Macroevolution is evolution at or above the level of a species. The starting point of macroevolution is speciation, the process through which a species evolves into two or more new species. A species is defined by its ability to interbreed with others within its group and its inability to interbreed with outsiders. For a new species to form, a group must become reproductively isolated from other groups. Reproductive isolation can occur at any step of reproduction. It can produce barriers that prevent fertilization and the formation of a zygote (a fertilized egg)
. Other barriers prevent a zygote from developing into a healthy, fertile adult. Once reproductively isolated, groups evolve independently as they experience different forces of microevolution, a change in allele frequencies within a population from one generation to the next. Over time, their gene pools may diverge, with each group developing traits that are unique to its population. If these two groups meet up again in a shared environment, they may not be able to produce healthy, fertile offspring. In this case, the two groups have become separate species. Speciation can happen gradually or abruptly.