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In creating categories, there is a major intellectual divide between two, conflicting, orientations:
For most people most of the time, these two different orientations do not produce any significant conflicts. Apples and oranges and bananas are all fruit, and go into the fruit category. Or apples and oranges are round, while bananas are long, and so apples and oranges go into the round category while bananas go into the long one. Oranges and lemons and melons are all classed as fruit, but they readily break down into citrus fruit as against melons in other contexts. And so forth and so on.
In the study of only partially understood fossil animals, however, it is often difficult to decide how things should be classified. If a new specimen is found that is “kind of” like known specimens and “kind of” different, should we tentatively regard it as the same or different? In the study of fossils (including all sorts of proto-humans and almost-proto-humans), specialists often contend vigorously about where species boundaries ought to lie.
Lumpers tend to assume that separation of animal populations is rarely complete, that mutations do not occur often, and that the process of speciation is therefore not fast. The best assumption is that similar forms are probably best classed in one category until there is some reason to do otherwise.
Splitters tend to assume that speciation occurs relatively quickly when breeding populations are separated, that separation is usually quite complete, and that therefore speciation is quite a rapid process. We should assume that there are and have always been lots of species out there, the vast majority of which are, of course, long extinct, and, in the absence of other evidence, any new specimen is likely to be an example of a different species from previous specimens.
No two specialists balance lumping and splitting in quite the same way, but it makes a huge difference to the terminology that they use, and to the historical possibilities they think about.
Lumping tends to reduce the number of genus and species terms. Most popular authors tend to be lumpers. It makes for easier reading and avoids celebrating potentially trivial differences or needlessly multiplying names. (For example, most popular authors speak of Homo erectus, and do not separate Asian forms as Homo erectus from very similar African forms called Homo ergaster.)
Splitting tends to increase the number of genus and species names, at least in the short term until they are clearly shown to be redundant. Specialists, unlike popular authors, tend to be splitters, lest they lose sight of variation that could turn out to matter. Splitting, although more confusing, is arguably intellectually safer than lumping when the evidence is ambiguous.
Cautionary Note: Nobody lumps or splits all the time. That would be stupid. The terms “lumper” and “splitter” refer to only to the default assumptions applied in cases that are ambiguous. One hears rumors of exam questions being designed with this in mind.
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