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Animals of a given species are alike because they inherit a certain set of genes from their parents. Sometimes, it happens that something goes wrong with the mechanism that transmits the genes, and an animal is born that doesn't resemble its father and mother. You probably know of examples in your own family.
If this accidental variation, or mutation, is helpful in the animal's struggle to survive, it gets passed on to succeeding generations. In this way, new species have risen.
Evolution proceeds from simpler forms to the more complex. The first animals were simple one-celled blobs of protoplasm. From them, the multi-celled organisms arose. It is easy to see how this process led inevitably to more complex forms.
One of the remarkable features of the biosphere is the existence of innumerable species of plants and animals, each one adapted to a particular environment in which it lives. Man, for centuries, has wondered about this diversity of life and has tried to explain how such diversity of life originated and came into existence. The formulation of the theory of organic evolution is the outcome of man's curiosity about the great variety of living forms. In other words, evolution is a concept which explains the nature and history of a vast array of animal and plant species that inhabit the earth.
The central theme of evolution is one of 'change with descent'. The theory of evolution emphasises that there have been profound and continuing modifications in the forms of life, generation after generation, over millions of years of geological time. Evolution, in other words, is concerned with the origins of the diversity of living organisms and it also accounts for the present diversity of organisms with reference to past events of organic development. Organic evolution, thus, is a modern, fundamental concept in biology.
Theories of Evolution
Scientists, ever since Aristotle, have been attempting to explain animal and plant diversity in terms of evolution. However, until the middle of the nineteenth century naturalists generally agreed that each species had been separately created either by the action of a Supreme Being, or by 'Spontaneous generation'. According to this theory, fully developed organisms were believed to have sprung up from water, soil or other non-living matter. The idea that complex animals and plants might have developed by gradual change from simpler forms had been suggested earlier. However, such thoughts were not given much consideration until about 1800, when it was restated by several progressive English and French naturalists.


Jean Baptiste Lamarck, a French zoologist, was one of the first to attempt to explain, seriously, the phenomenon of evolution. He observed the changes and adaptations of certain parts of animal bodies that were used continuously in certain ways. He noted that certain structures increased in size if used but grew smaller or atrophied if not used. He thus concluded that these gains or losses through use or disuse were passed on to the offspring in succeeding generations. However, Lamarck never experimented to ascertain whether or not these acquired changes were hereditary. Use and disuse may alter certain body characteristics, but the transmission of such acquired traits to future generations has not been proved.
Although the basic idea of his theory of evolution is no longer accepted, it did stimulate serious thought about evolution. Lamarck recognised the fact that the species are not 'immutable', but that they descended from other species by gradual change over many generations. Lamarck called this process 'transformism'; the word 'evolution' was not applied to the process until many years later. His theory was strongly criticised by leading naturalists of his time, and as a result never received the attention or credit it deserved. It was Darwin, 50 years later, who convinced the scientific world with the significance of transformism.
Erasmus Darwin (1731-1802), the grandfather of Charles Darwin, recognized that various types of organisms arose from each other. He also stressed the response of the animal to environmental changes as the basis for such modifications.
Sir Charles Lyell (1797-1875), a geologist, was another person who stimulated sound thinking along evolutionary lines through his publication, Principles of Geology (1830). He formulated the theory of uniformitarianism, in which he stated that the forces that produced changes in the earth's surface in the past are the same as those that operate upon the earth's surface at present. Such forces over long periods of time could account for all the observed changes, including the formation of fossil bearing rocks. This concept showed that the age of the earth was millions of years. This great geological work stimulated and influenced Charles Darwin's thoughts greatly and resulted in the formation of his theory of organic evolution.

Charles Darwin (1809 - 1882)
Charles Darwin (1809-1882) believed that evolution had occurred and proposed the mechanism by which evolution took place, namely, natural selection. The idea that evolution had occurred was not new to Darwin. His contribution was stating the theory of evolution by natural selection and supporting it with massive evidence. Darwin maintained that living species were descendants of similar, slightly different, forms that lived in the past.
An Illustration
Acquired and Inherited traits
Traits are qualities, features or other things that distinguish the organism. They can include things like hair color, tooth shape, beak shape, bone size, or muscle structure.
Traits come in two varieties: acquired and inherited.
Acquired Traits
Just like human beings, animals can acquire useful abilities. These acquired traits cannot be passed on genetically. You can't inherit your uncle's knowledge, skills, ideas or memories and it doesn't work that way with other organisms either. Acquired traits include things such as calluses on fingers, larger muscle size from exercise, etc. Behaviors that help an organism survive would also be considered acquired characteristics most of the time e.g., things like where to hide, what animals to hide from and other behavior like that and for plants acquired characteristics might include bending because of wind or growths resulting from insect bites.
Inherited Traits
There are certain traits that can be inherited. In organisms, inherited traits must come from a parent or other ancestor. A trait may seem to skip a generation or even two or three, but if a trait is shown it must have been present in an ancestor. Mutations are the exception to this rule. Inherited traits include things such as hair color, eye color, muscle structure, bone structure, and even features like the shape of a nose. Inheritable traits are traits that get passed down from generation to the next generation. This might include things like passing red hair down in a family. For animals it may include things like the stripes on a tiger, a skunk's ability to spray, or the flavour of fruit from different apple trees. All plants and animals that reproduce pass on traits to their offspring.

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