For once, journal authors are arguing about something in evolution for which they have evidence … the design (or self-organization?) of the mammalian visual system. And it’s fascinating too.
Science 27 April 2012:
Vol. 336 no. 6080 p. 413
Comment on “Universality in the Evolution of Orientation Columns in the Visual Cortex”
Yicong Meng1, Shigeru Tanaka2, Chi-Sang Poon
Kaschube _et al_. (Reports, 19 November 2010, p. 1113) argue that pinwheel density in three mammalian species follows a universal constant of p as predicted by their orientation-selective suppressive long-range connectivity model. We dispute their conclusions and suggest that a simple brain size-pinwheel density scaling law suffices in predicting the self-organized and disorganized orientation maps from primates to rodents.
Kaschube _et al_. (_1_ ) proposed that the emergence of orientation columns in visual cortex can be explained by a common design in a self-organized network with suppressive long-range connectivity.
Based largely on theoretical grounds under a set of assumptions, their model predicts that the pinwheel density, defined as the average number of pinwheels per orientation-hypercolumn area, approaches a universal constant of p. Kaschube_et al_. reported that the precisely measured pinwheel densities in three mammalian species were virtually identical and approached the predicted value of p. These findings, if true, would be of fundamental importance in implicating a universal law in the self-organization of orientation columns across the mammalian phylum. However, … several concerns arise that cast doubts on the validity of the proposed universal constant…
Reply from Keil et al:
Meng _et al_.
conjecture that pinwheel density scales with body and brain size. Our
data, spanning a 40-fold range of body sizes in Laurasiatheria and Euarchonta, do not support this conclusion. The noncolumnar layout in Glires also appears size-insensitive. Thus, body and brain size may be understood as a constraint on the evolution of visual cortical circuitry, but not as a determining factor.
In summary, studying animals of different V1 size confirmed pinwheel density as a genuine invariant of orientation map design. The interspersed organization found in Glires constitutes a distinct design type also insensitive to V1 size. Orientation columns so far have only been found in large visual areas and may not exist in very small brains such as those of many late cretaceous mammals. In their descendants, the K-T extinction event enabled an explosive increase in body and brain size millions of years after the major mammalian lineages had separated. It triggered the separate evolution of architectures for large visual areas in distinct lineages. Our theory of universality in network self-organization explains how they could independently develop a common design. It is the only known explanation for the quantitatively precise agreement of orientation column layouts in tree shrew, galago, ferret, and cat. Other mammalian lineages are predicted to adopt the same design when using qualitatively similar developmental mechanisms. Precise quantification of visual cortical architecture in mammals from the extremes of body and brain sizes in all therian clades will help to clarify this fascinating chapter of brain evolution with rigor and certainty.
The word “design” appears 12 times in the article.