flowered, and matured around the same time are harvested.
By repeating this process, the grower selects a portion of the population with a specific genetic makeup regarding its planting-to-maturity cycle.
Over several planting and harvest cycles, a specific maturity period is achieved. This creates a strong selection pressure, transitioning a wild species to a cultivated one.
This unconscious selection process is called automatic selection.
This entire process is called domestication, and the resulting plant is considered domesticated. (See diagram)
Changes in Plant Architecture
Domestication causes significant changes in plant architecture. Depending on the plant part subject to automatic selection, the results can vary:
- Loss of seed dormancy.
- Indehiscent fruits or infructescences.
- Determinate growth.
- Upright growth.
- Synchronous maturation.
- Sterile to fertile flower transfer.
- Increased inflorescence size.
- Decreased number of inflorescences.
- Larger seeds.
Plants and Animals
Plants have been domesticated more easily than animals. A certain capacity for taming is assumed to be necessary in animals.
In plants, there’s a spectrum of domestication degrees. Humans likely attempted to domesticate many species from both kingdoms, selecting those that responded best. Materials of interest for the future include ornamentals, medicinals, and industrial groups ranging from algae to orchids and various microorganisms.
Evolution of Domestication
Molecular biology reveals the narrow genetic base of cultivated plants, particularly in self-pollinating species, but less so in cross-pollinated species, which allow almost unlimited gene flow within the species. It appears that domestication often occurred from a single, or very few, wild populations.
However, cultivated forms of a species exhibit incredible diversity. The primary sources are mutation, recombination, and segregation, followed by genetic drift and selection.
In areas where a species was domesticated, there were no other gene sources besides the wild populations from which they were cultivated. Even in self-pollinating species, crosses occur, allowing hybrid formation.
Hybrid seeds were crossed with both wild and cultivated plants, leading to introgression of “domesticated” genes into the wild and “wild” genes (genetic diversity) into cultivated populations, which were later segregated and fixed by drift and selection.
Another pathway involves adaptation to agricultural environments. These plants resemble cultivated plants but retain wild characteristics (dehiscence, small seeds, dormancy, etc.). They behave like wild plants but thrive in cultivated settings without benefiting the farmer, instead competing for resources. These are called weed companions. They are a driving force for evolution but also difficult to control (e.g., wild rice).
Weed companions can only be eliminated through careful selection and planting in areas where the species has never been cultivated.
Primary, Secondary, and Transdomestication
Early farmers were meticulous about crop purity. For example, early wheat would be mixed with barley and other grasses. The species used allowed for separation of different types.
One such “impurity” in wheat fields was a grain now known as rye. It coexisted with wheat in the Middle East and migrated to other areas. In environments like the Asian steppes or Central European mountains (low limestone soils and cold temperatures), the proportion of rye increased until it became a new domesticated species, not through direct domestication from the wild, but from a previously domesticated material.
Thus, wheat is a primary crop, and rye is a secondary crop. Other examples include melon (secondary) and cucumber (primary), and sweet orange (secondary) and bitter orange (primary).
Transdomestication occurs when a new domesticated plant appears in a location different from the species’ origin, particularly on a different continent. For example, the tomato originated in South America, where it was cultivated, but wasn’t truly domesticated in Europe until the 17th century. (See drawing)
This illustrates that humans domesticated plants wherever they could.
Domesticated and Protected Species
There isn’t a clear distinction between cultivated and wild plants (varying degrees of domestication, weed companions, etc.). Protected species are those that coexist with humans without deliberate domestication efforts. For example, Rosa gallica grows wild and is left undisturbed. (See diagram)
Precursors: Darwin and de Candolle
Darwin coined the term “natural selection,” drawing a parallel with the artificial selection practiced by farmers. On the Origin of Species includes a significant discussion of artificial selection in plants and animals. In The Variation of Animals and Plants under Domestication, he examines the diversity of domesticated plants and animals, sometimes suggesting possible origins from wild species.
- As causes of evolution under domestication, he suggested:
- Segregation and the appearance of new plant forms, with traits fixed by artificial selection.
- Natural selection pressure: when individuals are in excess, only the fittest survive.
- While we now know that differences among individuals are due to mutations, Darwin attributed them to accumulation and reproductive and geographical isolation.
- He partially adopted the concept of use and disuse (Lamarckism), where acquired traits influence gametes.
De Candolle wrote Origin of Cultivated Plants, integrating data from botany, archaeology, history, linguistics, and ethnology.
He was the first to suggest that plant domestication wasn’t uniform in space or time (Australia, Patagonia vs. Middle East, China, etc.), anticipating Vavilov’s concept of centers of origin.
His criteria for studying the origin of a cultivated plant were:
- The presence of a wild relative in the hypothetical area of origin.
- Archaeological or paleontological evidence of the species.
- Confirmation from history, language, and related disciplines.
