Biologists have for a long time believed that each species of life did not have any change i.e. they remained unchanged and no new species had appeared ever since our world began. But however, this theory or belief was questioned by many biologists during the late 1700’s. They identified that farmers had started producing new varieties of plants and animals through selective breeding. Also, many voyagers of exploration revealed that isolated groups of plants and animals that contained many species which varied only slightly from one another. Biologists started thinking why there should be so many species with such little variations. Such observations and thoughts led many biologists to believe that species evolved over time or changed i.e gradually developed from other species over time.

During the early 1800,s, many biologists proposed explanations for how species evolved. The most convincing theory was eventually reached independently by two British naturalists – Charles Darwin and Alfred Russel Wallace. Alfred Russel Wallace was the first one to propose a theory of natural selection which prompted Darwin to propose his own theory. However, Darwin presented his ideas in a widely read book, and his work became better known.

Charles Darwin detailed his theory of evolution in the book ‘The Origin of Species(1859)’. As per Darwin’s theory, some organisms are born with traits that help them survive and reproduce. Such favourable traits are passed on genetically to offspring.  Other members of the same organism that have unfavourable traits are less likely to survive and reproduce. The organism with the unfavourable traits eventually died out. Darwin proposed that species evolve as more and more favourable traits appear and are passed from generation to generation. This was called as the process of ‘natural selection’. Darwin states in the theory that the evolution has to be gradual and cannot be a leap.

The Theory of Darwin, currently is a theory in crisis due to the tremendous advancements which we have made in the field of genetics, biochemistry and molecular biology. We have identified now that there are innumerable irreducibly complex systems at the cellular level. The heart, eye and ear are examples of irreducible complex systems even though this was not identified as such during Darwin’s days.