I presented my ideas on the limitations of linear evolution, versus tree thinking, in a presentation to a Department of Ecology and Evolution, and got an interesting comment. A colleague challenged my statement that the "morphological series" of eyes (at the time I presented a specific figure from Salvini-Plawen and Mayr on gastropod eyes, which will be the subject of a future post) was presented as a model of evolution. The commenter was actually was a colleague of Mayr at the time, and the commenter stated that Mayr and people at that time weren't thinking of this as a model for how evolution actually proceeded.
Well I am very grateful for this comment, because others may be thinking the same thing - that the linear series of eyes are not meant to represent a model of evolution per se. In fact, I believe this may be true, at least for the time when it was authored. However, as I will explain below, people do currently use eye series as models for how eyes actually did evolve.
So, why do I believe that there initially could have been different intent? Well, all these morphological series do is to test an ancillary assumption of the hypothesis that natural selection drove the origin of complex eyes. If natural selection did and could drive eye evolution, an ancillary assumption is that a series of intermediate steps exists between a simple and complex eye. This was Darwin's strategy in Origin - to point out support for this ancillary assumption by highlighting eyes of living animals at many different levels of complexity. Salvini-Plawen and Mayr refined Darwin's idea by providing such series in closely related groups of animals. True, a test of an ancillary assumption of natural selection may not have been intended as a model of how evolution actually proceeded. Incidentally, another ancillary assumption that natural selection produced complex eyes is that there was enough time for the process to work. Nilsson and Pelger tested this with a morphological model, and several calculations, but that is a topic for another post. Perhaps, Salvini-Plawen and Mayr were not equating the eyes of living animal eyes with ancestral states of extinct animal eyes (although this is not uncommon). Certainly testing an ancillary assumption is good science, but it also may not represent the proposal of an actual model.
Nevertheless, these morphological series are now taken as models of how evolution actually proceeded. For example, I just came across such a series in the textbook I use for my Macroevolution course. The text is Ridley's Evolution, 3rd Edition. Note that the caption begins, "Stages in the evolution of the eye".
At this point, I need to re-iterate, there is value to these series, in testing an ancillary assumption. But it could be dangerous to equate the eyes of living species, with those of species in a progessive series of evolution, from simple to complex eye. Furthermore, these series ignore the origins of variation. For example, the caption states "An eye is protected by adding a transparent cover of skin and part of the cellular fluid has differentiated into a lens." How does an eye add a transparent cover? Where does the variation come from? How does fluid differentiate into a lens?
Studying natural selection at the level of morphology alone is silent on these points, because it assumes variation is abundant, and that variation is then left to be anonymous. But exciting times are upon us, because we are know beginning to uncover the molecular basis for phenotypic changes. There is no designer - it is all mutation and natural selection - but by looking at the molecules, we can gain a new level of detail in our understanding of how complex traits evolve.
For one example, where we've gone beyond a morphological series, see a previous post of mine on the evolutionary origin of a cephalopod lens....