Human Efforts in Plant and Animal Breeding and Their Impact on Speciation

Throughout the history of agriculture, planting and animal breeding have played a crucial role in shaping our food sources and domesticated animals. While many efforts in these fields aim to improve the traits of crops and livestock, a subset of these activities has led to the formation of new species. This article explores whether human breeding efforts in plants and animals have ever led to speciation and the mechanisms behind such transformations.

Human Intervention in Plant Breeding for Speciation

Plant breeders, including myself, work with various techniques to modify and enhance plant species. One method commonly employed is the creation of polyploids, which are organisms with more than two complete sets of chromosomes. This can be achieved by artificial doubling of chromosomes (diploid to tetraploid) or combining different species (allopolyploids) through interspecific crosses. Allopolyploids, for example, are plants resulting from the combination of two different species, such as wheat.

Using advanced techniques, it is possible to create haploid plants (with a single set of chromosomes) or chromosome-doubled plants (with two complete sets), effectively creating reproductively isolated plants from their parent species. Synthetic bread wheat, for instance, has been recreated by combining different ancestral wheat species through laboratory techniques. This demonstrates how human intervention can lead to the formation of new species by altering the genetic makeup and reproductive capacity of plants.

The Biological Species Concept and Its Limitations

The high school definition of a species, often referred to as the biological species concept, states that a species consists of populations of organisms that interbreed and produce fertile offspring. However, this definition can be problematic when applied to plants. Plant breeders typically use a more practical but subjective classification system known as the gene pool classification. This system groups plants with similar genetic characteristics, even if they do not strictly adhere to the biological species concept.

Harlan and deWet have extensively discussed the complexities and limitations of applying a strict biological species concept to crops. For domesticated crops, it is often impossible to define a species with objective and clear-cut criteria. The emergence of new species in crops can result from hybridization during domestication, which, while beneficial for improving traits, can blur the lines between species.

Speciation in Domesticated Wheat Species

Many examples of speciation have occurred in domesticated wheat species. Hybridization between different species during the domestication process has resulted in the formation of new species. For instance, hexaploid wheat (Triticum sp.) emerged through natural hybridization between tetraploid cultivated varieties and diploid wild species. The hexaploid wheat species T. aestivum (AABBDD) is believed to have arisen from a hybridization between T. turgidum (AABB) and the wild species Aegilops tauschii (DD). Similarly, T. zhukovskyi (AAAAGG) resulted from hybridization between T. timopheevii (AAAAGG) and cultivated einkorn T. monococcum (AA).

The fact that no wild forms are known for T. aestivum or T. zhukovskyi suggests that these hexaploid species are unique cultivars formed through hybridization. This process of hybridization, coupled with human intervention, has created new genetic combinations and reproductive isolation, leading to the formation of new species.

Polyploidy in Oca and Other Crops

Polyploidy, a condition with more than two complete sets of chromosomes, can also play a role in the domestication of crops. Oca, a South American tuber crop, and perhaps some other crops, may have originated through polyploidization during domestication. The transformation of teosinte into maize through the continuous interspecific crosses and hybridization process provides another example of how genetic changes can lead to the formation of new species.

Maize (corn) is derived from a teosinte species. Although there are distinct differences between the two, their genomes can interbreed, albeit with limited efficiency. The contribution of different teosinte species to the maize genome further illustrates the complexity and fluidity of species boundaries in the context of domestication and hybridization.

Conclusion

Through advanced breeding techniques, human efforts have indeed led to the formation of new species in both plants and animals. These processes, such as polyploidy and interspecific crosses, demonstrate the potential for human intervention to reshape the genetic and reproductive structures of organisms, leading to the emergence of new species. Understanding these mechanisms is crucial for advancing both agricultural and evolutionary biology.