Heredity is the process of passing of traits from one generation to the next generation. Now, this process would not lead to any interesting results (and in turn, the astonishing variety of life on earth), if this process was 100% accurate or faithful. So, even though traits are transferred from parents to their off-springs, the traits are usually marginally altered.
This gives rise to off-springs who are like their parents but with very minor changes. These changes could either be a) beneficial b) harmful (deleterious) or c) neutral.
Harmful changes cause the organism to perish before it can pass on these changes to the next generation. Sometimes a trait that existed in the earlier might become deleterious due to a change in the environment and thus, undergo the process of elimination from the population.
Beneficial changes help the organism to adapt better to its surroundings and live longer and have more off-springs such that these changes are spread to more and more organisms in the population and are eventually get fixed.
A series of such changes in a subsection of a population eventually lead to a new species which is nothing like the older generation. This process of “fixing” or beneficial changes and removal of deleterious traits from a population is called evolution.
Interestingly, the changes that seem the most inconsequential, the neutral changes, are responsible for the bulk of evolution. Neutral changes along with geographical isolation lead to a process called genetic drift, which is considered by many to be the strongest driving force of evolution.
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