Life Historical Consequences of Natural Selection

The tremendous variation in the life-history patterns of organisms is best explained as adaptive.any organism has a limited amount of resources at its disposal, and these have to be partitioned between reproductive and nonreproductive activities. A larger share of resources to reproductive activities, that is, a higher reproductive effort at any age, leads to a better reproductive performance at that age; this may be considered a as profit function. This reproductive effort also leads to a reduction in survival and growth and consequent diminution of the reproductive contribution of the succeeding stages in the life history; this may be considered as a cost function. Natural selection would tend to an adjustment of the reproductive effort at every age such that the overall fitness of the life history would be maximized. A model of life history processes has been developed on the basis of these considerations. It leads to the following predictions: 1. If the form of the profit function is convex, or that of the cost function concave, the optimal strategy may be to breed repeatedly. Otherwise, the optimal strategy is to breed only once in a suicidal effort like a salmon (big-bang reproduction). 2. The value of reproductive effort continuously increases with age in the case of repeated reproducers. 3. If all the stages in the life history following a certain age are adversely affected, the age of reproduction will tend to be lowered in the case of big-bang reproducers, and the reproductive effort at all ages preceding that stage will tend to increase in the case of repeated reproducers. 4. As the reproductive potential increases with size at a slower rate, reproductive effort will be lower at maturity, reproductive effort will increase at a higher rate with age, and growth will continue beyond maturity. 5. A uniform change in the probability of survival from one age to the next at all ages would have no effect by itself, on the age of reproduction in big-bang breeders or on the distribution of reproductive effort with age in the repeated reproducers. 6. Such a change in survivorship would lead to a change in the equilibrium density of a population. If the population is resource limited, this would affect the availability of resources to the members, of the population in such a way that an increase in mortality would increase the availability of the resources. 7. For a resource-limited organism a greater availability of resources would lead to a lowering of the age of reproduction in the case of the big-bang breeders, and to a greater reproductive effort at ail ages for the repeated breeders.