Lamarck, Jean Baptist (1744-1829): A French biologist who joined military service initially but had to retire due to poor health. In 1778 he became Botanist in Jardin due Roi in Paris, where he prepared his Dictionary of Botany and later wrote Flora of France. With the help of Buffon he obtained a job in the Museum of Natural History and changed his field of study from Botany to Zoology and wrote 7 volumes of his best known work, Philosophie Zoologique. He studied fossils in detail and introduced several new terms, viz. ‘species’, ‘biology’, ‘invertebrates’ and ‘vertebrates’. He described animal kingdom in a series that graded from simpler to complex forms and gave the first diagram of evolutionary tree. He made use of inheritance of acquired characters to explain the theory of evolution, which had the following salient features:
He gave the example of the long neck of giraffe to explain his theory of use and disuse and appetency. The theory still remains to be demonstrated experimentally.
He married 4 times, had many children, was always poor and in later life became blind and died alone at the age of 85 in a pathetic state.
Cuvier, George (1769-1832): He is considered to be the father of comparative anatomy and palaeontology. He prepared the geological time table and believed that fossils are the result of catastrophe whence new creations are made. He believed in fixity of species and in special creation theory and opposed Lamarck’s theory. He was a scientific dictator of France, wielding unparallel scientific and political influence.
Later, Carl Gegnbaur (1826-1903) of Germany carried out extensive studies on comparative anatomy of all vertebrate classes and provided data in support of Darwinian Theory.
Saint-Hilaire, Geoffroy (1772-1844): A colleague of Curvier, he opposed his views and supported Lamarck and Buffon. He explained small variations and recognised effects of isolation, including physiological and geographical isolation. He had a public debate with Cuvier on the origin of squids and although correct in principle, lost owing to poor presentation and arguments. After French revolution he divided the Chair in Zoology with Lamarck.
Charles Darwin (1809-1882): After 5-year Voyage of the Beagle and studies on Galapagos Islands, he gave the theory of Natural Selection to explain evolution (see chapter on Darwinism for details).
Weismann, August (1834-1914): A German biologist he differentiated between somatoplasm and germplasm, which was the hereditary material carried from generation to generation. He believed in the continuity of germplasm, which he thought occasionally underwent change. He was of the view that struggle of existence was between the vital and invisible particles in the germplasm. A staunch supporter of Darwin, he was proficient in microscopic study of cells.
T.H. Huxley (1825-1895): He was foremost of Darwin’s defenders. By 12 years of age he had read all books in his father’s library. Later joined medical services on the ship Victory, during whose voyage he took extensive notes. After returning to England he wrote such large volumes that no publisher would publish them. Published Oceanic Hydrozoa and wrote on all branches of biology. He was not convinced of Lamarck’s work and supported Darwin so vigorously that he was nicknamed as “Darwin’s bulldog”.
deVries, Hugo (1848-1935): A Dutch botanist, he was interested in variations among organisms. He wrote a book in 1901, The Mutation Theory, which provided much needed explanation for the occurrence of variations. He studied variations in the evening primrose, Oenothera lamarckiana, which was introduced in Europe and USA, and in which several dwarf and giant types developed by mutation. He firmly believed that mutation controls all evolutionary events. Original mutation theory was gradually modified and refined, and is now accepted as a change in genes that bring about variations.
Ernst Haeckel (1834-1919): He made a connection between development of individual and evolution of race, which he called The Biogenetic Law, published in his book, Generalle Morphologie, in 1866. The idea was first proposed by von Baer (1792-1876) of Germany but later popularised by Haeckel by the famous phrase, “Ontogeny recapitulates phylogeny”. He argued that gill slits develop in the embryos of all vertebrates even when they are terrestrial lung breathers. Similarly heart, central nervous system, cartilaginous skeleton, notochord and even tail in the early development show remarkable similarity with one another in all vertebrates. All animals start their embryonic development from zygote, suggesting that all life evolved from a single cell. He believed that paleogenetic characters are ancestral traits that are retained in the embryos and coenogenetic characters are secondary, adaptive and non-ancestral that appear later in the development. However, Ernst Haeckel’s theory was criticised on three counts as follows: 1) Embryos of higher animals resemble only the embryos of lower animals and not the adults. 2) Ontogeny only shows affinity and not evolution. 3) Retaining ancestral embryonic stage has no selection value and therefore would be wasteful and time consuming. Modern gene theory explains the sequence of events and the subsequent end product of ontogeny in the right perspective and, therefore, Haeckel’s original theory has now been rejected.
The theorem of natural selection: R.A. Fisher (1932) gave the theorem of natural selection in his book, The cause of evolution, according to which the rate of increase in fitness of any organism is equal to its genetic variance at that time. More a species develops genetic heterozygosity, more fitness it would demonstrate during natural selection. Fisher studied replacement of grey form of industrial melanic moth in England by carbonaria form and thought that this was due to genetic variance in the population.
Concept of punctuated equilibrium: The concept was given by two American palaeontologists, S.J. Gould and N. Eldredge (1972), who thought that fossil record does not support Darwin’s concept of Gradualism, which suggests that species evolve by gradual changes over millions of years. They proposed that species evolved by sudden bursts of rapid change, punctuated by long periods of static equilibrium. Fossil records indicate that species have evolved rapidly within a few thousand years and once evolved they remained unchanged for millions of years. Origin of birds from reptiles and evolution of whales, ungulates, carnivores, Chelonia, grasses and flowering plants appears to have taken place suddenly.
Significance of isolating mechanism: Importance of isolating mechanism was first proposed by M. Wagner. Later Ernst Mayr (1971) and G.L. Stebbins (1976) recognised that isolation was the chief component in the production of new species. Isolation is any mechanism which prevents interbreeding between two populations. The concept has been dealt with in detail in a separate chapter.
Quantum evolution theory: The principle of quantum evolution was formulated by G.G. Simpson (1944) and Grant (1971). Ayala (1975) called it Saltation and Goldschimdt (1948) Macrogenesis. They proposed that macromutations give rise to entirely new kinds of organisms, sometimes monsters, which survive and perhaps move to new environment and give rise to new species or groups. However, Mayr (1970) did no support the idea.
Adaptive Radiation: H.F. Osborn (1902) coined the term Adaptive Radiation to explain emergence of several species from a single ancestor. G.G. Simpson (1953) said that adaptive radiation is rapid proliferation of new species from of single ancestor by occupation of varied ecological niches. Savage (1969) and Volpe (1985) thought that adaptive radiation was produced by macroevolution caused by major genetic changes. The phenomenon was first pointed out by Charles Darwin in finches after his visit to Galapagos Islands. Later, it was analysed by David Lack (1969). Several distinct species emerged from seemingly single population that was blown from South American coast to the Galapagos Islands.
Law of irreversibility: Also known as Dollo’s Rule, it was first proposed by the Belgian palaeontologist, Dollo (1895), who proposed that body structures once lost during the course of evolution could not be regained even if needed by the animal later. Terrestrial animals that returned back to water and became fish-like in appearance could not grow gills again for respiration. For example, extinct reptiles Ichthyosaurus and Plesiosaurus, dolphins and whales, although completely fish-like could not grow gills for respiration but continued breathing with lungs.
Principle of preadaptation: George Mivart (1871) believed that certain preadaptations are necessary for the animal to venture into new environment. Although these preadaptations may be non-adaptive in the existing environmental conditions, they are maintained in the population if not disadvantageous and become useful when the animal moves to a new environment. Evolution of horse is a good example of preadaptations in dentition and limb bones developing in Eocene and Oligocene horses although they were still living in forests.