ECOL3453 - Crop Ecology

Do crops represent a particular taxonomic grouping? - No.

The 127 crops, on which Table 1 is based, are from about 31 families and 91 genera, plus or minus a few each to allow for nomenclatural uncertainty. Including the minor crops treated in Simmonds (ed.)(1976) Evolution of Crop Plants, the numbers rise to 230 from 64 families and 180 genera. Although our crops are systematically diverse, they only represent about one sixth of the families of the Magnoliophyta and a smaller fraction of the 3000+ genera.

Most of the potential new crops come from families already providing crop species especially the three families that make up the order Fabales - Fabaceae, Caesalpiniaceae and Mimosaceae, i.e. the legumes.

The diversity reflects the diversity of human needs for the products. Broad categories of use are:- cereal, pulse, tuber, vegetable, oilseed, fruit, flavour, drug, industrial, fodder, fibre, and ornamental.

Every agriculture needs cereals, tubers, vegetables, fruits and fibres - these were developed from whatever plant species were available. So, important cereals and tubers originated from independent sources in widely seperated places showed parralel features. E.g. the tuberous aroids (Araceae) e.g. eddoe (Colocasia esculenta var. antiquorum), the yams (Dioscoreaceae) e.g. greater yam (Dioscorea alata), and the potato (Solanum tuberosum) from the Solanaceae.

Some families have produced several important crops:-

• Poaceae - cereals, fodders, ornamentals, sugar juices, perfume oils.
• Leguminosae (if considered a single family) - pulses and fodders.
• Brassicaceae - fodders, vegetables and oil seeds.
• Solanaceae - vegetables, tubers, drug plants and ornamentals

others only one, or two:-

• Musaceae - bananas including plantains
• Myrtaceae - cloves
• Lauraceae - avocado*

Multiple uses for a single crop species are not uncommon:-

• Linum usitatissimum - flax, linseed
• Cannabis sativa - hemp, drugs, pharmaceuticals
• Beta vulgaris - leaves, roots, sugar
• Brassica oleracea - vegetables including:- broccoli, cabbage, cauliflower, brussels sprouts, kohlrabi, and sprouting broccoli

Evolution

The main features of evolution, as applied to diploids, are as follows. Plant species commonly become geographically differentiated into subspecies, ecotypes, etc. Variability is maintained by heterozygosity supplemented by gene flow between populations; and heterozygosity is adjusted by various cytological and genetical mechanisms. Reproductive isolation between populations may lead to speciation and, since it generally develops gradually, speciation is a continuous rather than a discontinuous process. At any one time, a group of related plants is likely to comprise distinct biological species, which do not naturally hybridise or do so only with difficulty, and a variety of intra-specific groups between which various levels of genetic exchange are possible and do occur. In summary, adaptation is procured by successive gene substitutions in evolving populations, leading to local differentiation and, ultimately, to speciation.

Plants, unlike most animals, have another evolutionary resource - polyploidy. Polyploidy may sometimes be selectively advantageous per se. More generally its significance lies in:

• facilitating recombination that would be restricted at the diploid level
• in permitting adjustment of the mating system towards inbreeding
• in offering (in allopolyploids) an opportunity for permanent interspecific hybridity
• in offering (again in allopolyploids) an opportunity for long-term diploid differentiation by way of adjustment of duplicate loci.

The essential feature of micro-evolution therefore emerges as gene substitution; polyploidy is important (as shown by its frequent occurrence in wild plants) but only as a preliminary to gene substitution processes which would not otherwise have been possible. Cultivated plants obey the same evolutionary rules as wild ones.



* An account of domestication and early selection of the avocado was given by Smith (1966) Econ. Bot. 20:169-175.