A recent article in the journal Nature on the evolution of novel phenotypes in seahorses presents a great example of the Nature of Science (NOS) and Scientific Reasoning. This study is also exemplary of how changes in just a few genes can produce spectacular phenotypic adaptations and drive the evolution of new species.
Below are some key NOS passages from the article with my breakdown of the scientific reasoning process in action (H. comes is the particular seahorse species they studied).
“Loss of function of this gene in mouse leads to a failure of hindlimb formation as well as strong pleiotropic defects in lung and placental development. Expression of zebrafish tbx4 specifically in pelvic fins suggests a similar role in appendage patterning in fishes.”
Abduction – The stealing or borrowing of previous ideas and applying them to the current situation. In other words, analogical reasoning and hypothesis formation. In this particular case, the researchers are coming up with a generalizing hypothesis (a description of a pattern) and are looking for that same pattern in seahorses. The key word is “associated” because the researchers aren’t actually proposing a mechanism (an explanatory hypothesis) for how tbx4 works during embryogenesis:
“Given the major role of tbx4 in hindlimb formation in mammals, we hypothesized that its absence in H. comes might be associated with the loss of pelvic fins.”
Deduction – In science, the act of testing a hypothesis (or theory or law) with a set of methods and making a prediction of its consequences. In the planning stage, this usually includes a specific and measurable prediction. Since this is the publication of the study, they have left their prediction out, but one can infer it from this passage:
“To test this hypothesis, we generated a CRISPR–Cas9 tbx4-knockout mutant zebrafish line. Interestingly, unlike homozygous mouse Tbx4 mutants, which fail to develop a functional allantois, the homozygous zebrafish mutants are viable but completely lack pelvic fins without exhibiting any other gross morphological abnormalities in pectoral or median fins.”
Induction – The most slippery part of the reasoning process is generalizing from the specific. This is where huge mistakes in reasoning can be made by scientists (and students) if they fail to consider the possibility of alternative explanations for their results and the possibility of violating their major assumptions about the known universe (called auxiliary hypotheses). Indeed, false hypotheses can make true predictions. One can tell from the tentative nature of the following passage (“is consistent with” “suggests” “may”) that these researchers are experienced authors:
“This finding is consistent with the results of a recent study that showed that mutations in tbx4 are associated with the loss of pelvic fins in a naturally occurring zebrafish strain called pelvic finless. These results show that tbx4 has a role in pelvic fin formation in teleosts and suggests that the loss of pelvic fins in H. comes may be related to the loss of tbx4.”