A second key trade-off is that between the number of offspring and their individual fitness. At its simplest, this is a trade-off between the size and number of offspring, within a given total reproductive investment. That is, a reproductive allocation can be divided into fewer, larger offspring or more, smaller offspring. However, the size of an egg or seed is only an index of its likely fitness. It may be more appropriate to look for a trade-off between the number of offspring and, say, their individual survivorship or developmental rate.
Of the few genetic correlations between egg size and number that have so far been examined (dominated by domestic poultry), the majority have been negative, as expected (Lessells, 1991). Negative correlations have also been observed in simple cross-species or cross-population comparisons (e.g. Figure 4.23a, b), although it is unlikely in such cases that the individuals from different species or populations are making precisely the same total reproductive allocation. Moreover, this type of trade-off is especially difficult to observe through experimental manipulation. To see why, note that we need to ask the following kind of question. Given that a plant, say, produces 100 seeds each weighing 10 mg and each with a 5% chance of developing to reproductive maturity, what would be the seed size, and what would be the chance of developing to maturity, if an identical plant receiving identical resources produced only 80 seeds? Clearly, it would be invalid to manipulate seed number by altering the provision of resources; and even if 20 seeds were removed at or close to their point of production, the plant would be limited in its ability to alter the size of the remaining seeds, and their subsequent survivorship would not really address the question originally posed.
Sinervo (1990), though, did manipulate the size of the eggs of an iguanid lizard (Sceloporus occidentalis) by removing yolk from them after they have been produced, giving rise to healthy but smaller offspring than unmanipulated eggs. These smaller hatch-lings had a slower sprint speed (Figure 4.23c) - probably an indication of a reduced ability to avoid predators, and hence of a lower fitness. Within natural populations, this species produces smaller clutches of larger eggs in California than in Washington (typically seven to eight eggs with an average weight of 0.65 g, as against approximately 12 eggs weighing 0.4 g; Figure 4.23c). Thus, in the light of the experimental manipulations, the comparison between the two populations does indeed appear to reflect a trade-off between the number of offspring and their individual fitness.
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