A species is a group of organisms that can reproduce in nature to form healthy, fertile offspring. A common misconception surrounding British naturalist Charles Darwin's book On the Origin of Species is that the title refers to how life on Earth began. The title actually refers to the process through which a species evolves into two or more new species, which is called speciation.Speciation is part of macroevolution, which is evolutionary change at or above the species level. Like all evolution, speciation begins with the mechanisms of microevolution, which are changes in allele frequencies within a population from one generation to the next. A population is an interbreeding group of individuals of the same species. Small changes accumulate until a new species emerges. Understanding how new species form requires a formal definition of what a species is. Some species can be easily differentiated, such as a rose (e.g., Rosa mundi, a species of "old" rose) and a rhinoceros (e.g., Diceros bicornis, black or hook-lipped rhinoceros). However, much variation exists throughout life, and some of it is subtle. For example, separate species of birds, such as cuckoos and sparrowhawks, can have color markings that are nearly identical at first glance. Some individuals of the same species have very different forms, such as the sunburst cerulean-satyr butterfly (Caeruleuptychia helios), which is found in variations from bright blue to dull brown. Additionally, some organisms undergo a complete metamorphosis throughout a single lifetime. The first biologists examining caterpillars and butterflies of a given species may have initially suspected they belonged to two different species rather than different stages of the same species.