Microevolution and macroevolution
Evolution is change in heritable traits of biological populations over successive generations. Evolutionary processes give rise to diversity at every level of biological organisation, including the levels of species, individual organisms, and molecules. All life on Earth shares a common ancestor known as the last universal ancestor, which lived approximately 3.5—3.8 billion years ago.
The study of evolution can be performed on different scales. Microevolution reflects changes in DNA sequences and allele frequencies within a species over time. These changes may be due to mutations, which can introduce new alleles into a population. In addition, new alleles can be introduced in a population by gene flow, which occurs during breeding between two populations that carry unique alleles.
In contrast with microevolution, macroevolution reflects large-scale changes at the species level, which result from the accumulation of numerous small changes on the microevolutionary scale. An example of macroevolution is the evolution of a new species.
The species problem
Biologists frequently disagree about species, and even argue over how best to define the word species.
Darwin, Mayr, Simpson and others have taught us about species, but none has been broadly convincing on the basic questions of what the word 'species' means or how we should identify species. For its entire brief history, the field of evolutionary biology has simply lacked a consensus on these two related questions. Indeed, there was broader consensus before Darwin.
Last year, two papers were published proposing that our ancestors had sex with at least two kinds of archaic humans at two different times and places. Both Neanderthals and mysterious humans from Denisova Cave in Siberia interbred with ancient modern humans - and those liaisons produced surviving children, according to the latest ancient DNA research. But the researchers avoided the thorny question of species designation and simply referred to Neandertals, Denisovans, and modern humans as "populations." So were the participants in these prehistoric encounters members of separate species? Doesn't a species, by definition, breed only with others of that species?