Question

define darvin theory of evolution

Answer 1

Darwin's Theory of Evolution is the widely held notion that all life is related and has descended from a common ancestor: the birds and the bananas, the fishes and the flowers -- all related. Darwin's general theory presumes the development of life from non-life and stresses a purely naturalistic (undirected) "descent with modification". That is, complex creatures evolve from more simplistic ancestors naturally over time. In a nutshell, as random genetic mutations occur within an organism's genetic code, the beneficial mutations are preserved because they aid survival -- a process known as "natural selection." These beneficial mutations are passed on to the next generation. Over time, beneficial mutations accumulate and the result is an entirely different organism (not just a variation of the original, but an entirely different creature).
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Answer 2

Charles Darwin had collected innumerable specimens of plants and animals and depending upon the observation of those specimens; he published the theory of natural selection with preaches the survival of fittest.
Darwin says that all the organisms reproduce prolifically. all the organisms compete with each other in the life-threatening manner. in this competition only those organisms sustain which show the modification essential for wining the competition