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HOME :: CHAPTER 22 :: GENETIC ASSIMILATION :: GENETIC ASSIMILATION |
Genetic Assimilation
Although the fast-developing cannibalistic tadpoles of the spadefoot toad survive drought successfully, their survival rate after metamorphosis is lower than that of tadpoles that develop more slowly. If there is no significant trade-off for a morph induced by hostile environmental conditions, we might expect it to become the predominant form of the species.
C. H. Waddington and I. I. Schmalhausen independently made this prediction to explain how some species have evolved rapidly in particular directions (see Gilbert 1994). Both scientists were impressed by the calluses of the ostrich. Most mammalian skin has the ability to form calluses on areas that are abraded by the ground or some other surface.1 The skin cells respond to friction by proliferating. While such examples of environmentally induced callus formation are widespread, the ostrich is born with calluses where it will touch the ground (Figure 1). Waddington and Schmalhausen hypothesized that since the skin cells are already competent to be induced by friction, they could be induced by other things as well. As ostriches evolved, a mutation (or a particular combination of alleles) appeared that enabled the skin cells to respond to some substance within the embryo. Waddington (1942) wrote:
Presumably its skin, like that of other animals, would react directly to external pressure and rubbing by becoming thicker. … This capacity to react must itself be dependent upon genes. …It may then not be too difficult for a gene mutation to occur which will modify some other area in the embryo in such a way that it takes over the function of external pressure, interacting with the skin so as to'‘pull the trigger' and set off the development of callosities.
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| Figure 1 Ventral side of an ostrich; arrows mark the calluses. (After Waddington 1942.) (Click image to enlarge.) |
By this transfer of induction from an external inducer to an internal inducer, a trait that had been induced by the environment became part of the genome of the organism and could be selected. Waddington called this phenomenon “genetic assimilation,” while Schmalhausen (1949) called it “stabilizing selection.” Both scientists had used orthodox embryology and orthodox genetics to explain phenomena that had been considered cases of Lamarckian “inheritance of acquired characteristics.” Waddington (1961) defined genetic assimilation as “a process by which a phenotypic character, which initially is produced only in response to some environmental influence, becomes, through a process of selection, taken over by the genotype, so that it is found even in the absence of the environmental influence which had at first been necessary.”
A shift from environmental stimulus to genetic stimulus might explain sex determination in Menidia and caste determination in ants. Similarly, the preexisting developmental plasticity of arm length in feeding echinoderm larvae may have bridged the transition from pluteus (feeding) larvae to nonfeeding larvae that lack ciliated arms. This change in the allocation of resources between larval and juvenile structures parallels that seen in the food reserves stored in the egg. Thus, the changes already present as adaptations to external food resources may have become genetically fixed in those species whose larvae do not need to hunt for food (Strathmann et al. 1992).
If genetic assimilation is the genetic fixation of one of two or more phenotypes that had been adaptively expressed, then butterflies would be a good place to look for further examples. Brakefield and colleagues (1996) showed that they could genetically fix the different morphs of the adaptive polyphenism of Bicyclus, and Shapiro (1976) showed that the short-day (cold-weather) adaptive phenotype of several butterflies is the same as the single genetically produced phenotype of related species or subspecies living at higher altitudes or latitudes. Thus, the genetic assimilation of morphs originally produced through developmental plasticity may result in the origin of new species (West-Eberhard 1989; 2003).
West-Eberhard (2003) has noted that environmentally induced novelties may be extremely important in evolution. Mutationally induced novelties would occur in only a family of individuals; whereas environmentally induced novelties would occur to a population. Moreover, the inducing environment would most likely also be a selecting environment. Thus, there would be selection immediately for this trait, and the trait would be continuously induced. This type of evolution would indicate that the phenotype is produced first and then would later be “fixed” by the genotype. Recent computer modeling (Kaneko et al. 2002; Behera and Nanjundiah 2004) has shown that such an environmentally-induced genetic assimilation can produce rapid evolutionary change as well as speciation.2
Genetic Assimilation in the Laboratory
Genetic assimilation is difficult to "prove" in natural conditions. However, it can be demonstrated in the laboratory. C. H. Waddington, one of the first developmental geneticists was a person who did not believe that genetics, embryology, and evolution were separate sciences (he called the fusion of these disciplines "diachronic biology" and worked in each area). Waddington showed that Drosophila had a particular reaction norm in its response to ether. Embryos exposed to ether at a particular stage developed a phenotype similar to the bithorax mutation, and had wing-like halteres. Generation after generation was exposed to ether and those individuals showing the altered metathoracic development were selectively bred each time. By twenty generations, the mated Drosophila produced the mutant phenotype even when ether was no longer applied (Figure 2; Waddington 1953; 1956). Interestingly, the bithorax-like phenotype had been assimilated by other genes, not bithorax.
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| Figure 2 Phenocopy of the bithorax mutation. (A) A bithorax phenotype produced after treatment of the embryo with ether. The forewings have been removed to show the aberrant metathorax. This particular individual is actually from the "assimilated" stock that produced this phenotype without being exposed to ether. (B) Selection experiments for or against the bithorax-like response to ether treatment. Two experiments are shown (red and blue lines). In both cases, one group was selected for the trait and the other group was selected against the trait. (After Waddington 1956). |
In this experiment, Waddington took advantage of an experimental phenomenon that is the converse of genetic assimilation: the phenocopy. Phenocopies are environmentally produced phenotypes that mimic the genetically produced phenotypes. In the above-mentioned experiment, exposure to ether produced a phenotype that mimicked that of the bithorax mutation. One of the simplest ways of inducing phenocopies is by cold shock or heat shock at a particular time during development. (Each type of phenocopy has a particular time when the environmental factor works. This could be at the embryonic, larval, or pupal stages, depending on the phenocopy. During that critical period, however, several types of trauma may be equally effective at eliciting the same phenotype). Although Richard Goldschmidt (another of the first developmental geneticists and someone who tried to bridge development, genetics, and evolution) coined the word "phenocopy," the technique had been used since the 1890s (Merrifield 1890, 1893) to disrupt the pattern of butterfly wing pigmentation.
