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Mimicry, Saltational Evolution, and the Crossing of Fitness Valleys

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Mimicry, Saltational Evolution, and the Crossing of Fitness Valleys CHAPTER 16 Mimicry, Saltational Evolution, and the Crossing of Fitness Valleys Olof Leimar, Birgitta S. Tullberg, and James Mallet 16.1 Introduction saltationism. In terms of Adaptive Landscapes, if the mimic-to-be resides on one adaptive peak and The relative contribution of gradual and saltational the model on another, mimicry evolves in a single change to evolution has been debated ever since mutational leap, and the issue of natural selection Darwin (1859) emphasized gradualism in his the- only enters through the constraint that the peak ory of evolution by natural selection. The phe- jumped to should be higher than the starting peak. nomenon of mimicry was an important example This would apply both to Müllerian mimicry, where in this debate. In mimicry evolution, members of the starting point is an aposematic species, and to a population or species become similar in appear- Batesian mimicry, where the starting adaptive peak ance to an aposematic model species and thereby of the palatable mimic-to-be is determined by func- gain increased protection from predation. The num- tions other than aposematism, for instance crypsis ber of steps in the approach to mimicry could be or other protective coloration, like flash coloration few or many and their sizes either large or small. (Cott 1940; Ruxton et al. 2004), or partner choice. In 1915, Punnett published an influential book on In response to claims like those by Punnett (1915), mimicry in butterflies, in which he summed up and as part of his efforts to unify gradualism and his strong opposition to gradualistic accounts of Mendelian genetics, Fisher (1927, 1930) presented mimicry evolution. He dismissed previous sug- a fully gradualistic alternative. He envisaged a gestions by Poulton (e.g. Poulton 1912, 1913) that genetically variable population of mimics-to-be and mimicry could emerge in a sequence of steps, begin- proposed that individuals with trait values deviat- ning with the appearance of a rough likeness of the ing from the population mean in the direction of would-be mimic to its model, followed by further the traits of the model species would be slightly improvement in resemblance. Punnett’s main argu- favored over deviations in the opposite direction, ment was representative of the thinking of the early because of a slightly higher probability of being Mendelians, who often pointed to a lack of interme- mistaken for the model by predators. The outcome diates between existing variants, inferring that the could be a gradual shifting of the mean trait val- variants had originated as mutants in a single step, ues in the direction of improved resemblance. It as opposed to being molded by natural selection appears that Fisher intended his gradualistic sce- through successive replacements of intermediate nario to apply both to Batesian and Müllerian rela- forms. As supporting evidence, Punnett used exam- tions, but it gained more attention in the latter case. ples of female-limited polymorphic mimicry, for In terms of (frequency-dependent) Adaptive Land- instance the one found in the butterfly Papilio poly- scapes of Müllerian mimicry evolution, Fisher’s tes, where the developmental switching between process would be a gradual shifting of two peaks, female morphs was known to be controlled by a until they overlap and, approximately, merge to a small number of Mendelian factors, later shown to single peak. be alleles at an autosomal locus (Clarke and Shep- Fisher’s (1927, 1930) proposition was not gener- pard 1972). Punnett’s position was thus a radical ally accepted. The strongest opposition came from 259 OUP CORRECTED PROOF– FINAL, 3/5/2012, SPi 260 PART V DEVELOPMENT, FORM, AND FUNCTION Goldschmidt (1945a,b), who ended his examina- predators, selection tends to operate on deviations tion of the issue by coming to the conclusion of appearances from the current population mean. that “Punnett’s interpretation of polymorphism by The selective peaks could be constrained, and in mutation (saltation) agrees better with the facts part formed by the aesthetics of predator learning, than Fisher’s neo-Darwinian theory.” Although not to particular regions of phenotypic space, as well as subscribing to the saltationism of Punnett and being influenced by selection towards the current Goldschmidt, even Fisher’s close associates (for mean. If the population mean appearance changes instance E. B. Ford and P. H. Sheppard), who to explore new aesthetic combinations, predator were preoccupied with the problem of Batesian learning need not always act to bring the mean back mimicry evolution, came to deviate in their views to a previous value. This implies that the mean may from Fisher’s original proposition (see Turner 1985 perform a random walk over evolutionary time, for an overview). Instead the so-called two-step exploring parts of the relevant trait space, and