1.3 Local adaptation of rare sapphire rockcress plants. When plants of the rare sapphire rockcress from low‐elevation (drought‐prone) and high‐elevation sites were grown together in a common garden, there was local adaptation: those from the low‐elevation site had significantly better water‐use efficiency as well as having both taller and broader rosettes.
Source: From McKay et al. (2001).
the balance between local adaptation and hybridisation
On the other hand, local selection by no means always overrides hybridisation. In a study of Chamaecrista fasciculata, an annual legume from disturbed habitats in eastern North America, plants were grown in a common garden that had been derived from the ‘home’ site or were transplanted from distances of 0.1, 1, 10, 100, 1000 and 2000 km (Galloway & Fenster, 2000). The study was replicated three times: in Kansas, Maryland and northern Illinois. Five characteristics were measured: germination, survival, vegetative biomass, fruit production and the number of fruit produced per seed planted. But for all characters in all replicates there was little or no evidence for local adaptation except for transplant distances of 1000 km or more. There is ‘local adaptation’ – but in this case it was clearly not that local.
We can also test whether organisms have evolved to become specialised to life in their local environment in reciprocal transplant experiments: comparing their performance when they are grown ‘at home’ (i.e. in their original habitat) with their performance ‘away’ (i.e. in the habitat of others). In his meta‐analysis of 74 reciprocal transplant studies (50 concerning plants, 21 animals, two fungi and one protist), Hereford (2009) reported that local adaptation was common (71% of studies) but not ubiquitous. On average, local populations had 45% greater fitness than non‐local populations. And crucially, there was a small but significant positive association between fitness differences and the magnitude of environmental differences between parental sites (‘environmental distance’ measured using composite values for up to four environmental variables, such as soil moisture, annual rainfall, elevation and frequency of predation) (Figure 1.4). The magnitude of local adaptation does not seem to be correlated with geographic distance (Leimu & Fischer, 2008), so Hereford’s results emphasise the role of ecological factors, not separation itself, as drivers of adaptive differentiation.
Figure 1.4 Meta‐analyses reveal generalities about local adaptation. Regression of local adaptation on environmental distance between sites in a meta‐analysis of reciprocal transplant experiments (P = 0.003). Local adaptation is the difference in relative fitness between a native population and a non‐native population in the native’s environment. To standardise measures of environmental difference between sites, Euclidean distances from the means of environmental variables were calculated for all sites in each study.
Source: From Hereford (2009).
1.2.2 Genetic polymorphism
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.
