The Pace Of Replacement

Organisms approach reproduction in various ways. Evolutionary biologists have sometimes characterized these in two general categories as r and K "strategies." This nomenclature derives from an equation for the dynamics of population size, relating the rate of change in size of a population now of size N, to its intrinsic growth capability of the population (r), and the carrying capacity (maximum sustainable population) (K) of the local environment:

This is the logistic growth equation. At low relative population size, K - N is large, (K - N)/K is nearly equal to 1 so that growth occurs nearly at the maximum capacity (a factor of r per generation); but as population size nears the carrying capacity, K nearly equals N, so that (K - N)/K is nearly zero, and growth tapers off.

In this conceptualization, a species that reproduces rapidly is said to be using an r strategy. Its offspring mature quickly and shed large numbers of gametes. Insects and fish that produce thousands of eggs are examples. They can tolerate massive die-off of their young and still have net successful reproduction during their lifetime. The parent typically does not invest much care or effort into the maturation of the offspring, but there are exceptions (for example, among mammals, mice are rapid reproducers, but they also nurse their young until self-sufficiency). The race for success is run by producing many offspring, increasing the odds that at least some will survive. Animals lower in the food chain, whose lives tend to be risky and short, often reproduce rapidly and can be more adaptable. Any individual offspring is, more or less, expendable in terms of the parents' evolutionary interests. On average, in a stable environment, there is no net population growth after all this struggle, but an r strategy is able to recover more quickly from losses in numbers.

In contrast are the lumbering K strategists. Elephants, albatrosses, and some trees would be examples. So are humans and other primates. These organisms are, so to speak, near their carrying capacity or at least their ability to respond in terms of growth. They reproduce only irregularly, after a long juvenile time period, or only produce a small number of slow-growing offspring. In some cases, there is a need for longterm parental care so offspring can survive to reach reproductive age. Carnivores high in the food chain are fewer in number than their prey and may need more training for life. Because their reproduction is slow, maximizing the probability of survival of each individual offspring is important. A lost offspring is devastating to its parents' net reproductive output.

Mathematical theory has been developed to show how these reproductive behaviors might evolve genetically through the action of natural selection. Clearly, however, there is no one way to succeed in reproducing, nor is any way restricted to particular taxa, nor is a clade of taxa necessarily restricted to one way of doing business. But we should remember that while populations may fluctuate in size, no matter what their "strategy," in the long term the net reproduction on average for members of a population is just at the replacement level: one offspring produced per individual. In Chapter 3 we described various aspects of the way selection might act in regard to life history, to differentiate success among genotypes. The only invariant and merciless principle is that if it works, it works.

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