There’s probably a list of this somewhere, but I can’t find it.
What genus has the most species? In another thread it was mentioned that there are over 40,000 species of spiders. I sort of remember hearing that crabs have about 20,000, as do orchids. What about beetles? Pines?
And what genus has the fewest? The one I can think of is elephants, with only two. Is there a genus with only one?
I’m pretty sure most is beetles.
Dunno about least – do you wish to include only current species? Because some of the megafauna have higher counts if you include their extinct kin.
There are hundreds of genera with only one species. Plenty of families with only one species. Even orders with only one species. Heck, there are phyla that have only one species.
ETA: Also “spider” or “crab” wouldn’t be examples of genera, but rather orders: order Areana within class Arachnidia within phylum Arthropoda, and order Decapoda within class Crustacea within phylum Arthropoda.
Are there any other extant species in Homo? That’s the genus I think of first.
Well, at least in fossils, there’s homo habilis and homo erectus.
I believe the Aardvark is the only member of its family, order, and genus, and I don’t think there are even fossil relatives considered to be members of the family.
ISTR they found a water cave critter that was a new genus, and I suspect it is still the only one. It might have even been a level or two up, making it really special, if not exactly what the OP is asking.
I’ll let the google fu experts and biolgists find that one.
There is one living species in the order Sphenodontia.
For the sake of simplicity, yes.
Two, according to Wikipedia.
The first example of a single-species genus that comes to my mind is the Venus Flytrap, Dionaea muscipula. The order that contains it also has things like carnations and spinach, as well as more closely related carnivorous plants.
By the way, the name you are looking for is “monotypic” genera. Wikipedia has a list of over 1,000 of them.
Maybe extant doesn’t mean what I think it means.
There are probably more monospecific genera than polyspecific genera. So we could list thousands of them.
That’s what I get. I originally wrote “two” but then misread a Wiki entry and corrected it to “one.”
I stand by my first post. 
Extant is the antonym of extinct. An extant taxon is one that has living members.
There is only one extant species of the genus Homo: Homo sapiens. Unless we want to go all radical and lump chimpanzees into Homo. That would invalidate all sorts of genera for extinct hominids.
As for the most, my Googling isn’t coming up with much.
The greatest number I’ve found so far is Acacia which has somewhere north of 1300 species. “Orchids” and “beetles” etc comprise loads of genera.
The modern elephants are actually in two different genera: Loxodonta and Elephantus.
Elsewhere in the animal kingdom, the two species of tuatara are the only member of their class, order, family, and genus.
But including all living things, I think the champion is probably the ginkgo tree, with only one extant species (Ginkgo biloba) in the entire division (equivalent to phylum).
Taxonomists are almost always willing to split a genus if its member list is growing to an unwieldy size.
Heck, most of them are willing to name a new genus if it can get them another paper published.
There are literally hundreds of monospecific genera (genus-es with only one species).
I have no clue as to what genus actually has the most species. Some WAGs:
– among all life, probably Bacillus, the capsule-shaped bacteria.
– among plants, genus Poa, the common grasses, probably far exceeds any other genus in number of species
– among insects, Musca, the houseflies and allies. While there are more species of beetles than almost anything else you can name, there are a lot of beetle genera, none of which to my knowledge have a large number of species.
– among mammals, my guess would be Mus or Rattus.
– among birds, Passer, the sparrows, with at least 27 distinct species. While there are dozens of species of warblers, they belong to a wide assortment of nont-particularly-closely-related genera.
As for the ginkgo, it’s definitely monospecific, and that quite far up the taxonomic hierarchy. But depending on who’s doing the classifying, Ginkgoformes may be only an order among Gymnosperma.
I was thinking of Formica - but on checking I see that there are fewer than 200 species.
Wikipedia says Poa has only about 500 species, less than half that of Acacia. In fact there are said to be almost 1000 species of Acacia in Australia alone.
Really? I hadn’t even heard that there were two distinct subspecies of African elephant (presumably, the two species some are arguing for are at least widely recognized as subspecies, right?).