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transient polymorphisms

On a finer scale than ecotypes, it may also be possible to detect levels of variation within populations. Such variation is known as polymorphism. Specifically, genetic polymorphism is ‘the occurrence together in the same habitat of two or more discontinuous forms of a species in such proportions that the rarest of them cannot merely be maintained by recurrent mutation or immigration’ (Ford, 1940). Not all such variation represents a match between organism and environment. Indeed, some of it may represent a mismatch, if, for example, conditions in a habitat change so that one form is being replaced by another. Such polymorphisms are called transient. As all communities are always changing, much polymorphism that we observe in nature may be transient, representing the extent to which the genetic response of populations to environmental change will always be out of step with the environment and unable to anticipate changing circumstances.

the maintenance of polymorphisms

Many polymorphisms, however, are actively maintained in a population by natural selection, and there are a number of ways in which this may occur.

1 Heterozygotes may be of superior fitness, but because of the mechanics of Mendelian genetics they continually generate less fit homozygotes within the population. Such ‘heterosis’ is seen in human sickle‐cell anaemia where malaria is prevalent. The malaria parasite attacks red blood cells. The sickle‐cell mutation gives rise to red cells that are physiologically imperfect and misshapen. However, sickle‐cell heterozygotes are fittest because they suffer only slightly from anaemia and are little affected by malaria, but they continually generate homozygotes who are either dangerously anaemic (two sickle‐cell genes) or susceptible to malaria (no sickle‐cell genes). Nonetheless, the superior fitness of the heterozygote maintains both types of gene in the population (that is, a polymorphism).

2 There may be gradients of selective forces favouring one form (morph) at one end of the gradient, and another form at the other. This can produce polymorphic populations at intermediate positions in the gradient. Females of some damselfly species come in distinct colour morphs: gynomorphs and male‐mimicking andromorphs. The andromorph form may provide benefit by reducing harassment of the females by males, allowing more time for foraging, but this may be at the expense of being more obvious to predators (Huang & Reinhard, 2012). Takahashi et al. (2011) have described a geographic cline in this polymorphism in Ischnura senegalensis over a latitudinal range of 1100 km in Japan (Figure 1.5). Such clines suggest that the fitness advantage of each morph changes differentially along an environmental gradient such that the balance of advantage switches around a mid‐point where each phenotype has equal fitness. In this case, Takahashi et al. (2011) determined that the reproductive potential of gynomorphs (related to ovariole number, body size and egg volume) was indeed higher in the south and lower in the north compared with andromorphs.

3 There may be frequency‐dependent selection where each of the morphs of a species is fittest when it is rarest (Clarke & Partridge, 1988). This is believed to be the case when rare colour forms of prey are fit because they go unrecognised and are therefore ignored by their predators.

4 Selective forces may operate in different directions within different patches in the population. A striking example of this is provided by a reciprocal transplant study of white clover (Trifolium repens) in a field in north Wales. To determine whether the characteristics of individuals matched local features of their environment, Turkington and Harper (1979) removed plants from marked positions in the field and multiplied them into clones in the common environment of a greenhouse. They then transplanted samples from each clone into the place in the sward of vegetation from which it had originally been taken (as a control), and also to the places from where all the others had been taken (a transplant). The plants were allowed to grow for a year before they were removed, dried and weighed. The mean weight of clover plants transplanted back into their home sites was 0.89 g but at away sites it was only 0.52 g, a statistically highly significant difference. This provides strong, direct evidence that clover clones in the pasture had evolved to become specialised, such that they performed best in their local environment. But all this was going on within a single population, which was therefore polymorphic.

Figure 1.5 The frequency of andromorphs of local damselfly populations in Japan increases with latitude. The inset shows the logistic regression with latitude (t = 8.15, df = 21, P < 0.001), excluding the northernmost population (solid black plot). This population had been recently established in a newly created pond by gynomorphs, and showed 100% gynomorph frequency because of the founder effect.

Source: From Takahashi et al. (2011).

no clear distinction between local ecotypes and a polymorphism

In fact, the distinction between local ecotypes and polymorphic populations is not always a clear one, as illustrated by a study involving the marine snail Littorina saxatilis. This common inhabitant of North Atlantic shores is remarkably polymorphic with reproductively isolated ecotypes in microhabitats where crabs are either present and wave action is weak (crab ecotype), or on wave‐swept rocky surfaces where waves are strong and crabs are absent (wave ecotype) (Johannesson, 2015). The crab ecotype is large and robust, with a thick shell, a high spire and a relatively small aperture, while the wave ecotype is only about half the size of its crab counterpart, has a thin shell, a relatively large aperture and a low spire (Figure 1.6). The same pattern is observed in different parts of the snail’s range and, for example, in both Sweden and Spain, snails of each ecotype are fitter in their native microhabitat than if moved to the other microhabitat. In contact zones, however, snail morphologies represent a continuum from one morph to the other, with all possible intermediate stages. Even though the spatial scale of distribution of the two ecotypes may be very small, the forces of selection are clearly able to outweigh the mixing forces of hybridisation – but it is a moot point whether we should describe this as a small‐scale series of local ecotypes or a polymorphic population maintained by a gradient of selection.

Figure 1.6 Contrasting ecotypes of the periwinkle Littorina saxatilis from Sweden and Spain. Swedish crab ecotype (top left) and wave ecotype (top right), and Spanish wave ecotype (bottom left) and crab ecotype (bottom right).

Source: From Johannesson (2015).