Briefly discuss the evolution of the following crops: (i) Brassica spp., (ii) Triticum aestivum, (iii) Nicotiana tabacum, and (iv) Gossypium hirsutum.

(i) Brassica spp.
The evolution of Brassica species, particularly those of agricultural importance, involves several key events. Brassica oleracea, for example, is the progenitor of many cultivated varieties such as cabbage, broccoli, and kale. The species originated from wild forms in the Mediterranean region. Through selective breeding and domestication, distinct morphological forms were developed. Brassica rapa, another important species, includes crops like turnips and bok choy. Evolution in Brassica spp. is characterized by interspecific hybridization and polyploidy, notably the formation of the allopolyploid species Brassica napus (rapeseed) and Brassica juncea (Indian mustard). These hybrids were formed through the crossing of B. oleracea and B. rapa, with subsequent chromosome doubling stabilizing the hybrids into new species.

(ii) Triticum aestivum
Triticum aestivum, commonly known as bread wheat, is a hexaploid species resulting from the hybridization of three different wheat ancestors. The evolution of T. aestivum involves the combination of the genomes from Triticum urartu (A genome), Aegilops tauschii (D genome), and an intermediate species with the B genome. The earliest forms of bread wheat were tetraploid (Triticum durum), which then underwent a second hybridization event to become hexaploid. This species was domesticated in the Fertile Crescent around 10,000 years ago. Selective breeding over millennia has led to the development of various wheat cultivars adapted to diverse climates and agricultural practices.

(iii) Nicotiana tabacum
Nicotiana tabacum, the species from which most commercial tobacco is derived, is a hybrid species resulting from the cross between two diploid species: Nicotiana sylvestris and Nicotiana tomentosiformis. This hybridization likely occurred naturally in South America. N. tabacum exhibits allopolyploidy, having multiple sets of chromosomes from its progenitors. The domestication of N. tabacum began with its use by indigenous peoples for ceremonial and medicinal purposes. It was later cultivated for commercial tobacco production, with selective breeding enhancing its yield, disease resistance, and leaf quality.

(iv) Gossypium hirsutum
Gossypium hirsutum, commonly known as upland cotton, is one of the most widely cultivated cotton species. It is a tetraploid species (4x) with a genome resulting from hybridization between two diploid ancestors: Gossypium herbaceum (A genome) and Gossypium raimondii (D genome). This allopolyploidization event led to the formation of the AADD genome configuration in G. hirsutum. The species was domesticated independently in the Old and New Worlds, with evidence of early cultivation in Mesoamerica and the Indus Valley. Through selective breeding, G. hirsutum has been developed into high-yielding cultivars adapted to various environmental conditions and cotton production systems.