Why h2 does not always equal V A/V P?

Over the last decade, there has been a rapid growth in the application of quantitative genetic techniques to evolutionary studies of natural populations. Whereas this work yields enormous insight into evolutionary processes in the wild, the use of modelling techniques and strategies adopted from animal breeders means that estimates of trait heritabilities (h(2)) are highly vulnerable to misinterpretation. Specifically, when estimated using animal models, h(2) will not generally be comparable across studies and must be interpreted as being conditioned on any fixed effects included in the model. Failure to realize the model dependency of published h(2) estimates will give a very misleading, and in most cases upwardly biased, impression of the potential for trait evolution.