See the remark above Definition of G.
G is the sum of evolutionary ability. That is: if a person's performance over a wide enough range of abilities is summed, this indicates the amount of G in that person. This makes sense as abilities tend to correlate positively; they do so because they have been selected throughout evolution by the same criterion: survival. Abilities giving greater evolutionary advantage get higher correlations with G than those giving less advantage. This hierarchy itself may evolve in the course of evolution, be it slowly and generations behindhand. An ability's correlation with G reflects, with delay, its evolutionary value; the ability hierarchy forms a "fossil record" of evolution.
An analogon: imagine a tray filled with dice. On each side of each die is a random number between (including) 1 and 99. Now suppose someone takes a die, looks at a random side of it, and throws it into a second tray if he sees a 1, 2, 3, 4, 5 or 6. Otherwise, he throws it back into the first tray. He repeats this indefinitely. A second person does the same between the second and third trays, etcetera.
Now suppose at the umpteenth tray someone tests all arriving dice for "1-ness", "2-ness", etc.; meaning he looks if they have a 1, 2 etc. on them. Obviously he will find positive correlations between the properties 1-ness, 2-ness... 6-ness. That is: a die with a 1, 2... 6 on it is more likely to also have one or more 1s, 2s... 6s on its other sides than it is to have any particular other number on its other sides. For instance, the correlation 4-ness vs 5-ness will be higher than 4-ness vs 11-ness.
Now imagine in the chain of trays each person selects for only one number, and more are selecting for 1-ness than for 2-ness than... 6-ness. At the end of the chain we now find a correlation hierarchy, with 1-ness on top. That is, if each arriving die is tested for the six properties, and the 15 intercorrelations computed, 1-ness has the highest average of correlations, followed by 2-ness, 3-ness... 6-ness. Also, if for each die its 6 scores are summed - 1 for having a property, 0 for not having, so maximum score is 6 - 1-ness will show the highest correlation with this sum, followed by 2-ness... 6-ness.
A simple model of the hierarchy of evolutionary ability (educated guess):
Detail:
1 Meta-mental - Resulting from unaware complex mental work (mainly reasoning). We get the result in our head, without knowing where it came from. An Überordnung of mental ability. The evolutionary advantage of these abilities lies in their value regarding group survival; only with a sufficient amount of altruism a group - and so its gene pool - can survive. Evolution is about group survival. By being dishonest and criminal, the individual can survive, but not the group; not the genetic material. Therefore egoistic abilities will always stay low in a surviving group; there will never be more than a few bad apples in the basket, or it wouldn't have made it to the store. It is thinkable humans are still to make progress in this field, resulting in abilities yet unknown.
Meta-mental ability is the summit of evolution, having of all abilities the highest correlation with G. In practice this means people with higher G levels are more moral, and above a certain G level immorality, including lying, cheating etc., becomes biologically impossible. Note the morality meant here "comes from within" and is not learnt from educators or related to law, culture or religion.
2 Mental - I discussed these in "Definition of G". Traditional IQ testing deals almost exclusively with mental ability. This makes sense as mental ability in humans likely has the highest correlation with G of all objectively measurable abilities. Still, it might be worth considering including physical ability in tests. The famous White-Black difference in mental ability, considered proven by leading psychometricians (not by geneticians though) might decrease. Also, a test's correlation with G would rise with other than mental abilities included, even though the additional abilities themselves have lower correlations with G. In general, the wider a test's range of abilities, the higher its correlation with the general factor. Finally, the popularity of these tests among the wider public would rise, as more people would see their ability profile recognized. E.g., people who are "good with their hands" now strongly dislike IQ tests.
3 Emotional - The evolutionary value of emotion is it incites one to perform tasks needed for survival; e.g. take care of children, family or oneself. This was most needed in the early days, when humans did not yet have our awareness and higher mental ability. Nowadays still part of our behaviour is governed by emotion, be it we are more or less able to mentally decide if to follow it.
Emotional ability must not be confused with fashionable fallacies like "EQ" ("emotional" or "social" "intelligence"). EQ is a set of immoral tools to manipulate people. Meant here is emotion itself; the ancient evolutionary "propulsion" of man's behaviour.
4 Peripheral - Between mental and physical; linking mind to matter, so to speak. These abilities are vital not only for "working with your hands", but also for art forms like visual, graphic, music, dance, acting etc. For anything requiring body action and perception (including time perception). Note musical ability involves sense of time; both rhythm and pitch are "time" phenomena. Peripheral ability is measurable, but excluded from traditional IQ tests.
5 Physical - Most of these go without saying. Pulchritude definitely offers an evolutionary advantage and is unequally distributed, so it qualifies as an ability that contributes to G. Physical ability is measurable, but excluded from IQ tests.
6 Autonomous - These functions are usually taken for granted and not given much thought. We have no direct control over them, but indirect influence is possible via relaxation methods, exercise and food.
Of these six ability types, only three appear suitable at this moment for inclusion in a test for G: Mental, Peripheral and Physical. These are objectively measurable. I have created a test called Grail Test, that measures a practical selection from those. This test is individual and supervised; it takes about four hours.
Apart from measuring and researching G and its components, it is tempting to think of ways for the individual to improve his G level by maximizing each ability type, within the limits of the given biological\genetic material. A systematic approach to train the elementary features of each ability type. More about that later.