Botany online 1996-2004. No further update, only historical document of botanical science!
Evolutionis the process that led to the existence of the great variety of existing and extinct organisms. Nowadays, it is regarded as a fact, that each species has developed from more primitive predecessors, that complex systems evolved from simpler ones, and that the adaptation of organisms is continuously improved. Mankind recognized rather early in its cultural history that such thing as a heredity of characters exists, while the connection between heredity and evolution was not made out until much later. The belief that the variety of species existed right from the beginning of life and that the features of species are constant lasted till the last century.
Gradually, proof for changes during earth history accumulated and favoured the theory that the formation of living things is a historical process. The next section shows, how the ways in which such changes occur and why they occur at all were tried to explain. The theory of selection, that was finally drawn up by CHARLES DARWIN is today – after decades of controversial discussions – regarded as assured. It has to be viewed as one of the most important foundations of general biology.
It states the following, that will be explained in more detail in the next section.
Species are not immutable. They developed in a continuous succession of generations that began with the origin of life and finds its current expression in the manifold recent species.
Individuals of one species are not alike. Each feature of a species is subject to considerable variations.
Each individual is exposed to natural selection, also called selection pressure. Only those individuals adapted best to their environment have a chance to propagate and to survive.
2. and 3. show that the individual and not the species is the basic unit of selection. The often heard term ‘species conservation’ is therefore actually nonsensical. It makes, nevertheless, sense when applied to the survival of whole species, since the individuals of one species do usually have far more in similarities than differences and selection pressure may be exerted on the common characteristics, so that all individuals of a given species may be equally concerned.
Changes and restructuring of genetic information (mutations in the broadest sense and recombination) on the one hand, and selection on the other are the two equally important causes of evolution. Mutations are non-directional events while it seems that selection has an orientation. If evolution is regarded as a sum of small subsequent steps, and if it is tried to reconstruct the processes of evolution, directed tendencies can indeed be recognized. Selection is nevertheless always a process that orients itself on a given present state: everything that has no advantage in a given situation will not be developed any further and can actually also be lost. Selection is therefore not oriented on the future. Structures and functions that could be of value in the future have no selection advantage.
Evolution leads to an accumulation of valuable, i.e. effectively used genetic information causing an increasing complexity of advantageous structures and an improved success in reproduction also described as an increased fitness. Such tendencies are regarded as an advance. Advance is always based on change, but not every change is advantageous. Tendencies resulting an a poorer fitness are called regression. Evolution is an opportunistic principle, only a very small percentage of changes does actually lead to improvements. Most reduce the fitness. A grave change does often result in an inability to reproduce and is consequently lost. A slight reduction of the reproductive success may nevertheless spread in the gene pool of a given species and may over a period of several generations finally add to the extinction of the species. E. MAYR (1975) estimates, that the percentage of extinct species is as high as 99.99 percent.
Whether the particular change is of advantage, depends on the respective situation. A valuation has always to apply to a standard. Better or of advantage means more efficient, more often or more complex than before. The wide variety of different environmental factors has resulted in many different trends of evolution. The diversification of organisms is a consequence. During evolution, living things have occupied all habitats available on earth. Some of these, like symbiosis or parasitism developed themselves as a consequence of diversification.
The ability of organisms to gain, store and process evermore subtle information about the surrounding (environment) in order to be better equipped for highly probable and / or regularly occurring situations like, for example, the weather changes caused by seasons is regarded as highly developed. The deduction, that organisms were during evolution able to acquire an increased independence of their surrounding (environment), is nevertheless only true for the humans. They are the only species that is to a large extend able to control its surrounding and to change this surrounding according to its needs.
Plants perceive a rather limited amount of their surrounding’s parameters. Among them are light, gravity, moisture, fungal infections and several more. They react stereotypically, for example with differentiated growth or, in the case of protozoa, with active movements towards a source of light or another stimulus.
The gain of independence from one factor causes usually the dependence on a new one. The change from wind pollination to insect pollination is a suitable example. It can still be regarded as an advance, since it led both to a new success in pollination (it decreases the loss of pollen) and it offered the possibility to open up new habitats with just little wind. Wind-pollinated plants were now also able to form populations of widely spread individuals.
Research into evolution is, since quite a long time now, not concerned anymore with the question whether evolution did take place, but is interested in the details of how evolution occurred. It is mainly dependent on circumstantial evidence like fossils. The exact time of events will therefore never be known with absolute certainty. It is nevertheless possible to deduce the most probable sequence of events, when taking into account all existing evidence. Evolutionary research is an integrating science. Interactions between organisms, their adaptation to the inanimate world, and the organization and expression of their genome have all to be taken into account in order to understand causal connections. Among the most interesting problems are:
|Why do species exist?|
|How did species evolve?|
|On which mechanisms are the isolating barriers between species based?|
|What properties enable an individual (or a species) to survive in a constant or in a changing environment and how can it secure its propagation?|
|Why are some groups of organisms (taxa) more successful than others (i.e. they have more individuals or more species) and why occurs evolution faster in some groups than in others?|
|What is the function of polyploidy and chromosomal mutations?|
|Are there more possibilities to change a genome?|
|What is the importance of repetitive DNA?|
|How large is the percentage of actually used (valuable) genetic information when compared to the total genome of an individual?|
For the reconstruction of the past, evolutionary research depends on circumstantial evidence. The physico-chemical laws, on the other hand, have most likely always remained the same. If, in addition, we leave the period of the origin of living systems aside, then we can assume, that inheritance follows rules, that can be elucidated by studying living organisms. Also, we assume, that the diversity of environmental influences was formerly not larger than it is today.
>Numerous examples illustrate, that evolution is a continuous process. Partial developments can today, too, be observed or understood by experiments.
The fast progress in genetics is largely based on the choice of suitable test objects. The reasons for the choice of Drosophila melanogaster have already been given. Drosophila is nowadays also known to be an ideal object for evolutionary studies:
|Numerous Drosophila-species plus numerous species of tightly related genera exist.|
|Drosophila-species are in nature characterized by large numbers of individuals.|
|Drosophila-species occur world-wide in the most different biotopes.|
|All species have giant chromosomes, i.e. changes in chromosomal patterns are easily recognized.|
|Fast biochemical techniques (electrophoresis) for the simultaneous capture of allotypes and enzyme activities monitoring the state of genes of a great number of individuals exist.|
|The effect under laboratory conditions of certain selective factors can be imitated and analyzed quantitatively.|
Not all strategies that proved to be successful in one group of organisms are successful for others, too. Induction of polyploidy is, for example, an essential cause in the formation of many angiosperm species. In Drosophila, and in the animal kingdom in general, it plays no or only a very limited part.
Evolutionary research is interested in the development of the great variety of structures and species. The classification of these structures is the task of systematics, while the drawing up of practicable classification systems is done in taxonomy. Taxonomy tries to arrange organisms in natural systems mirroring the lineage and relationships of the single taxa as realistic as possible. Research into evolution is therefore an essential support of taxonomic decisions.
Essay:The National Academy of Sciences US - Teaching about Evolution and The Nature of Science