Male and female in one individual or separate individuals

Let us start with the first question. We will take a broad fitness-based approach first outlined by Charnov et al. (1976). They viewed the question of whether to express both sexes in the same individual or in separate individuals as analogous to asking if an organism should specialize in one role or

-^Convex

a

b /

Female fitness

Concave^

Investment in gender x Male fitness

Fig. 5.1 The evolution of hermaphrodites, following Charnov et al. (1976). On the left are fitness curves detailing how much fitness derives from a given investment in resources. Curve 'a' is asymptotic, meaning that each extra investment gives less extra fitness return, whereas curve 'b' accelerates, meaning that only a high level of investment in that sex will return much fitness. On the right are fitness trade-offs between the sexes. Curve 'a' translates into a convex trade-off, meaning that switching investment to the alternative sex does not greatly reduce the fitness of the other sex. For such curves, hermaphroditism is optimal (black circle), because it gives the greatest total fitness derived from both sexes. If the fitness curve accelerates (b), trade-offs become concave, and the optimum is to be either male or female but not both (open circles).

Investment in gender x Male fitness

Fig. 5.1 The evolution of hermaphrodites, following Charnov et al. (1976). On the left are fitness curves detailing how much fitness derives from a given investment in resources. Curve 'a' is asymptotic, meaning that each extra investment gives less extra fitness return, whereas curve 'b' accelerates, meaning that only a high level of investment in that sex will return much fitness. On the right are fitness trade-offs between the sexes. Curve 'a' translates into a convex trade-off, meaning that switching investment to the alternative sex does not greatly reduce the fitness of the other sex. For such curves, hermaphroditism is optimal (black circle), because it gives the greatest total fitness derived from both sexes. If the fitness curve accelerates (b), trade-offs become concave, and the optimum is to be either male or female but not both (open circles).

be a generalist, assuming more than one role. We will encounter this problem again in a more ecological context in Chapter 8, so the discussion here serves as a useful introduction. First, imagine a trade-off between investment in male versus female fitness. This can be a linear trade-off, convex or concave (Figure 5.1). It is simple to show that when the curve is convex, cosexuality is stable: a pure female, reallocating resource to 'male-ness', gains more fitness than it loses, because the cosexual is almost as good a female as the pure female itself while there is some male-ness added that also gives a fitness return. Similarly, a pure male allocating resources to female-ness would be almost as good as being male, and yet, get a bit of fitness through female function too. What might cause the trade-off to be convex? We can understand this from an examination of the relationship between investment in one of the sexes and fitness. If the added fitness from extra investment in one sex tends to decline, selection will increasingly favour investing in the other sex (Figure 5.1). This means that just a bit of investment in either sex will make the plant both a perfectly good male and a perfectly good female, and the fitness trade-off will be convex.

The distribution of cosexuals in plants makes sense in this framework. Cosexuality, for example, is associated with animal as opposed to wind pollination, and with wind as opposed to animal-dispersed seeds (e.g. Bawa 1980; Charnov 1982; Vamosi et al. 2003). All these associations can be related to the shape of the fitness gain curves. For example, wind-dispersed seeds need to be light and therefore fitness asymptotes strongly with extra investment. Animal-dispersed seeds, however, need to be resource-rich to attract animals, and may even display an accelerating gain curve (Figure 5.1). Thus, selection favours dioecy in animal-dispersed plants, and cosexuality in wind-dispersed plants. With pollination, however, it is the reverse: successful wind pollination demands pollen en mass, thus, the male gain curve is unlikely to saturate at all strongly, thus selecting for dioecy. Although this approach can claim some intuitive success, theory of the evolution of cosexuality verses dioecy remains poorly tested in general: it is very challenging to measure the gain curves necessary to confirm or refute hypotheses, and many hypotheses are consistent with a single set of gain curves (see Charnov 1982). We will encounter similar problems in later chapters.

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