CORRELACIÓN ENTRE LA COMPOSICIÓN DE LA UVA Y LA RELACIÓN SFT/P

Additive speciation is the commonest mode of speciation, which adds to the diversity of living organisms. Mayr (1963) suggested the occurrence of reductive speciation, whereby two previously independent species fuse into a third, new species, themselves becoming extinct. Hybridization likewise produces new species but this always leads to an addition in the number of species.

It is impossible to imagine that two evo- lutionary species can actually fuse to pro- duce a third species and themselves become extinct. This may happen in a particular re- gion, but not over the entire range of these species. The various modes of additive spe- ciation are described below:

1. Allopatric speciation: Lineage inde- pendence and consequent speciation result from geographical separation of lineages, i.e. the actual physical sepa- ration of two relatively large popu- lations of a single species. Over a period of time, such separation would enable these geographical races to de- velop and maintain gene combinations controlling their morphological and physiological characters. The develop- ment of reproductive isolation would sooner or later result in the establish- ment of distinct species (Figure 6.2 B). Allopatric speciation may also result from the development of new species along the boundaries of a large central population. These marginal populations (races) get separated from the main population during environ-

mental differentiation. They then undergo adaptive radiations to develop physical and physiological differences, which sooner or later get genetically fixed (ecotypes). With further morpho- logical and physiological differentia- tion, they form distinct varieties (or subspecies). Development of reproduc- tive isolation establishes these as dis- tinct species that will retain their identity even if a future chance should draw them together (Figure 6.2 A). 2. Allopatric introgressive speciation:

Although origin of species through hybridization is commonly results from sympatric species, examples of speciation involving two allopatric spe- cies, which had contacts in the past, are also reported. Quercus brandegei, now confined to Cape Region of Baja California, extended to west in Terti- ary times and had a narrow zone of contact in Edwards Plateau escarp- ment area, with Q. virginiana of S. E. Coastal plain of U.S.A. Allopatric introgression between the two species occurred at the contact zone, but the genes spread slowly because of limited contact of parental species and pre- dominantly rhizomatous propagation. The introgressed population, now in contact with only Q. virginiana, is suf- ficiently stabilized to be classified as a distinct species Q. fusiformis. 3. Allo-parapatric speciation: Such

speciation occurs when two populations of an ancestral species are separated, differentiate to a degree that is not sufficient for lineage inde- pendence, and then develop lineage independence during a period of parapatry (limited sympatry). It differs from allopatric speciation in the sense that speciation is completed after a period of sympatry and the process of attaining lineage independence is potentially reversible because it is possible that two partly differentiated populations could form a single evolu- tionary lineage showing clinal varia-

tion after they meet rather than the period of sympatry reinforcing differ- ences between them.

4. Parapatric speciation: This occurs when two populations of an ancestral species differentiate despite the fact that no complete disjunction has oc- curred. The daughter species may share a small fraction of their respec- tive ranges and interbreed within this narrow contact zone and yet still dif- ferentiate.

5. Stasipatric speciation: This is simi- lar to parapatric speciation except that it results from spontaneous chromo-

somal modifications. The resultant chromosome arrangement must be fully viable in the homozygous state but of reduced viability in the hetero- zygous state.

6. Sympatric speciation: The examples of sympatric speciation due to hybridi- zation and apomixis have been dis- cussed under abrupt speciation. The process of ecological sympatric speciation is a slow one of gradual speciation. The ecological differences in the habitats result in adaptive radiations in populations which gradu- ally evolve into new species.

Over the last few decades, the affinities be- tween plant groups have been redefined as more and more information is accumulated from various sources. Newer approaches in recent years include (a) increasing reliance on phytochemical information (Chemotax- onomy); (b) studies on ultrastructure and micromorphology; (c) statistical analysis of the available data without much a priori weighting and providing a synthesis of all the available information (Taxometrics); and (d) analysis of phylogenetic data to con- struct phylogenetic relationship diagrams (Cladistics). The aforesaid disciplines con- stitute the major modern trends in tax- onomy. Data continues to flow from differ- ent disciplines, so that the process of analy- sis and synthesis is an ongoing activity. Tax- onomy (Systematics) is as such a field of unending synthesis. The following disci- plines have contributed to a greater or lesser extent to a better understanding of taxo- nomic affinities between plants.

MORPHOLOGY

Morphology has been the major criterion for classification over the last many centuries. The initial classifications were based on gross morphological characters. During the

last two centuries, more and more micro- scopic characters of morphology were incor- porated. Although floral morphology has been the major material for classifications, other morphological characters have also contrib- uted in specific groups of plants. The diver- sity of morphological features has already been discussed in detail under Descriptive terminology in Chapter Four.

Habit

Life-forms—though of little significance to taxonomy—allow a means of estimating adaptiveness and ecological adjustment to the habitat. In Pinus, bark characters are used for identification of species. Woody and herbaceous characters have been the pri- mary basis of recognition of Lignosae and Herbaceae series within dicots by Hutchinson (1926, 1973).

For several decades it was believed that trees or shrubs with simple leaves repre- sented the most primitive condition within angiosperms. Increased evidence over the last decade, however, is pointing towards the assumption that the perennial herbaceous condition in paleoherbs such as Cerato- phyllaceae, Nymphaeaceae and Piperaceae represents the archetype of the most primi- tive angiosperms.

Chapter 7