He explained the origins of 244 species.
Vavilov
Vavilov assembled large collections of varieties. Observing high variability for a given species in specific regions, he proposed that these areas could be the origins of cultivation. He called these centers of origin. Vavilov defined a variety as the difference in endemic forms, those unique to a region (e.g., wheat in the Iberian Peninsula vs. the East).
Regions rich in endemic forms were called centers of origin, and those rich in non-endemic forms were secondary centers.
Vavilov’s criteria were:
- Prior knowledge of botanical classification.
- Identifying past geographic distributions of the species and regions with the highest number of endemic forms.
- Examining traits from a genetic perspective. Primary centers were where species were domesticated, and secondary centers where they diversified.
He derived two conclusions:
- The law of homologous series: a trait in one species can be found in related species, suggesting a common ancestor.
- Emancipation of recessive traits: dominant alleles are found in the center of origin, while recessive alleles are found in peripheral or isolated areas.
- By mapping centers of origin for many crops, he identified key areas. He established primary centers of origin (where species were domesticated) and secondary centers (where they diversified).
He made some errors in assigning multiple centers of origin to the same species (e.g., wheat). Today, we refer to centers of diversity, which are areas where species flourished.
Modern Perspectives
Harlan observed high heterogeneity in the Middle Eastern center of origin and identified rich areas within it, which he called microcenters.
Detailed study of crop distribution patterns (e.g., sorghum in Africa) suggested that domestication wasn’t focal but diffuse and acentric.
He simplified Vavilov’s centers into three major centers and three large areas where domestication occurred throughout the region:
- Near East and Central Africa.
- North Central China, not Southeast Asia and the South Central Pacific.
- Mesoamerica, not the South American center.
Harlan’s system shows that centers coincide with areas of significant human impact.
Agricultural empires existed in the Middle East, the Valley of Mexico, and the Andean slopes of Peru.
In regions with dense populations, distinct centers are not observed, but rather diffuse domestication across time and species (African savannas, Polynesia).
Conclusion
The number and density of domesticated species are a function of human activity, not a predisposition of the land or climate. Areas richer in agricultural species:
- Domesticated plants and animals originated based on human needs.
- Domestication occurred where and with what was possible.
- Places of origin:
- China: soybean, apricot.
- Middle East: wheat, barley, olives.
- Mediterranean: lettuce, carrots, garlic.
- Importance:
Besides seeking desirable traits, early landraces or wild species hold valuable genetic diversity, primarily found in centers of diversity.
Initially, improvement focused on practical traits (disease resistance, vitamins, ease of harvest).
Brücher
Brücher argued that the origins of cultivation are diffuse in both space and time. A crop originating in a limited area can change significantly as it spreads due to the introduction of germplasm from wild relatives and selection pressures applied by different peoples.
Maize, for example, was domesticated in southern Mexico but exhibits great diversity in Guatemala, Colombia, and Peru.
He concluded that domestication wasn’t a simple process.
Crop Classification and Evolution
- Endemic: originating in a limited area and recently expanded.
- Semi-endemic: originating in a definable center with limited dispersal.
- Monocentric: a definable center of origin and wide dispersal.
- Oligocentric: a primary center of origin, wide dispersal, and one or more secondary centers of diversity.
- Noncentric: variety patterns suggest domestication across a wide area.
Periods of Improvement
- Automatic Selection:
The first varieties were nearly identical to their wild parents but differed in key traits, with their reproduction controlled by humans.
Natural mutation and crossbreeding with wild relatives created numerous differences among individuals.
Plants with advantageous traits like stronger stems, larger seeds, and robust spikes, or simply more aesthetically pleasing ones, were selected.
Improved varieties likely arose in different areas, with different starting materials and varying human preferences.
Local varieties, or landraces, were developed, maintained, and improved by farmers.
These landraces possess high genetic variability (mixed plants), representing a valuable gene bank for the future, highlighting the importance of their preservation. During this period, the farmer was also the breeder.
- Scientific Improvement:
Based on the discovery of plant sexual reproduction, crosses between varieties were made.
By the late 18th century, there was evidence of wheat hybridization.
New selection methods were explored, and the biological basis of heredity began to be studied (Mendel).
The establishment of seed and seedling trading houses was crucial.
Commercial houses eliminated the need for farmers to save their own seeds, impacting the number of local varieties as only the highest-performing ones were sold.
This established the separate profession of breeder.
- Modern Breeding:
The rediscovery of Mendel’s laws enabled the development of genetic improvement.
This allowed the creation of profitable and competitive varieties using techniques like chromosome manipulation and mutagenesis.
The term “variety” came to represent highly uniform crops.
Breeders select a trait and introduce it into their variety or line through crossing and subsequent selection.
In 1970, recombinant DNA technology allowed genes to be isolated and transferred between species (Green Revolution).