A hyostylic jaw suspension in chondrichthyans allows protrusion of the upper jaw independent of the lower jaw

Question

Evolutionary biologists have assumed that the ability to protrude the upper jaw is advantageous for feeding—in other words, that it is an adaptive derived character. An alternative hypothesis is that a hyostylic jaw suspension is a neutral ancestral character (i.e., one that is neither advantageous nor disadvantageous). These two hypotheses (i.e., hyostylic jaw is advantageous/hyostylic jaw is neutral) generate different predictions about the phylogenetic distribution of hyostyly. What are those predictions, and which one is supported by the phylogenetic distribution of hyostyly?

Answer 1

If a hyostylic jaw is a neutral character it probably evolved only once, but if it is advantageous it might have appeared several times. In other words, the hypothesis of a neutral value would be supported by a single appearance of hyostyly in the phylogeny of sharks, whereas the hypothesis of an adaptive value would be supported by multiple appearances of hyostyly.
A phylogenetic analysis shows multiple appearances of hyostyly, providing support for the hypothesis that this type of jaw suspension is advantageous. That part of the answer is straightforward, but there's a second component that is more subtle. A single appearance of hyostyly cannot rule out the possibility that it was advantageous for the lineage in which it appeared. In other words, a single origin would support the hypothesis that hyostyly is an ancestral character that was inherited by descendant lineages, but it would not provide information about the adaptive value of that type of jaw suspension.