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 All these evidences point to the fact that evolution occurs. The exact mechanism of evolution, however, is not yet conclusively proven. But constant study and research in the above fields is continuing for answers. |
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Asexual reproduction results in identical offspring whereas sexual reproduction results in variation among the offspring and also between parents and the offspring. |
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Heredity may be defined as the transmission of characteristics through generations. |
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Variation may be defined as differences among individuals of a species. |
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Both heredity and variation play important roles in evolution of organisms. |
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The study of the mechanism of transmission of characteristics from parents to the next generations is called genetics. |
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Gregor Mendel was the first to carry out scientific studies on transmission of characteristics from the parent to the offspring. |
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Based on the experiments he conducted, Mendel came to the conclusion that there are certain physical entities called the factors that are responsible for the transmission of characters. These factors are now called genes. |
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There are various laws of genetics also based on Mendel's work. |
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These laws explain the dominance of certain traits, the haploid nature of gametes and their independent assortment. |
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Inheritance, according to the chromosomal theory, is due to the passage of genes that are present on the chromosomes that are passed on from the parents to the offspring. |
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The genetic material is made up of the chromatin or the chromosomes. These contain the genes that are arranged in a linear fashion. Each gene is a short chain of nucleotides arranged in a specific sequence. |
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Each chromosome consists of two strands called the chromatids that are held together by the centromere. |
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The location of the centromere decides the shape of the chromosome. |
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Each organism contains a fixed number of chromosomes in each of its cells. The chromosomes in each cell are of two types - autosomes and sex chromosomes. Males have X and Y sex chromosomes whereas females have X and X sex chromosomes. |
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Variations are of many types. Based on whether they are phenotypic or genotypic, they are called somatogenic and blastogenic. |
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If the variation among a group of organisms is large and very obvious, it is called discontinuous and if it is small, it is called continuous. |
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Variations play an important role in evolution, especially those that are inheritable. |
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Evolution may be defined as a gradual development of more complex species from pre-existing simpler forms. Some of the formal theories on evolution are Lamarckism, Darwinism, theory of inheritance of germplasm and neo-Darwinism. |
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According to Lamarck, parts that are continuously used develop well and those that are not used get degenerated. These acquired characters are then inherited. This theory is not very widely accepted. |
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Weismann proposed the theory of continuity of germplasm, only the changes that affect the germ cells and the germplasm (the collection of genes) will be inherited by successive generations. |
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Darwin proposed the theory of natural selection. According to this theory, the reproduction in large numbers resulted in a struggle for existence and competition. This leads to survival of the fittest. Thus the best of the genetic combinations is preserved and passed on to the next generation as only the species that survive reach the reproductive age. |
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Due to the recent scientific advances by man, there are many more evidences that are coming to light. A study of all these evidences taken from various fields such as paleontology, comparative anatomy, geological distribution, embryology, etc. in search of the exact mechanism of evolution constitutes neo-Darwinism. |
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