Convergence and common descent

Summary of problems with claim:

This is another instance where Explore Evolution uses ambiguous language to confuse students, rather than bring clarity to a subject. It is entirely predictable that shared selective pressures would produce superficial similarities from dissimilar anatomical structures. We expect the underlying anatomy to reveal underlying evolutionary relationships; superficial similarities are not expected to be evolutionarily informative.

Full discussion:

Explore Evolution claims:

For other scientists, the phenomenon of convergence raises doubts about how significant homology really is as evidence for Common Descent. Convergence, by definition, affirms that similar structures do not necessarily point to common ancestry. But if similar features can point to having a common ancestor and to not having a common ancestor how much does "homology" really tell us about the history of life?
EE, p. 48

It is not clear who the "scientists" are who advance this argument. As discussed above, scientists see nothing surprising about similar selective pressures producing superficial similarities between structures. The similarities that scientists consider evidence of common descent are similarities of underlying structures and developmental processes.

The point that EE raises here is not a terribly complex point, and closing the chapter with that question is hardly educational. Indeed, EE here passes up an opportunity for genuinely inquiry-based learning. It would not be difficult to prepare an exercise in which students would be asked to examine actual organisms, and to propose investigations which would test whether certain traits are homologous. Scientists perform such tests routinely, and a simplified example would allow students to understand that process. In doing so, students would come to understand that identifying similarities or differences between a particular structure in two species is the first step in a scientific inquiry. Students would learn that testing a hypothesis about homology requires comparison with other structures in other species. Students would also see that certain traits tend to vary rapidly in response to an organism's environment (coloration, for instance), while other traits are remarkably consistent (the number of bones in the limb). Students could then be given a new set of organisms to examine, and see how scientists use knowledge from previous research to inform new assessments of homology and homoplasy.

Instead of encouraging scientific inquiry, EE misuses terminology, makes false claims about the current state of the science, and then closes the discussion with a question to the students, without having given any indication of how students ought to go about addressing the question. This is a poor model of how science works, and a poor way of teaching any subject.

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