Where do species come from? A simple answer to this question is that microevolution gives rise to macroevolution. Populations continually undergo changes in allele frequencies and adaptive change. Eventually, populations that used to be similar diverge enough to be classified as no longer belonging to the same species. There are many mechanisms that prevent hybridization between divergent populations and species, and thus help keep them separate. The greater the differences in the habitats of the populations and their selection pressures, the greater the rate of the divergence. We intuitively recognize the concept of species: no one would argue that a giraffe and an oak tree belong to the same species, and the distinction between an oak tree and a maple tree is also clear. A precise definition of species is more difficult, however. We define species on the basis of shared characteristics and degree of interbreeding, but there are many gray areas. It is impossible to say precisely at what point two divergent populations become two separate species. The process is a continuum. When we look at the millions of species with which we share this planet, we see the result of billions of years of populations diverging under different selective forces in new and changing environments.