Humans are curious critters. We keep trying to pick apart reality to figure out how it works. Part of that is to break reality up into smaller chunks so it makes sense. Abstractions improve understanding and allow further refinements to the model. It's what science is based on.
Biology is a continually vexing problem, though. In many ways, it's a continuous function we keep attempting to turn into a discrete maths problem. Taxonomy, the naming of species, is a great example of this. Early classification methods relied on similar morphology to determine relatedness, and that gave us a nice family tree. Then we figured out how to sequence genomes and we learned how wrong we were; they're now moving whole species/phylum branches around. It turns out nature sometimes solves the same problem the same way through completely unrelated species.
What sparked the rearrangement? A new way to classify. A new method was picked to be more accurate, and changes had to be made.
Topically, take a look at OS classification of non-mobile consumer computing devices (what used to be called desktop-OS). You can see this on any web-visitor analytics platform. Some break it down like:
- Windows
- OSX
- Linux
Others get more specific, breaking it down to versions within OS:
- Window XP
- Windows 7
- Windows 8
- OSX 10.6
- OSX 10.7
- OSX 10.8
- OSX 10.9
- OSX 10.10
- Linux
For some reason they don't break apart the Linux versions. Perhaps because it's such a small segment of the market and highly fragmented at that. Still more detailed charts go down to Windows service-pack levels. OS version is a discrete space, but in order to provide a brief chart some simplifications are made. Each analytics application makes its own classification decisions.
Less topically, lets take a look at a fictional made-up species, the Variegated Civet. Take the physical sex of this critter. The original population study was done in 1906 and an odd sex ratio was observed, 1.3 females to every male. As with all studies of the time, external morphology studies were used to determine sex with a few dissections as a cross-check.
Fast forward a bunch of years and genetic studies become financially doable for an appropriate sample-size of the population. It reveals a funny thing. Some of the females are genetically male. This raises eyebrows and further studies reveal the cause. A significant percentage of males undergo gonadogeneis at puberty, not in-utero, which skewed the original study's sex ratio.
A new classification technique, genetics, reveals an interesting feature in a specific population. It also raises the question of what how to differentiate pre-puberty males with fully formed gonads from those who will do so later. A third sex may need to be created to explain this species.
We're undergoing an attempt to change the cultural classification method for gender in humans. For ages it has been based on physical morphology and came in two types. Nature being nature, there are plenty of ambiguous presentations to make the classifier problem harder (intersex); but not enough to prompt the creation of a third gender. Those weird-cases were assigned into one or the other, which ever was closer, in the opinion of the classifier (sometimes nudging things along with a bit of surgery). For ages gender was a synonym of sex.
That's beginning to change, and it's not been an easy thing to bring about. For one, gender is becoming more widely seen as discrete from sex. For another, gender is at the early stages of redefining its classifier away from external morphology and/or chromosomes and into self description. Self-description brings it away from a discrete function (binary, trinary) and into more of a multi-axis graph.
It takes a long time for a change like this to take hold, and there are fights being had. Vital Records only record one of these, and it's still legally entangling both sex and gender. Maybe in some future time driver's licenses will have both sex and gender fields on it. Or maybe those fields will be left off all together (the better option, in my opinion). Chromosomes are not truth, as nature's continuous function ensures there will always be an exception (complete Androgen Insensitivity Syndrome is the big exception to the XY = Male 'truth').
The work continues.