as much as 15–25% of the genome. Many plant secondary metabolites such as the colour and flavor components of saffron and the chemotherapeutic drug taxol are of culinary and medical significance to humans and are therefore of commercial importance. In plants they seem to have diversified using mechanisms such as gene duplications, evolution of novel genes and the development of novel biosynthetic pathways. Studies have shown that diversity in some of these compounds may be positively selected for. Cyanogenic glycosides may have been proposed to have evolved multiple times in different plant lineages, and there are several other instances of convergent evolution. For example, the enzymes for synthesis of limonene – a terpene – are more similar between angiosperms and gymnosperms than to their own terpene synthesis enzymes. This suggests independent evolution of the limonene biosynthetic pathway in these two lineages. Mechanisms and players in evolution While environmental factors are significantly responsible for evolutionary change, they act merely as agents for natural selection. Some of the changes develop through interactions with pathogens. 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. Polyploidy is a very common feature in plants. It is believed that at least half plants are or have been polyploids. Polyploidy leads to genome doubling, 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. It is not known though, why genome doubling is such a frequent process in plants. One possible 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, leading to polypoidy. 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 is less, 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 found clustered. It has been proposed that these miRNA families have expanded by duplications of