pos- process, where a large mutation first achieves sibly different peaks of an Adaptive Landscape, approximate similarity to the model, after which in a shifting balance process (Wright 1977; Mal- smaller changes can improve the likeness, became let and Singer 1987; Coyne et al. 1997; Mallet and accepted as describing Batesian mimicry evolution. Joron 1999; Mallet 2010; see also Chapters 2 and The idea is often credited to Nicholson (1927), 4–6). This would correspond to a random walk to a although Poulton (1912) had already suggested it. new adaptive peak. The phenomenon will be more Over time, the two-step process became accepted pronounced in small local populations, and/or in also in the context of Müllerian mimicry (Turner populations with limited predation pressure, where 1984; Sheppard et al. 1985). In terms of Adap- selection towards the current mean will be weak- tive Landscapes, the process entails a mutational ened. The process could be responsible both for leap from the adaptive peak of the mimic-to-be, mimicry evolution and for the rapid diversification protected as it is by predator learning of that of novel aposematic signals that occurs in many phenotype, to somewhere on the slope of the aposematic groups (Mallet and Joron 1999; Mallet higher, more protective peak of the model, thus 2010). crossing a fitness valley, followed by a series of In the following, we extend this brief review of modifications climbing the higher peak. Because the history of ideas, outlining some recent work mimicry often involves several traits, which at on mimicry evolution. The concept of an Adap- least initially can be genetically independent, this tive Landscape will be central in the presenta- first mutational leap is clearly a less demanding tion, in particular the question of how a transi- assumption than a saltation as argued for by Pun- tion from one adaptive peak to another can come nett and Goldschmidt. Even so, the assumption about. The landscapes we consider depict fitness needs to be backed up by arguments or observa- as a function of the phenotypic traits of individ- tions making it likely that predators in fact would uals, rather than as a function of genotype or avoid attacking the first, quite imperfect, mutant allele frequencies (see Chapters 1–3, 5, 7, 18 and 19 mimic. for expositions and discussions on different types A different kind of ingredient in explanations of of Adaptive Landscapes). It should be kept in mimicry evolution is that, possibly only in a partic- mind that, for aposematism and mimicry, Adaptive ular region or period of time, evolutionary forces Landscapes are strongly density- and frequency- other than mimicry may modify the appearances dependent, in the sense that their shapes depend of mimics-to-be, fortuitously bringing about suffi- on the traits and population sizes that are present cient resemblance to a model to start off mimicry in a prey community (this is also true for many evolution. Examples could be selection in relation other types of Adaptive Landscapes; see Chap- to mate choice or thermoregulation (Mallet and ter 7). The reason for the frequency dependence Singer 1987). Random genetic drift in small pop- is that predator behavior is influenced by learn- ulations is another general category of this kind. ing and generalization about the properties of the In aposematism, with learnt attack avoidance by community. OUP CORRECTED PROOF– FINAL, 3/5/2012, SPi MIMICRY, SALTATIONAL EVOLUTION, AND THE CROSSING OF FITNESS VALLEYS 261 16.2 Transitions between adaptive the most commonly used and empirically substan- peaks tiated in animal psychology (Ghirlanda and Enquist 2003). Similarly, the assumptions about predator Predators learn about prey, for instance to avoid learning follow broadly accepted ideas of associa- attacking those with particular appearances if they tive learning (Rescorla and Wagner 1972). have been found to be distasteful or otherwise Restricting attention to a single trait, a two- unprofitable in previous attacks. Generalization is step process of Müllerian mimicry evolution is another crucial aspect of predator psychology: if a illustrated in Fig. 16.1b. Because large-effect muta- predator encounters prey with a different appear- tions are known to occur, at least for certain ance than previous prey, the reaction to the new traits, for instance for the hue or intensity of pig- prey may be a generalized version of the learnt mentation (Socha and Nemec 1996), and because reaction to similar-looking prey. From assumptions large-effect alleles have been found in Müllerian about learning and generalization by predators in a mimicry systems (Joron et al. 206b; Baxter et al. predator–prey community, an Adaptive Landscape 2009), it follows that a two-step process involving of prey survival as a function of phenotype can be a single-locus mutation could well produce shifts defined. Fig. 16.1 shows a generalization function between adaptive peaks in one-dimensional trait and an Adaptive Landscape for a one-dimensional spaces. trait space.
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