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Basics of Characters • A taxonomic character is any expressed attribute of an organism that can be evaluated and that has two or more discontinuous states or conditions - for example the number of petals on a flower - can be in 3's, 4's or 5's - thus distinct states and they are discontinuous • The taxonomic value of a characteristic is increased if the biological significance of the characteristic has been determined
Anatomy (Internal) • Anatomical characteristics are of most importance in studies for classification – especially when trying to determine relationships among large groups of plants, often at higher taxonomic levels - they are less often used for identification and aren't often mentioned in floras
Physiology • Physiology is less important in classification, but can be of use for classifying parasitic and saprophytic plants which lack chlorophyll and thus don't do photosynthesis - this separates the dodders (genus Cuscuta) from morning glories (Convolvulaceae)
Indian pipe - Monotropa uniflora- parasitizes mycorrhizal fungi
Chemistry • Chemistry can be important because some plants produce distinctly different chemical compounds - aromatic mints (Lamiaceae) and the parsleys (Apiaceae) were separated into unique groups by Dioscorides in the 1st century A.D. due to aroma "chemistry"
Starch grains from Sorghum (left) and Fescue (right) - Poaceae
Ecology • Ecology and geography of plants provide data of much relevance to plant taxonomy because each taxon exhibits a certain pattern of distribution which is one aspect of its definition • Coincidence or not of areas occupied by related taxa has bearing on classification of a group - especially when considering its evolutionary history
Patterns of Geographic Distribution • Geographic differentiation exists between taxa at all levels and in all degrees of spatial separation - effective spatial separation varies greatly in absolute terms because different taxa may possess quite different abilities to migrate, either as pollen or as seeds • Phytogeography is the study of patterns of distribution by plants
Patterns of Geographic Distribution • Allopatric taxa occupy mutually exclusive geographic areas - i.e. they do not overlap in their distributions • Sympatric taxa occupy overlapping geographic areas • Sympatric taxa usually show different types of genetic, ecological and structural differentiation from those shown by closely related allopatric taxa - this is because geographic separation is a reproductive isolating mechanism and is effective in maintaining species differences
Floristic Elements • If the distribution of a large number of taxa is analyzed, certain geographic patterns are found to recur consistently - such recurring patterns and the taxa which exhibit them are called "floristic elements" • In Illinois the tall grass prairie could probably be divided into 3 floristic elements - wet prairies dominated by sedges, bluejoint reedgrass, New England aster, marsh muhly grass, cowbane, prairie cordgrass • mesic prairies dominated by big bluestem, prairie dropseed, sky-blue aster, showy sunflower, false sunflower, Indian grass • dry prairies dominated by little bluestem, big bluestem, needlegrass, heath aster, prairie coreopsis, prairie goldenrod
Geographic vs. Ecological Distribution Patterns • Geographic distribution – based on region/area of occurrence • Ecological distribution – based on habitat of occurrence • Sometimes may be hard to distinguish
Disjunct Distributions • Most taxa are fairly continuous throughout their region of distribution, but some have distribution patterns which are interrupted by considerable areas from which the taxa is absent - such patterns are said to be disjunct distributions • Disjunct distributions can arise in several ways – • 1) long range dispersal of a taxon from one area to another • 2) current distributions represent relics of former, wide continuous distribution patterns where the intervening areas have lost the members of the taxa
Disjunct Distributions • 3) it is possible that the taxon arose independently in the disjunct areas by convergent or parallel evolution - usually this is not thought to be likely, especially if potential ancestor species are not present
Spartina maritima • Spartina maritima (Small Cordgrass) is native to the coasts of western and southern Europe and western Africa, from the Netherlands west across southern England to southern Ireland, and south along the Atlantic coast to Morocco and also on the Mediterranean Sea coasts. There is also a disjunct population on the Atlantic coasts of Namibia and South Africa - Wikipedia
Vicariance • Another geographic distribution pattern is vicariance - when two similar taxa occupy separate geographic (or ecological) areas – • the distinction between disjunct and vicariant taxa is subtle
Vicariant Distribution of Cedrus Distribution: Cedrus atlantica in red, C. brevifolia in blue and C. libani in purple
Endemism • Taxa which occur in single restricted geographical areas are known as endemics • Endemism is a relative concept, but is normally applied only where there is considerable restriction in area of distribution • Often the term is used in the same way as indigenous which means native to an area, but to a taxonomist, it should just mean geographically restricted
Senecio cambrensis – groundsel - Asteraceae S. vulgaris
Centers of Diversity • If the distribution for every species in a genus is drawn on a map, it is usually found that there are one or more areas with a concentration of species - these areas are called centers of genetic diversity for that genus • You can usually draw similar maps for higher levels of taxa as well • Usually there is just a single center of diversity for a particular taxa and you find progressively fewer species as you move away from the center of diversity
