Genetic Assimilation: Evolutionarily Significant?

Lecture by A. R. (Rich) Palmer

BIOL 606 Session, University of Alberta, January 20, 1999

Rapporteur: Mark V. H. Wilson

Rich Palmer began by posing the question "How often do environmental effects on animal form yield significant evolutionary change?" A brief review of the history of thinking about mechanisms of evolution included mention of the ideas of Lamarck, Darwin, and Mendel. Between 1896 and 1905 James M. Baldwin, Lloyd Morgan, and H.F. Osborn independently suggested that characters individually acquired might eventually, under selection, be replaced by similar hereditary characters, a process called "organic selection" by Baldwin and "coincident selection" by Osborn. A similar concept was later called "genetic assimilation" by Waddington. These ideas continue to circulate because environmental effects on morphology can be striking. Examples include developmental changes in morphology to predator presence in rotifers, bryozoans, and barnacles, induction of head and tail spines in Daphnia , and scale thickness and apertural teeth in intertidal snails. Exposure to different foods causes modifications of claws in crabs and of pharyngeal teeth in fishes.

Does phenotypic plasticity retard evolution in such cases? Traditionally, environmental effects on form were believed to reduce the predictable resemblance of offspring to their parents. If so, this would retard the rate of evolution.

Waddington proposed the term "genetic assimilation" for a phenomenon observed in a series of experiments on fruit flies exposed to a brief 40C heat shock as pupae. Initially the shock caused a 40-50% incidence of loss of cross-veins in the wings. If these cross-veinless flies were selectively bred, for up to 23 generations, the incidence approached 90% (or 10% if opposite phenotypes were selected). In some later generations, subsets of flies, raised without heat shock, displayed 30-40% cross-vein loss compared with virtually 0% in early generations. A similar experiment was published by Waddington in 1956 using ether stress to induce bithorax-like phenotypes in Drosophila . After only 8 generations, some offspring were bithorax-like in the absence of ether stress.

Palmer argued that one could examine clades of living species in which environmentally induced variation in lateral symmetry is present to determine whether fixed asymmetries have evolved most often directly from symmetrical ancestors or if they have evolved most often through an antisymmetrical intermediate (phenotypically plastic, dominant side determined by external environmental stimulus). If the latter, then those species or groups that evolved fixed asymmetries from antisymmetrical ancestors are probably examples of genetic assimilation. The methods are outlined and the data are compiled and discussed in Palmer (1996).

Examples of antisymmetrical groups included that of some lobsters, which when fed soft food develop few crushing claws, but when fed hard food develop crushers in an approximately 50:50 Right:Left ratio. Phallostethid fishes are an example of a group with symmetrical ancestors, but with most species in the clade being antisymmetrical. Three derived species of phallostethids display fixed asymmetries, in different directions. Thus, in this group the fixed asymmetrical species appear to have evolved (independently) from antisymmetrical intermediates (and more remote symmetrical ancestors).

Overall, in Palmer's (1996) compilation of cases, 1/3 to 1/2 of fixed asymmetries apparently arose from antisymmetrical ancestors. If these are indeed cases of genetic assimilation, biologists appear to have vastly underestimated the power of genetic assimilation, he suggested.

References:

Palmer, A. R. 1996. From symmetry to asymmetry: phylogenetic patterns of asymmetry variation in animals and their evolutionary significance. Proc. Natl. Acad. Sci. USA, 93 :14279-14286.

See also this list of Genetic Assimilation References.

Discussion

Discussant: Curtis Strobeck

Curt Strobeck began by asking what the discussion of the retarding of evolution by environmentally-induced phenotypic plasticity had to do with the reality of genetic assimilation, suggesting that it was a side issue. Palmer suggested that it is usually assumed that environmental effects on development and morphology reduce the rate of evolution, whereas genetic assimilation may actually be facilitated by existence of phenotypically plastic intermediate ancestors.

Other participants suggested that these plastic cases could be explained by selection, now or in the past. In such cases subsequent selection might lead to fixation of one or the other trait. Thus they might not be examples of genetic assimilation. Strobeck argued that without a genetic model, and specified parameters for a computer simulation, one doesn't have a useful explanation for such evolutionary events. One must know what the selection pressure is, and exactly what genetic mechanism can lead to fixation.

A long discussion, initiated by Dick Fox, ensued about the proverbial 98 lb. weakling with sand kicked in his face, and how lifting weights doesn't lead to muscular offspring. Strobeck later noted that some individuals might have a greater capacity to produce muscle mass, and that this capacity could be heritable and thus respond to selection.

Palmer asked whether all cases of fixation from antisymmetrical intermediates represent examples of genetic assimilation. The critical question is: how commonly does antisymmetry reflect negative frequency-dependent selection on genetically-determined right and left phenotypes? If anyone can think of examples, then the claim that genetic assimilation is common might need re-evaluation.

Several examples were noted. First, what keeps the ratio at 50:50 in antisymmetrical populations such as scale-eating fishes? The answer may be that rare-sided individuals have an advantage, when sneaking up on their victims. How can heritability be established in such cases? In the example of scale-eating fishes, heritability was established by examining the young fish being brooded in the mouths of females.

Second, Sean Graham mentioned flowers in which asymmetry helps to ensure outcrossing. Palmer speculated that some of these examples could be examples of frequency-dependent selection. If so, not all examples of antisymmetrical to fixed asymmetrical transitions represent genetic assimilation.

A third example mentioned was that of hermit crabs, dependent on coiling of their host snail shells. Because the vast majority of snail shells are dextral, directional deviations from symmetry may have been induced initially in hermit crab ancestors, by environmental effects. In this case, directional (fixed) asymmetry may have arisen via genetic assimilation.

Dick Fox asked whether there is evidence for fixed asymmetry in fiddler crabs. Palmer replied in the affirmative. Only one species in the clade has lost antisymmetry. The loss in this case is explained by selection against injury in male-male combat (like-sided males are less likely to injure each other).

Is there any way to evolve from antisymmetrical, or from fixed asymmetrical, to symmetrical populations? Palmer said that in his surveys he found almost no examples of this (apparent exceptions occur in gastropods that have lost their shell). This pathway is therefore thought to be very unlikely.

Discussion by some participants continued later at a watering hole, but an incompatible activity (imminent band rehearsal) prevented your scribe from recording those proceedings. They probably wouldn't have shed much more light on the subject anyway.*

* I'm told that much light was, in fact, shed.