Biological phenomena are so complex that they can be logically explained only in the light of evolution. Evolution therefore has to be brought into picture while studying any aspect of zoology, namely, anatomy, embryology, cytology or physiology. The fact that animals and plants change and do not remain static during the course of time is the backbone of evolutionary reasoning. Dobzhansky’s view that nothing makes sense in biology except in the light of evolution outlines the importance of this branch of science. The word evolution simply means a change with time, and organic evolution means change in plants and animals from generation to generation, which Darwin called, “descent with modification”.
Evolution is the outcome of interaction of organisms with their environment. As animals try to adapt to the changing environment, adaptive structures arise by gradual change in the existing organs, which over a period of time accumulate to form large anatomical changes that we call marvels of nature.
Evolutionary changes in reality occur by chance or as short term advantages of variations. Adaptations do not evolve because they benefit species but evolution produces patterns of variability from which some animals might derive benefits while others not.
Natural selection is the executing arm of evolution which thrives on excessive reproduction and genetic diversity caused by sexual reproduction and mutations. Therefore, genetic composition of a population changes with the passage of each generation and all organisms existing today are modified descendants of the ancestors that existed sometime in the past,
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