BIOLOGY Volume 3 - A Guide to General Biology - 2004

26. EVOLUTION, OR THE HISTORY OF LIFE ON EARTH

26.4. The Theory of Evolution

The Theory of evolution occupies a unique place in The Study of The history of life. It has become the unifying theory that provides a foundation for the whole of biology. Evolution implies a universal, gradual development that is both ordered and sequential. With regard to living organisms, evolution can be defined as the development over time of complex organisms from pre-existing simpler ones.

The Concept of Evolution did not originate with Darwin and his book On THE ORIGIN OF Species. Long before Darwin, attempts to explain the apparent Water/126.html">Diversity of the living world paradoxically drew scientists' attention to Structural and functional similarities among organisms. Various evolutionary hypotheses were proposed to explain these similarities, and these hypotheses themselves underwent an "evolution" as science developed in the post-Darwinian era.

As the history of evolutionary theory shows (Table 26.1), METABOLISM/2.html">THE CONCEPT OF continuity, or the gradual development of more complex species from pre-existing simpler forms, was proposed by several philosophers and naturalists even before the formal Introduction of evolutionary hypotheses in the early 19th century.

Class="center">Table 26.1. History of evolutionary ideas

Ancient China

Confucius

Life arose from a single source through gradual unfolding and branching

Antiquity and the Middle Ages

Diogenes

All creatures are the result of the differentiation of a single substance and are akin to it

Empedocles

Air, earth, fire, and water are the four roots of all things. Life arose through the action of attractive and repulsive forces between these four elements. Attempted to explain the Origin of the universe, plants, animals, and humans (the first to develop the idea of organic evolution)

Democritus

Living organisms arose on Earth from silt by spontaneous generation

Anaxagoras

Organisms arose from germs floating in the air

Thales

(640—546 BC)

All living things originated from water

Anaximander

Plants, then animals, and finally humans arose from the mud of the forming Earth

Aristotle (384-322 BC)

Formulated a theory of the continuous and gradual development of life from inanimate matter, based on his observations of animals

Middle Ages (400-1400 AD)

Theories based on earlier concepts listed above, or acceptance of creationism

Age of Speculative Thought (1400-1790)

John Ray (1627-1705)

Established the Concept of the species

Carl Linnaeus (1707-1778)

Introduced binomial nomenclature. Believed that genera were created separately and that species are variants within genera

Buffon (1707-1788)

Expressed the opinion that Different types of animals have distinct origins and arose at different times. Recognized the Influence of the environment and the inheritance of acquired characteristics

James Hutton (1726-1797)

Theory of uniformitarianism. Estimated the age of the Earth in millions of years

Age of Theory Building

(1790-1900)

Erasmus Darwin (1731-1802)

Life arose from a single "filament" created by God. Did not accept separate human creation. The "filament" evolved through the inheritance of acquired characteristics

Jean-Baptiste Lamarck (1744-1829)

Inheritance of acquired characteristics (Environmental Impact on organisms and transmission of phenotypic changes to offspring). The concept of use and disuse of Organs

Georges Cuvier (1769-1838)

Utilized the fossil record. Fossils are the result of "catastrophes," after which new species arose

William Smith (1769-1838)

Objected to Cuvier's catastrophe theory, based on the continuous distribution of similar species in strata of similar age

Charles Lyell (1797-1875)

Demonstrated the presence of progressive Changes in the fossil record

Charles Darwin (1809-1882)

Influenced by the ideas of Lyell and Malthus. Formulated the theory of evolution by natural Selection

Alfred Russel Wallace (1823-1913)

Formulated a theory similar to Darwin's, but did not include humans in it

Hugo de Vries (1848-1935)

Discovered the existence of heritable Mutations forming The basis of discontinuous variation; believed that species arise through mutation

August Weismann (1834-1914)

Showed that Germ Cells are segregated from the rest of the body and are therefore unaffected by influences acting on somatic Tissues

Gregor Mendel (1822-1884)

Genetic studies (published in 1865), which gained recognition only after 1900. Discovered the laws of heredity

Twentieth Century (Neo-Darwinism)

W. L. Johannsen

Phenotypic traits are determined by genotype and environmental factors

T. Hunt Morgan

Established the chromosomal theory of heredity based on genetic and cytological data

H. J. Muller (1927)

The genotype can be altered by X-rays; induced mutations

R. A. Fisher (1930)

Changes studied by geneticists are analogous to those revealed in the fossil record

G. W. Beadle and E. L. Tatum (1941)

Continuing the work of Garrod (1909) and J. Haldane (1935), elucidated the GENETIC BASIS OF biosynthetic processes

J. Lederberg and A. D. Hershey (1951)

Demonstrated the value of Bacteria as subjects for studying genotypic variation

J. Watson and F. Crick (1953)

Proposed a model for the molecular Structure of DNA and its Replication mechanism

F. Jacob and J. Monod (1961)

Discovered the mechanism regulating Gene activity

26.4.1. Lamarck's Theory of Evolution

In 1809, the French biologist Lamarck put forward a hypothesis on The Mechanism of evolution based on two conditions: the use and disuse of organs, and the inheritance of acquired characteristics. Environmental changes, in his view, can lead to behavioral changes, creating a need to use certain organs or structures in new ways or more intensively (or to stop using them altogether). With intensive use, the efficiency and/or size of an organ will increase, whereas disuse may lead to its degeneration or atrophy. These traits, acquired by an individual during its lifetime, are, according to Lamarck, inherited—that is, passed on to offspring.

From the standpoint of Lamarckism, the long neck and legs of the giraffe are the result of many generations of once short-legged and short-necked ancestors feeding on tree leaves, which forced them to stretch higher and higher.

The slight elongation of the neck and legs occurring in each generation was transmitted to the next until these body parts reached their present length. Webbed feet in waterfowl and the body shape of flatfish were explained in the same way. Webbing arose from the constant spreading of the toes and stretching of the Skin between them while swimming in search of food or escaping predators, whereas the flattened body of the flatfish resulted from lying on its side in shallow water. Although Lamarck's theory helped pave the way for the acceptance of the evolutionary concept, his views on the mechanism of change were never widely accepted.

However, Lamarck was correct in emphasizing The Role of environmental conditions in producing phenotypic changes in an individual. For example, Physical Exercise increases Muscle mass, but these acquired traits affect the phenotype and are not genetic; having no effect on the genotype, they cannot be passed on to offspring. To prove this, Weismann cut off the tails of mice over many successive generations. According to Lamarck's theory, the forced disuse of tails should have led to their shortening in descendants, but this did not happen. Weismann postulated that traits acquired by the soma (body) and altering the phenotype do not directly affect the germ cells (Gametes) through which traits are transmitted to the next generation.



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