Extinction

Extinction refers to the disappearance of an animal group (e.g. a species), from the evolutionary period. It is not an unusual event, but rather a natural phenomenon. It is estimated that 99.9% of all species that ever lived on earth are now extinct. 

Extinction appears to be the ultimate fate of all species. The rate of extinction spikes in occasional mass extinction events. The 2 ways in which species may become extinct are:

• negative role of environmental selection in evolution (species develops a way of life such that a change in environment would prevent its persistence). 
• a species may become extinct as it is transformed into another. 

Major Extinction events:

1. Permian-Triassic extinction - Though it is less known, it was the most severe and approximately 96% of species were driven to extinction. 
2. Cretaceous-Tertiary extinction - The non-avian dinosaurs went extinct.
3. Holocene extinction - Ongoing mass extinction associated with humanity's expansion across the globe over the past few thousand years. Present-day extinction rates: 10-1000 times greater than the background rate. 

Theories of Evolution

Theory	Proponents	Brief note
(Greek and Roman Thinkers)
6th Century BC	Anaximander of Miletus (Greek)	Proposed that the first animals lived in water, during a wet phase of the Earth's past, and that the first land-dwelling ancestors of mankind must have been born in water, and only spent part of their life on land. He also argued that the first human of the form known today must have been the child of a different type of animal (probably a fish), because man needs prolonged nursing to live. Man evolved/originated from fish.
Theory of spontaneous generation/ creation or Abiogenesis	Aristotle, Thales, Plato, Von Helmont, Empedocles Democritus	The theory of spontaneous generation is as old as human thought. It is well known that life arises only from pre-existing life (principles of bio-genesis) and assumes that life originated from inert, inorganic matter as a result of a series of physico-chemical conditions which must have existed at a given moment during the evolution of earth. The theory contends that life had originated repeatedly from inanimate materials or non-living things in a spontaneous manner. Aristotle thought that fireflies originated from morning dew and mice from the moist soil spontaneously.
(Medieval & others)
Creationism/ Theory of special creation/ Theory of Divine creation 17th and 18th century	Father Suarez (1548-1671) A Spanish Monk	Proposed this theory. It was based on the Biblical book of Genesis. According to Genesis, of Old Testament of Bible, the world was created by the supernatural power (God) in six natural days. Since all species were made individually by god, the theory does not accept the idea of origin of new species from ancestral forms. The created organisms exist unchanged from the day of their creation.
	James Ussher (1581-1656) Archbishop of Northern Ireland	Fixed the date of creation at October 23, 4004 B.C.
	Dr. Charles Lightfoot(1889-1961)	Added the exact time of creation, i.e., 9 a.m. on October 23, 4004 B.C.
mid 18th century	Carl Linnaeus (1707-1778) Swedish Botanist	Designated each living organism two Latin names (binary nomenclature), one for Genus and the other for Species. Thus, from the days of Linnaeus, Man has been scientifically known as Homo Sapien.his immortal work “The Systema Nature”
Late in the 18th century	Comte de Buffon (1707-1778) French scientist	Suggested strongly that life forms are not fixed.He strongly believed that this could be the influence of the environment on living organisms. He explained this in his voluminous work, “Historic Naturelle”. He had more clear ideas on the physical features of man than Linnaeus. He explained them in his book “Varieties Humanies”. He argued that although catastrophic events do occur, they are rare and so “have no place in the ordinary course of nature.” Instead, the earth’s history is mainly explained by “operations uniformly repeated, motions which succeed one another without interruption”. Thus, much of the earth’s geological history could be explained by normal, everyday, uniform processes—the things taking place before our eyes, such as erosion and deposition of sediments in water. For such processes to account for all the changes recorded in the earth’s strata, however, the earth would have to be older than 6,000 years. Buffon also published Les Epoques de la Nature (1788) where he openly suggested that the planet was much older than the 6,000 years proclaimed by the church, and discussed concepts very similar to Charles Lyell's "uniformitarianism" which were formulated 40 years later.
18th Century	Erasmus Darwin (1731-1802)	Suggested through his work the evolutionary aspects of animals and strongly contended that the earth and life on it must have been evolving for millions of years and the history of mankind is the latest.
Lamarckism	Jean Baptiste Lamarck(1744 – 1829)	Was the first evolutionist who confidently put forward his ideas about the process leading to biological change in the organism.
Catastrophism/ Theory of Catachysm   17th Century/ 18th Century	Nicholas Steno (1638-1686)	Steno and Hooke, still believed in a biblical chronology. To Steno, the water-deposited layers of the stratigraphic sequences represented two events—the original water-covered earth on which God created land and plants and animals (Genesis 1) and the waters of Noah’s flood (Genesis 6-8). The geological record, however, shows a vast amount of change, and the Bible provides only 6,000 years of the earth’s history. So much change in such a short time, thought Steno and Hooke, required the presence of global catastrophic events such as earthquakes and volcanoes. Steno and Hooke and others who subscribed to this explanation are often referred to as catastrophists.
	Robert Hooke (1635-1703)
	George Cuvier (1769-1832) French scientist	Objected to Lamarck strongly. This is the extension of the theory of special creation. This theory assumes that life is originated by the creation and it is followed by catastrophe due to geographical disturbances. Each catastrophe destroyed the life completely whereas each creation forms life different from the previous one. His observation was based on the fossil remains of varied organisms. According to him, the earth had to face severe natural calamities at different times for which many animal species have been destroyed. But each time when the earth settled after a great Catastrophe, relatively higher forms of animals appeared to replace the situation. Cuvier did not believe in continuous evolution. To him the species never evolved by modification and re-modification; a series of Catastrophes were responsible behind changes where previous sets of living creatures get replaced by new creatures of complex structure.
