Alison Arrak

Critique of:

Martin, A. 1993. Hammerhead shark origins. Nature, 354:494.

The hypothesis of this journal, as stated clearly in the introduction is as follows: knowledge of the phylogenetic relationships among hammerhead sharks allows the estimation of the course of cephalofoil evolution. The introduction of this journal is easily digestible for the laymen, defining the evolutionary parameters concerning the explored states of affairs and setting up the problem that the journal is trying to address quickly and clearly. Immediately, the author launches into explaining the current hypothesis concerning the phylogenetic relationships among the different hammerhead species and referencing it, this shows the author is knowledgeable on the subject and ready to argue his hypothesis.

In the body of this journal the author goes on to explain how he tests his hypothesis; mitochondrial DNA analysis of eight hammerhead species. Evidence from this testing proves a monophyletic origin of the cephalofoil and also goes on to prove a different sequence of evolution versus what was previously thought. The basis of his argument is that small head species are derived, whereas the large head species are ancestral. This is where the journal is lacking in data. The author claims that his data supports his hypothesis but there is no DNA related data presented to prove this. Although the author states in his paragraph under figure 1 that DNA data is available upon request, not enough data is presented to explain how and why this evidence fully supports his argument.

The visual graphics presented in this journal provide a nice visual description of his phylogenetic hypothesis, it clearly shows the evolution of the cephalofoil providing body and head measurements. The author again references other scientists involved in creating this particular phylogenetic tree. This is where the phylogenetic tree data becomes very technical, hard to understand, but referencing does ensure that there are other resources available to explain the information.

Finally, in the conclusion of this piece, the problem has been stated and it has been addressed... but the author does not tie them together in his conclusion. Instead he goes into explaining the function of the cephalofoil and the different organs within it responsible for certain functions. It is here where another hypothesis is put into the picture, "identification of at least two different functions provides the opportunity for selection to act in different directions, a possibility that may explain the existence of multiple cephalofoil designs." (Martin, Nature, August 1993) The author leaves us at somewhat of a loss as far as the focus of his hypothesis is concerned as it is fragmented. Overall this journal is clear in the introduction and quite accessible. However, figures, data and conclusion are abundant and sometimes fun to look at but as a professional scientific piece is concerned, it is my opinion that the author did not adhere to the rules of convention.