CONCEPT OF SPECIES It is already been stated that individual organisms which have many features in common and able to interbreed in natural conditions are encompassed by the term species. Species is the smallest basic unit of classification. Its proper characterization is, therefore, of vital importance for a 'Natural' scheme of classification. Identifying species on the basis of reproduction, John Ray (1693) considered all individuals produced by parents of the same types as members of the same species. Linnaeus and Some others distinguished species merely on the basis of structure. Morphological Concept of Species This is the most practical and widely used species concept to describe species since people began to classify organisms. Taxonomists use this concept mostly by default because they know most of the species on Earth only from their morphology. New species are usually named on the basis of morphological species concept; morphological descriptions are all that we need
Charles Darwin gave the theory of Natural Selection that is applicable to both plants and animals.
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| Darwin's theory of evolution |
Charles Robert Darwin ( 1809-1882 ) was a British naturalist who made significant discoveries of biology. He became the unpaid naturalist in 1831 and set out on a voyage when he was 22 years old. He developed the idea of organic evolution by natural selection on his five-year expedition around the world. When he returned to England, he didn't leave again and stayed at home. He conducted various experiments and formulated the hypothesis stating that evolution occurred due to natural selection.
Darwin gave the theory of evolution called The Theory of Natural Selection in his book The Origin of Species. The Theory of Natural Selection in his book The Origin of Species. The Theory of Natural Selection suggests that the best adapted organisms are naturally selected to pass on their traits to the offsprings.
Darwin's theory of evolution can be explained as follows.
1- Natural variation occurs within any population.
2- The populations of species even on producing a large progeny remain fairly constant naturally. This occurs as members of same species and different species compete for food, Shelter and mate.
3- The unfit organisms are left out in struggle for survival within populations. This results in population of fit organisms showing favorable variations that help them to survive and reproduce. This is referred to as natural selection.
4- The Favorable variations of organisms are inherited by the offsprings in the successive generations.
5- The origin of a new species occurs when the variations are accumulated over a period of time.
Let's try to understand Darwin's theory of evolution by natural selection with the help of an example. In an ecosystem, there are some giraffes having long necks and others are having short necks. Fig (A). If due to some unfavorable environmental conditions, the low-lying shrubs die, then there won't be enough food available for giraffes with short necks. This will results in a gradual decrease in the number of giraffes having short necks. Fig (B). As a result, after a few generations, all the giraffes would have long necks. Fig (C). In this way, the nature selects the giraffes with long necks to survive in the existing conditions. Hence, the giraffes having short necks have given rise to giraffes with long necks as a result of evolution mechanism.
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| Darwin's theory of evolution |
Natural selection can hence be defined as the process of evolution of species wherein the traits that help the organisms to reproduce and survive are passed on from one generation to another.
Darwin's theory of evolution was widely accepted. However, it was criticized as it could not provide an explanation about how the variations arise.
Let's now study about evolution of eyes, evolution of feathers and evolution by artificial selection.
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