 Although the work of Charles Darwin (see the entry on  Darwinism) is usually the starting  point for contemporary understandings of evolution, interestingly, he does not use the term in the first edition of On the Origin of Species, referring instead to “descent with modification”. In the early-mid 20th century, the “modern synthesis” gave birth to  population genetics,  which provided a mathematization of Darwinian evolutionary theory in light of Mendelian genetics (see also the entry on  ecological genetics).  This yielded a prevalent—probably the most prevalent—understanding of evolution as “any change in the frequency of alleles within a population from one generation to the next”. Note, however, that this definition refers to evolution only in a microevolutionary context and thus doesn’t reference the emergence of new species (and their new characteristics), although it is intended to underlie those macroevolutionary changes (see the entry on    philosophy of macroevolution). 