Modern evolutionary synthesis

The modern evolutionary synthesis is a union of ideas from several biological specialties which forms a logical account of evolution. This synthesis has been generally accepted by most working biologists. The synthesis was produced over about a decade (1936–1947), and the development of population genetics (1918–1932) was the stimulus. This showed that Mendelian genetics was consistent with natural selection and gradual evolution. The synthesis is still, to a large extent, the current paradigm in evolutionary biology.

Julian Huxley invented the term, when he produced his book, Evolution: The Modern Synthesis (1942). Other major figures in the modern synthesis include R. A. Fisher, Theodosius Dobzhansky, J.B.S. Haldane, Sewall Wright, E.B. Ford, Ernst Mayr, Bernhard Rensc, Sergei Chetverikov, George Gaylord Simpson, and G. Ledyard Stebbins.

The modern synthesis solved difficulties and confusions caused by the specialization and poor communication between biologists in the early years of the twentieth century. Discoveries of early geneticists were difficult to reconcile with gradual evolution and the mechanism of natural selection. The synthesis reconciled the two schools of thought, while providing evidence that studies of populations in the field were crucial to evolutionary theory. It drew together ideas from several branches of biology that had become separated, particularly genetics, cytology, systematics, botany, morphology, ecology and paleontology.

Modern evolutionary synthesis is also referred to as the new synthesis, the modern synthesis, and the evolutionary synthesis.

Fossil discoveries
In the past thirty or so years there have been excavations in parts of the world which had scarcely been investigated before. Also, there is fresh appreciation of fossils discovered in the 19th century, but then denied or deprecated: the classic example is the Ediacaran biota from the immediate pre-Cambrian, after the Cryogenian period. These soft-bodied fossils are the first record of multicellular life. The interpretation of this fauna is still in flux.

Many outstanding discoveries have been made, and some of these have implications for evolutionary theory. The discovery of feathered dinosaurs and early birds from the Lower Cretaceous of Liaoning, N.E. China have convinced most students that birds did evolve from coelurosaurian theropod dinosaurs. Less well known, but perhaps of equal evolutionary significance, are the studies on early insect flight, on stem tetrapods from the Upper Devonian, and the early stages of whale evolution.

A shaft of light has been thrown on the evolution of flatfish (pleuronectiformes), such as plaice, sole, turbot, halibut by recent work. Flatfish are interesting because they are one of the few vertebrate groups with external asymmetry. Their young are perfectly symmetrical, but the head is remodelled during a metamorphosis, which entails the migration of one eye to the other side, close to the other eye. Some species have both eyes on the left (turbot), some on the right (halibut, sole); all living and fossil flatfish to date show an 'eyed' side and a 'blind' side. The lack of any intermediate condition in living and fossil flatfish species led to questions and debates about the origin of such a striking adaptation. The case was considered by Lamark, who thought flatfish precursors would have lived in shallow water for a long period, and by Darwin, who predicted a gradual migration of the eye, mirroring the metamorphosis of the living forms. Darwin's long-time critic St. George Mivart thought that the intermediate stages could have no selective value, and in the 6th edition of the Origin, Darwin made a concession to the possibility of acquired traits. Many years later the geneticist Richard Goldschmidt put the case forward as an example of evolution by saltation, bypassing intermediate forms.

A recent examination of two fossil species from the Eocene has provided the first clear picture of flatfish evolution. The discovery of stem flatfish with incomplete orbital migration refutes Goldschmidt's ideas, and demonstrates that "the assembly of the flatfish bodyplan occurred in a gradual, stepwise fashion". There are no grounds for thinking that incomplete orbital migration was maladaptive, because stem forms with this condition ranged over two geological stages, and are found in localities which also yield flatfish with the full cranial asymmetry.