Table 1.
Proposed analytical trait types
| Type 1 |
| A trait that differs in two taxa because its presence and/or expression are downstream consequences of differences in the PI of its cells and its resultant effects on local pattern formation. Type 1 traits are fixed by directional and/or stabilizing selection because their primary functional features have a real effect on fitness (65), and result largely from a direct interaction between genes expressed during field deployment and the functional biology of their adult product. Example: the superoinferior shortening of the ilium in hominids. |
| Type 2 |
| A trait that is a collateral byproduct of field changes whose principal morphological consequences are selected. Type 2 traits differ in two taxa because of differences in pattern formation (as in Type 1 traits), but their functional effect is so minimal as to have had no probable real interaction with natural selection. Unlike Types 4 and 5, they represent true field derived pleiotropy. Example: the superoinferiorly shortened pubic symphyseal joint of hominids (for discussion see text). |
| Type 3 |
| A trait that differs in two taxa because of modification of a systemic factor that affects multiple elements, such as an anabolic steroid. Example: Body size and its allometric effects. Growth hormone indirectly regulates growth by controlling levels of secondary signals such as insulin-like growth factor 1. Zaire pygmies exhibit reduced levels of growth hormone binding protein and, therefore, lower levels of insulin-like growth factor 1 (reviewed in refs. 66 and 67). In giant transgenic mice, altered growth hormone levels increase the growth rate without changing the period over which growth occurs (68). This generates shape changes by allometric scaling effects: i.e., differential growth rates of elements (66). Such growth response mechanisms are also likely to be controlled by highly conserved SAMs (69). Allometric shifts, therefore, usually reflect slight changes of systemic control factors during development: e.g., small modulations of growth hormone and/or its related factors can generate fully coordinated morphological change. |
| Type 4 |
| A trait that differs between taxa because its presence/absence and/or “grade” are attributable exclusively to phenotypic effects of the interaction of SAMs and environmental stimuli. Such traits have no antecedent differences in pattern formation and therefore have no value in phyletic analysis. They are epigenetic and not pleiotropic. However, they provide significant behavioral information and are therefore of expository or evidentiary value in interpreting fossils. They result from habitual behaviors during development. Example: the bicondylar angle of the femur. |
| Type 5 |
| Traits arising by the same process as Type 4 traits but that have no reliable diagnostic value with respect to significant behavior. Such traits are not consistently expressed within species and often show marked variation of expression within individuals and local populations. Example: femoral anteversion. |