Variation is common, and often lingers in places where it is unexpected. The drawing to the left is from West-Eberhard’s Developmental Plasticity and Evolution(amzn/b&n/abe/pwll), and illustrates six common variations in the branching pattern of the aortic arch in humans. These are differences that have no known significance to our lives, and aren’t even visible except in the hopefully rare situations in which a surgeon opens our chests.
This is the kind of phenomenon in which I’ve become increasingly interested. I work with a model system, the zebrafish, and supposedly one of the things we model systems people pursue is the ideal of a consistent organism, in which the variables are reduced to a minimum. Variation is noise that interferes with our perception of common underlying mechanisms. I’ve been thinking more and more that variation is actually a significant phenomenon that tells us something about where the real constraints in the system are. It is also, of course, the raw material for evolution.
Unfortunately, variation is also relatively difficult to study.
There is some good work in the scientific literature that emphasizes variation. The drawing to the right is from a well-known paper on the morphology of amphibian limbs. There was a fortuitous (for us, not them) mass kill of newts in a small pond in California — a sudden freeze catastrophically killed the entire population. Shubin et al. (1995) collected more than 500 dead rough-skinned newts (Taricha granulosa), and counted and characterized the pattern of bones in their limbs. The drawing is of the canonical pattern. Only 70% of the population actually had limbs like this: the rest had missing bones, additional bones, and fused bones; interestingly, some of the patterns were atavisms, in that they resembled the limb structure of related but more primitive species.
Variation is common and important, and it’s all around us. The questions are about the source of that variation, and as I’m finding, how to study it. It’s hard to capture and isolate noise in a way that allows one to study it with the same consistency and convenience as regularities.
West-Eberhard discusses another particularly instructive paper, some observations made by Slijper in the 1940’s on a bipedal goat. This poor beast was born with missing forelimbs. Surprisingly, it lived for a year before it died in an accident (presumably unrelated to its deformities), and adapted in unexpected ways. It could hop around on its two hind legs, and developed additional peculiarities: enlarged hind limbs, a curved spine, and large neck.
At left are the bones of the hind limb of a normal goat (A) and one that had been born with congenitally absent forelimbs (B). The differences are obvious, and are related to the changes in posture the goat had to maintain.
Pelvic musculature of a normal (A) and two-legged (B) goat. The gluteus muscle (“gt”) has a long anterior extension, which has been reinforced anteriorly by novel tendons (“t”). These are new anatomical structures that were generated in the absence of any direct genetic specification.
The pelvic bones responded to the unusual stresses imposed on them with changes in shape, as well. To the left is the pelvic skeleton of a normal (A) and two-legged (B) goat (“i”=ischium). Slijper noted that the dorsoventral flattening and elongation of the ischium resembled the forms seen in kangaroos, a naturally bipedal animal.
These are not genetic changes — the goat was ordinary domestic stock, and presumably had perfectly ordinary genes that, under normal circumstances, would have generated more typically goatish morphology. These are instead the indirect consequences of a plastic phenotype, responding to a radical change in its environment.
The point of the two-legged goat example
…is not to argue that these handicapped individuals might have given rise to durable novelties, for it is unlikely in the extreme that such individuals would outperform normal individuals in nature. Rather, the point is to dramatize how a change in one aspect of the phenotype — in this case the front legs — can lead to correlated changes that show a degree of complexity and functional interaction that we usually assume to require generations of natural selection and genetic change at many loci. Clearly, the phenotype can be elaborately restructured, due largely to adaptive plasticity, without a proportional restructuring of the genome.
I think there are several important messages here. One is that genes are not “for” some feature; the absence of forelimbs did not conjure a gene “for” gluteal tendons into existence. Rather, cellular patterns of gene expression are regulated in response to the environment, and in turn modulate the environment of other cells and tissues. We have been conditioned by years of good (but selective) results in genetics and molecular biology to view gene expression as an end result. It is not. In development, gene expression is part of a process that produces an end result in collaboration with multiple other factors.
Another message is that the flexibility of an individual’s genome is great. It’s easy to slip into the trap of thinking that the phenotype is a predictable consequence of the genotype, that development is a process of translating genes into morphologies, in a way that is just more prolonged and elaborate than the way transcriptional machinery translates gene sequences into proteins. This isn’t the case. Developmental plasticity means that a single genome contains the potential to generate multiple morphologies.
Shubin N, DB Wake, and AJ Crawford (1995) Morphological variation in the limbs of Taricha granulosa (Caudata: Salamandridae): evolutionary and phylogenetic considerations. Evolution 49(5): 874-884.
West-Eberhard MJ (2003) Developmental Plasticity and Evolution. Oxford University Press.