Theory of Uniformitarianism   19th Century	Charles Lyell (1797-1875) English lawyer and geologist	Disproved Cuvier’s Catastrophism. Lyell, in his three volume book on ‘Principles of Geology’ (1830-1833), documented the fact that the earth must be considerably old; and natural processes through time, like, erosions, earthquakes, glacial movements and volcanoes have changed the shape of the earth and its living units. He provided conclusive evidence for the theory of uniformitarianism. He explained this saying that the present would be the key for understanding the past. He argued that the natural changes were the same in the past and the present. This theory on one hand discarded the “theory of Catastrophism” and on the other hand nullified the “theory of divine Creation”.
Through the work of Hooke, Steno, Hutton, Smith, and Lyell—and many others—the study of the earth shifted from the supernatural to the natural. Scientists sought data about earth’s history from the earth itself, not from the presuppositions of belief systems. As a result, by the early nineteenth century, our world was viewed through the interacting perspectives of constant change brought about by observable processes over vast amounts of time. Lyell put these ideas down in his three-volume Principles of Geology, first published between 1830 and 1833. Among those weighing Lyell’s ideas was a young British naturalist, Charles Darwin, who took the first volume of Lyell’s book with him as he embarked, in 1831, on a round-the- world voyage of scientific exploration.
Theory of Eternity of Present Conditions   19th Century	Preyer (1880)	This is an orthodox theory. It believes that some organisms were there from the very beginning of the Universe. Those organisms still exist and will be continued in future in addition to some new forms. According to this theory, the original forms are eternal, and they have been preserved automatically.This theory assumes that life had no beginning or end. It believes that life has ever been in existence and it will continue to be so ever. It further believes that there is no question of origin of life as it has no beginning or end. The theory is also known as steady state theory.
Theory of Cosmic Origin of life   19th Century	Richter (1865)	Richter developed this theory and he was supported by Thomson, Helmholtz (1884), Von Tieghem (1891) and others. This theory advocated that the first life seed had been transported through the cosmic particles from other planet. According to them the meteorites that travelled through the earth’s atmosphere, contained embryos and spores in them; those gradually grew and evolved into different types of organisms.
(organic evolution)
Organic Evolution :	The theory that more recent types of plants and animals have their origins in other pre-existing forms and that the distinguishable differences between ancestors and descendents are due to modifications in successive generations.
Lamarckism/ Theory of Inheritance of acquired characters  18th /19th Century	Jean Baptiste Lamarck(1744 – 1829) French naturalist/biologist	He opined that the structure of a living being is dependent on its function. He used the example of Giraffe, saying that it got long-neck structure for its constant use of reaching to higher foliage. Based on this observation, he propounded two theories, namely: i) use and disuse of characters and ii) the acquired characters are inherited. He presented a complete theory of evolution in his book ‘Philosophie Zoologique’, published in 1809.
Neo-Lamarckism	McDougall, Spencer, Cope, Packard, Kammere, Sumner, etc.	A group of evolutionary biologists further studied and modified the Lamarckism. They carried out experiments to find evidences for the inheritance of acquired characteristics. This modified version came to be known as Neo-Lamarckism.
Darwinism/ Theory of Natural Selection & the Origin of Species 19th Century	Charles Darwin (1809-1882) English naturalist, geologist and biologist	The start of new era for understanding biological evolution through genetic mechanisms. His book ‘The Origin of Species’ was published  in the year 1859. He proposed the term ‘Organic Evolution’ which signifies ‘descent with modification’, the hypothesis that all organisms on Earth are connected by bonds of genealogy and have changed through time.
Neo-Darwinism   (Also- Modern theory of origin of species/ developed into Modern Synthetic Theory)	Weismann, Earnest Heckle, Lyell, Huxley, Wallace and Simposon.	Supported the natural selection.
	R.A Fisher, Sewall Wright, and J.B.S Haldane.	Explained natural selection by modern synthesis. Natural selection is differential reproduction, plus the complex interplay in such phenomena as heredity, genetic variation, and all other factors that affect selection and determine its results.
Mendelism/ Laws of inheritance  19th Century	Gregor Mendel (1822-1884)	Mendelism is used to refer to the theoretical principles of heredity of the single-gene trait, which is derived from the principle put forward by Gregor Mendel, known as Mendel’s laws. Mendel’s laws came to be known as Laws of inheritance, which are as follows: Law of Dominance, Law of Segregation, Law of Independent Assortment.
Mutationism 19th Century	Hugo de Vries(1840 – 1935) Dutch Botanist	The theory states that evolution is a jerky process where new varieties and species are formed by mutations (discontinuous variations) that function as raw material of evolution.
Synthetic Theory/ Modern Synthesis 20th Century		Present understanding of the process of evolution. With more complete understanding of mechanism of inheritance, the biological sciences now generally define evolution as- The sum total of the genetically inherited change in the individuals who are the members of the gene pool of a population.
Neo-Mutationism 20th Century	Masatoshi Nei(1931-)	The contemporary view corresponding to Mutationism. A main feature of this theory is how single mutations can have significant effects to influence evolution.