"Grandfather Effects" on Offspring Size in the Eastern Mosquitofish, Gambusia holbrooki

Abstract

Offspring size at birth can be an important determinant of initial success (see reviews in Roff, 1992): larger may have advantages in predator avoidance (Reznick and Endler, 1982; Weeks and Gaggiotti, 1993), growth (Kaplan, 1980), and starvation resistance (Bagenal, 1969; Marsh, 1986). However, the production of larger has a significant energetic cost to the parent (Blaxter, 1969; Sinervo, 1990), and it is commonly assumed that producing larger necessitates the production of fewer overall offspring, resulting in a trade-off between size at birth (hereafter offspring size) and number (Smith and Fretwell, 1974; McGinley et al., 1987; Winkler and Wallin, 1987). Several authors have suggested that, within any habitat and specific to any particular species, there will be natural selection for an size (Smith and Fretwell, 1974; Morris, 1987; Winkler and Wallin, 1987). This optimum is the size at which the increase in fitness with increased size begins to plateau (Smith and Fretwell, 1974). Producing of this size should balance the benefits to the parent by producing as many as possible, while allocating enough energy to each to provide them a good probability of survival (Morris, 1987). Thus, it is predicted that there should be an identifiable optimal size for an organism living in a particular habitat and that natural populations should be at or near this optimal size because of directional or balancing selection toward this optimum. The above statement implies that natural selection has the appropriate genetic variation with which to work. For natural selection to be