<P> While environmental factors are significantly responsible for evolutionary change, they act merely as agents for natural selection . Change is inherently brought about via phenomena at the genetic level: mutations, chromosomal rearrangements, and epigenetic changes . While the general types of mutations hold true across the living world, in plants, some other mechanisms have been implicated as highly significant . </P> <P> Genome doubling is a relatively common occurrence in plant evolution and results in polyploidy, which is consequently a common feature in plants . It is estimated that at least half (and probably all) plants have seen genome doubling in their history . Genome doubling entails gene duplication, thus generating functional redundancy in most genes . The duplicated genes may attain new function, either by changes in expression pattern or changes in activity . Polyploidy and gene duplication are believed to be among the most powerful forces in evolution of plant form; though it is not known why genome doubling is such a frequent process in plants . One probable reason is the production of large amounts of secondary metabolites in plant cells . Some of them might interfere in the normal process of chromosomal segregation, causing genome duplication . </P> <P> In recent times, plants have been shown to possess significant microRNA families, which are conserved across many plant lineages . In comparison to animals, while the number of plant miRNA families are lesser than animals, the size of each family is much larger . The miRNA genes are also much more spread out in the genome than those in animals, where they are more clustered . It has been proposed that these miRNA families have expanded by duplications of chromosomal regions . Many miRNA genes involved in regulation of plant development have been found to be quite conserved between plants studied . </P> <P> Domestication of plants like maize, rice, barley, wheat etc. has also been a significant driving force in their evolution . Research concerning the origin of maize has found that it is a domesticated derivative of a wild plant from Mexico called teosinte . Teosinte belongs to the genus Zea, just as maize, but bears very small inflorescence, 5--10 hard cobs and a highly branched and spread out stem . </P>

The evolutionary trend from the ancestral algae to the more recently evolved seed plants shows