This technique has provided some surprising results: color patterns that develop after temperature shock sometimes mimic the normal genetically controlled patterns of related races or species living at different temperatures. This was first seen by Standfuss (1896) who demonstrated that the heat-shocked phenocopy of the Swiss subspecies of Iphiclides podalirius resembled the normal form of the Sicilian subspecies. Similarly, heat shocking the central European form of Papilio machon produced some individuals that resembled the Syrian subtype, and some that resembled the Turkish variety. Goldschmidt (1938) produced one of the most striking phenocopies. He observed that heat-shocked specimens of the central European subspecies of Aglais urticae produced wing patterns that resembled the Sardinian subspecies, while cold-shocked individuals of the central European variety developed the wing patterns of the subspecies from northern Scandinavia (Figure 3).
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| Figure 3 Temperature shocking Aglais urticae produces phenocopies of geographic variants. (A) Usual central European variant; (B) heat-shock phenocopy resembling Sardinian form; (C) a Sardinian form of the species. (After Goldschmidt 1938.) |
Recent observations by Shapiro (1976) on the mourning cloak butterfly (Nymphalis antiopa) and by Nijhout (1984) on the buckeye butterfly (Precis coenia) have confirmed the view that temperature shock can produce phenocopies that mimic genetically controlled patterns of related races or species existing in colder or warmer conditions. Chilling the pupa of Pieris occidentalis will cause it to have the short-day phenotype (Shapiro 1982), and this phenotype is similar to that of the northern subspecies of pierids.
Genetic assimilation can function by genetically fixing traits that had a phenotypic plasticity or it can genetically fix a phenocopy. For a phenocopy to be fixed, the population must be exposed to environmental conditions that repeatedly induce the phenocopy and there must be selective pressure such that the phenocopy is advantageous (produces higher fitness) in that environment (Rachootin and Thomson 1981). In this way, advantageous traits may be selected and incorporated into a population at a higher frequency that would be expected by random chance. As Waddington (1961) pointed out, a combination of orthodox Darwinism and orthodox embryology can give the results that look like the inheritance of acquired characteristics.
1Until the late twentieth century, writers could be recognized by the calluses on their fingers. Thus, in The Red Headed League, Sherlock Holmes correctly surmised that the red-headed man had been hired as a scrivener.
2Darwin based his concept of fitness quite literally on the metaphor of something fitting into an environment. Indeed, he envisioned metal wedges being driven into wood, displacing other wedges. At about the same time, another metallic analogy of fitting, the lock-and-key model, was being proposed for enzyme substrate interactions (Gilbert and Greenberg 1984). Today, both models have been criticized. The lock-and-key model has given way to the “induced fit” model, wherein the substrate interacts with the active site in order to fit; and the wedge model appears to be giving way to the notion of “niche construction” and “induced fitness” wherein the environment and organism interact such that the organism can be fit (Oldling-Smee et al. 2003; Gilbert 2005).
Literature Cited
Goldschmidt, R. B. 1938. Physiological Genetics. McGraw-Hill, NY.
Merrifield, F. 1890. Systematic temperature experiments on some Lepidoptera in all their stages. Trans. Entomol. Soc. Lond. 131-159.
Merrifield, F. 1893. The effects of temperature in the pupal stage on the colouring of Pieris napi, Vanessa atalanta, Chrysophanus phloeas, and Ephyra punctaria. Trans. Entomol. Soc. Lond. 425-438.
Nijhout, H. F. 1984. Color pattern modification by cold-shock in Lepidoptera. J. Embryol. Exper. Morphol. 81: 287-305.
Nijhout, H. F. 1991. The Development and Evolution of Butterfly Wing Patterns. Smithsonian Institution Press, Washington, D.C.
Rachootin, S. P. and Thomson, K. S. 1981. Epigenetics, paleontology, and evolution. In Evolution Today, Proceedings of the Second International Congress on Systematic and Evolutionary Biology. G. G. E. Scudder and J. L. Reveal (eds.), Hunt Institute for Botanical Documentation, Pittsburgh.
Shapiro, A. M. 1976. Seasonal polyphenism. Evol. Biol. 9: 259-333.
Shapiro, A. M. 1982. Redundancy in pierid polyphenisms: Pupal chilling induces vernal phenotype in Pieris occidentalis (Pieridae). J. Lepidopt. Soc. 36: 174-177.
Standfuss, M. 1896. Handbuch der palearctischen Gross-Schmetterlinge fŸr Forscher und Sammler. G. Fischer, Jena.
Waddington, C. H. 1953. Genetic assimilation of an acquired character. Evolution 7: 118-126.
Waddington, C. H. 1956. Genetic assimilation of the bithorax phenotype. Evolution 10: 1-13.
Waddington, C. H. 1961. Genetic assimilation. Advances Genet. 10: 257-290.
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