Assessing non-metric skeleton characters as a morphological tool

The appearance of non-metric skeletal characters in vertebrates results mainly from basic genetic control, as proved and documented for house mice and humans. Although the heritability of non-metric traits, mainly the presence of foramina and similar structures for blood vessels and nerves or dental occlusal variants, have been evaluated as rather low, the simultaneous consideration of several traits allows estimation of epigenetic variation in time and space as a result of genetic relationship. Thus, the main use of non-metric characters has been aimed at assessing epigenetic variability and divergence among populations. Applications extend from the problem of genetic isolation of populations, the lack of reproductive contact, detection of genetic drift, systematic studies to clarify species taxonomy, to phylogenetic interpretation. Additionally, non-directional deviations from bilateral symmetry in non-metric characters, e.g. fluctuating asymmetry, could be caused by current environmental conditions in general. Fluctuating asymmetry is regarded as a measure of developmental instability to indicate the presence of genomic changes or the influence of contamination and/or habitat deterioration, and is also used as a further population parameter with integrated information. However, standpoints on the use of fluctuating asymmetry are quite inconsistent, fluctuating themselves between considering it a powerful biomonitoring tool to being merely a curious scientific toy.