OPTIMAL INITIAL SIZE AND ADULT SIZE OF ANIMALS - CONSEQUENCES FOR MACROEVOLUTION AND COMMUNITY STRUCTURE

Evolution of adult size and offspring size is considered with the aid of an optimal energy allocation model in which, in contrast to existin g allocation models that apply a purely energetic definition of fitnes s, the amount of energy allocated to reproduction is divided into quan ta dependent on offspring size, and net reproductive rate is maximized . This approach enables the connection between adult and offspring siz e to be identified: larger offspring make it optimal for their mothers to have larger adult size. Optimal offspring size exists in the range of sizes for which the ratio of production rate to mortality rate is concave upward with respect to body size. If such a range does not exi st, it is optimal to produce the smallest viable offspring. Optimal ad ult size exists in the range of sizes for which the ratio of productio n rate to mortality rate is concave downward. If such a range does not exist, it is optimal to have the largest viable adults. The shape of the function representing the ratio of these two rates changes if a ne w size-specific predator invades the system: then, a macromutation abr uptly changing either initial size or adult size can be preferred by n atural selection. Possible mechanisms of such macroevolutionary change s are discussed. In the modern world, in which many small and large sp ecies with various offspring sizes exist, replacement of one species b y another is expected after invasion by a size-selective predator.