Phenotypic plasticity

A life history is not a fixed property that an organism exhibits irrespective of the prevailing environmental conditions. An observed life history is the result of long-term evolutionary forces, but also of the more immediate responses of an organism to the environment in which it is and has been living. This the scheme explains much - but leaves as much unexplained

Figure 4.30 Broadly speaking, plants show some conformity with the r/K scheme. For example, trees in relatively K-selecting woodland habitats: (a) have a relatively high probability of being iteroparous and a relatively small reproductive allocation; (b) have relatively large seeds; and (c) are relatively long lived with relatively delayed reproduction. (After Harper, 1977; following Salisbury, 1942; Ogden, 1968; Harper & White, 1974.)

Iteroparity -

Semelparity

Perennials, including trees -

Wild annuals -

10 20 30

Net reproductive allocation (%)

Open habit, short grass Woodland margins Woodland ground flora Woodland shrubs Woodland trees

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500 1000

Herbs • Shrubs Trees (angiosperms) Trees (conifers) ▲ Semelparous

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ability of a single genotype to express itself in different ways in different environments is known as phenotypic plasticity.

One of the most important questions we need to ask about phenotypic plasticity is the extent to which it represents a response by which an organism allocates resources differently in different environments such that it maximizes its fitness in each. The alternative would be that the response represented a degree of inevitable or uncontrolled damage or stunting by the environment (Lessells, 1991). Note, especially, that if phenotypic plasticity is governed by natural selection, then it is just as valid to seek patterns linking different environments and the different responses to them by a single individual, as it is to seek patterns linking the habitats and the life histories of genetically different individuals.

In some cases at least, the appropriateness of a plastic response seems clear. For example, kestrels (predatory birds) in the Netherlands vary in the quality of their territory, the size of their clutch and the date on which they lay it (Daan et al., 1990). The differences appear not to be genetically determined but to be an example of phenotypic plasticity. Is each combination of clutch size and laying date optimal in its own territory?

The optimal combination is, as usual, the one with the highest total reproductive value - the value of the present clutch plus the parent's RRV. The value of the present clutch clearly

Growing season

Habitat property Measured by Short Long

Climate variability s2/* frost-free days per year 3.05 1.56

Competition Biomass above ground (g m-2) 404 1336

Annual recolonization Winter rhizome mortality (%) 74 5

Annual density variation s2/x shoot numbers m-2 2.75 1.51

Plant traits

Days before flowering Mean foliage height (cm) Mean genet weight (g) Mean number of fruits per genet Mean weights of fruits (g) Mean total weight of fruits (g)

T. angustifolia

T. domingensis

44 162 12.64 41 11.8 483

Table 4.7 Life history traits of two Typha (cattail) species, along with properties of the habitats in which they grow. 's2/x' refers to the variance : mean ratio, a measure of variability. The cattails conform to the r/K scheme. (After McNaughton, 1975.)

increases with clutch size, and the value of each egg also varies with laying date. What, though, of RRV? This declines with increases in 'parental effort' (i.e. the number of hours per day in flight spent hunting in order to raise a clutch of chicks), and parental effort, in turn, decreases with increases in the 'quality' of a territory: the number of prey caught per hunting hour. Thus, RRV is lower: (i) with larger clutches; (ii) at particular, less productive times of the year; and (iii) in lower quality territories. On this basis, the total reproductive value of each combination of clutch size and date in each territory could be computed, the optimal combination predicted (Figure 4.31a) and the predicted and actual combinations compared in territories of different quality (Figure 4.31b). The correspondence is impressive. Each individual apparently comes close to optimizing its clutch size and laying date as an immediate response to the environment (territory) in which it finds itself.

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