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Darwin's book, On the Origin of Species met immediate popularity. It sold so well that the publishers undertook a second printing a mere month after the first. However, the ideas contained in Origin were not immediately accepted. This disparity arose in large part because of the simple fact that the world had not yet discovered genetics. Darwin's theory rested greatly on the assumption of the inheritance of traits, but no one at the time knew how such inheritance took place. The situation changed in the early 1900's with the birth of the field of genetics. Evidence from this field and others combined with Darwin's framework formed the modern theory of evolution, called the modern synthesis. Read the SparkNote on Darwin.

In this SparkNote, we will discuss the main points of the modern synthesis. Evidence from experimental genetics, mathematical modeling of populations, direct observation of natural populations, and study of the fossil record all made contributions to the modern synthesis. We will also examine the mechanism of natural selection as it can now be understood from the standpoint of genetics. We will see how mutation gives rise to new alleles, which may then convey greater fitness to individuals bearing them, and thus be spread throughout a population.