Just doing some SQL against the Catalogue of Life ’09 database and came up with some figures unrelated to what I set out to do. I can’t decide if they are useful or not so they are going here.
| Species | % of species | Genera | % of genera | |
| Species in monotypic genera | 41792 | 3.6 | 41792 | 38.3 |
| Working memory >1 and < 10 | 184811 | 15.92 | 46918 | 43 |
| Class Size >9 and <30 | 217306 | 18.72 | 13286 | 12.18 |
| > 29 species | 716609 | 61.75 | 7114 | 6.52 |
| Totals | 1160518 | 100 | 109110 | 100 |
I wondered what the mean/median genus size is but got distracted into wondering “Given any one species what does knowing the genus tell me about that species?”. I figured that we (being human) have rough classes of sizes of groups of thing. Firstly there are the unique things – monotypic genera. Then there are the small groups of things that fit in our working memory (7+/- 2). Then there are groups of things that are large but memorable like the size of a class of school children (in the UK) up to about 30. A typical teacher can probably list them and a taxonomist who was monographing such a group could probably list the species. Over 30 I figure we have the kind-of-things category where we just give up and may know set individuals but are more likely to think of set characteristics. This size includes anything over 30 right up to the super genera with thousands of species.
What does this mean in practice? I don’t know. Maybe something will come to you.
Looking back at this 81% of genera have < working memory species in them. So if you have a genus name the chances are you have access to a list of things you can hold in your mind. Either that or you have access to a list of things you can't hold in your mind at one go but can probably generalise about. 80/20 rule?
I wonder if the working memory limit has historically influenced how many species we are willing to lump together to define a genus?
@Karen Cranston
The more I think about this the more I think they are related. Take any accepted genus and there is an 81% chance it will be the size that we can fit in working memory and a 93% chance it will be a size we can get our heads round with a bit of study – or smaller.
I did a few queries on the genera in grassbase (http://www.kew.org/data/grasses-syn) a few years ago, and the number of species per genus seems to follow a power law (ie if you plot logs of no of speces per genus against the number of genera that have this number you get a straight line. Long tail strikes again 😉