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The Modern Synthesis

• Intraspecific Patterns of Variation
• Phenotypic and Genetic Variation; Ecotypes
• Karyotypes: Chromosome Numbers, Chromosome Shapes, and Phylogenetic Relations
• Polyploidy
• How to Define a Species ? - Origin of new Species - Hybridization - Reproductive Isolation - Adaptive Radiation
• Literature
  

Though C. DARWIN made the basic attempts to explain the origin of the diversity of species in 1859 already, he knew little about the actual mechanisms of speciation. He regarded the inheritance of characteristics as a fact, but had no idea of its causes. He used the term selection, but could not quantify it. His ideas about the efficiency and the velocity of evolutionary processes remained vague. He was able to interpret the extinction of non-competitive species and the selection of useful adaptations, but could neither explain the emergence of new species, nor the development of new adaptations.

With the advent of modern genetics at the beginning of the 20^th century, all these questions became the subject of new approaches. T. DOBSZHANSKY and J. HUXLEY presented the first comprehensive surveys on the connection between genetics and evolutionary biology in their works

Just like most other scientific disciplines, evolutionary biology progressed through several stages, and its discoveries depended to a large extent on the progress of other fields.

1. The second half of the 19^th century was still concerned with intraspecific variability and the impact of the environment on the phenotype.

2. During the 20^th century, the variability of a species’ genotype became a focus of interest.

3. The discovery of the constant species-specific numbers of chromosomes by E. STRASBURGER lead to the question, whether the karyotype might provide information about the systematic position of a species.

4. When polyploidy and hybridization of wild progenitors were recognized as the basic mechanisms of the formation of cultivated plants (cultivars), the next step was to examine, whether the hybridization of species is of a general importance for plant speciation in nature, too. It emerged, that speciation and the stability of new species are not caused by just a single factor. A number of factors have to coincide, before a new combination of genes becomes a new species. These questions led to a reconsideration of the term species.

5. Many problems avoided in classical genetics and cytology are now approached with the methods of molecular biology. Among these methods are the visualization of chromosomal segments by the banding technique, the quantitative determination of a species’ DNA, and the calculation of the ratio of coding and non-coding DNA. Indeed, these new techniques helped elucidating certain evolutionary trends, that had escaped notice of more traditional procedures, at the molecular level.

Besides these questions, evolutionary processes were tried to capture by model construction and mathematical analysis, an approach that came from population genetics. R.A. FISHER’s work

(published 1930, reprinted in 1953) became an early standard.

The origin of life, the development of increasing complexity and performance, i.e. the progress of evolution are as important problems as speciation leading to taxonomic and systematic topics.