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showed that species with internal fertilization and presumably high rates of
fertilization display similar patterns of egg variation to those species with freely
spawned gametes, thus they suggest that factors other than gamete encounter might be
determinant for egg size evolution across different modes of reproduction.
McEdward and Morgan (2001) analyzed the relationship between size of eggs
and the energy contained in them using published data for 47 species of echinoderms
(Table 4.1). They found that among echinoderms, larger eggs contain more energy,
suggesting a general pattern in which energy scales very nearly in direct proportion to
the volume of the egg across a significant range of egg sizes, both within and among
different modes of development. The only exception is among species with
planktotrophic larval development, where there does not appear to be a clear scaling
relationship. However, there were wide confidence intervals around the estimated
regression parameters in all of the analyses performed by McEdward and Morgan
(2001). In addition, in all cases the predictive power of the regression was poor,
requiring large differences in egg size in order to produce significantly different
predictions of energy content. Therefore they concluded that egg size is of limited
value for the quantitative prediction of egg energy content and should be used with
caution in life-history studies.
Natural selection is considered to drive the level of egg provisioning towards
reproductive strategies with high fitness. If a single maximum is observed in the
fitness curve, then selection will be expected to direct towards that best adaptation.
However, if the fitness curve possesses another shape, for example curved upward
(concave), then an adaptive valley between two optima might exist, and selection will
be expected to be disruptive across that region.
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