The external environment of a given region induces variation in plant form.
The internal environment of the plant - the multi-step process by which the coded information in a plant's genes is converted into the various components in its cells - is another source of variation.
Numerous genes are responsible for the difference in leaf and plant shapes between species. While the number and relative effect of genes can be determined through quantitative analysis, the identity of all of these genes is still unknown.
In the development of dissected leaves, like those of this Japanese maple (Acer dissectum 'Inaba Shidare'), the formation of leaflets and lobes happens only during a brief, early phase in the development of the cells of the leaf margins.
Compound leaves are thought to be either elaborations of simple leaves or fundamentally different. Certain genes are known to regulate plant development and when the coded information they contain is abnormally converted in cells they can cause changes in leaf shape, one example being excessive leaf compounding in tomatoes.
Genetic differences within a species are the result of gene flow and selection pressure.
Gene Flow
Gene flow, through sexual means between populations of the same plant species, is basically restricted by the distance that their pollinators travel. This tends to prevent genetic differentiation because the exchange of genes between plants caused by pollinators only re-mixes the genes that already exist, sometimes in new ways, but using pre-existing genes. Over time, the offspring of these populations become genetically similar unless new sources from different environments are added to the mix.
This doesn't mean that groups of plants of the same species remain similar in appearance because the recombination of existing genes can result in mutations and other novelties that promotes variability within a plant population. Many of our most interesting ornamental cultivars are the result of this type of gene flow; a different leaf or flower form discovered by an observant gardener in an otherwise homogeneous group of plants in the garden. Different forms can also be purposely bred by hand pollination. When these sports are grown on to establish their ability to retain the new feature over time and propagation, we have a new cultivar for our gardens.
Selection Pressure
Gene flow can be restricted by selection pressures that include environmental barriers, latitude, altitude, climate, soil and land surface features (physiography), local pathogens, plant eating creatures and pollinators.
Different selection pressures between locations encourage variations in the appearance or function in plants that are the same genetically. Some of the characteristics that can be modified by environment are:
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