Fitness can fundamentally be achieved by two different strategies: long life (stability) or fast reproduction (multiplication, replication). These strategies are to some degree dependent: since no organism is immortal, a minimum amount of reproduction is needed to replace the organisms that have died; yet, in order to reproduce, the system must live long enough to reach the degree of development where it is able to reproduce. On the other hand, the two strategies cannot both be maximally pursued: the resources used for fast reproduction cannot be used for developing a system that will live long, and vice-versa. This means that all evolutionary systems are confronted with a development-reproduction trade-off: they must choose whether they invest more resources in the one or in the other.
How much a given system will invest in one strategy at the expense of the other one depends on the selective environment. In biology, this is called r-K selection: in an r-situation, organisms will invest in quick reproduction, in a K-situation they will rather invest in prolonged development and long life. Typical examples of r-species are mice, rabbits, weeds and bacteria, which have a lot of offspring, but a short life expectancy. Examples of organisms undergoing K-selection are tortoises, elephants, people, and sequoia trees: their offspring are few but long-lived. In summary, r-selection is selection for quantity, K-selection for quality of offspring.
Selection for many offspring is most useful in an uncertain, dangerous environment, where most offspring will die anyway, whether the parents invest much resources in their development or not. The more offspring there is, the more chances that at least one of them will survive and continue the lineage. Selection for prolonged development is most useful when the environment provides a stable, predictable supply of resources, without great dangers. In that case, the one most likely to survive the competition with others will be the one that has had most time to develop its strength, experience or size.
In a uncertain environment, reproduction is basically a a lottery: you cannot predict or influence which of your offspring will survive; the only way to increase your chances that at least one of them will survive is to produce as many as possible (like you can increase your chances of winning only by buying more lottery tickets). In a predictable environment, on the other hand, reproduction is more like a game of chess: the best way to win is to make few but well-prepared moves, rather than quickly making a lot of moves at random. K-selection, therefore, is selection for increasing control over the environment, whereas r-selection is caused by an environment that is intrinsically difficult to control.
The names r and K come from a mathematical model of population growth, which is typically a sigmoid curve. For small populations, growth is exponential as represented by the r parameter. When the population becomes larger, growth slows down as the population reaches the maximum carrying capacity (represented by the K parameter) of the environment. r-selected populations are typically far from their carrying capacity, and thus able to grow exponentially using an abundance of available resources. However, because of the dangers in the environments (diseases, predators, droughts, etc.) the population is regularly decimated so that it never actually reaches the carrying capacity. K-populations are well-protected against such disasters and therefore remain close to the carrying capacity. In that regime, resources are limited, and there is strong competition among the members of the population. This competition allows only the strongest, largest, most developed or most intelligent members of the species to survive and reproduce.
It must be noted that the selective environment is not objectively given, but dependent on the specific system, whose organization and behavior determines its specific niche within the larger physical environment. Rabbits and tortoises may well share the same physical environment, but tortoises are shielded from dangers by their shell, and by their slow metabolism, which allows them to survive without food for a much longer time than a mouse would. Therefore, it pays for a tortoise to grow a large and strong shell and to have efficient repair mechanisms that allow it to live long, because this will increase its chances to produce offspring that will itself survive and reproduce. Rabbits, on the other hand, are easily killed by predators or temporary lack of food, and therefore do best to make sure they reproduce before such a calamity has struck, without investing too much energy in developing a body that is theoretically capable of living long, but that will in practice be killed long before this limit age (see the Evolutionary causes of aging and death).
This evolutionary principle, which states that organisms will determine their position on the development-reproduction trade-off according to the security of their environment, has many practical, observable applications. The main prediction that can be made is that organisms that are otherwise similar, but confronted with different environments, will put either more emphasis on development and survival or on reproduction. An example of such a prediction was recently confirmed: a variety of opposum that lives on an island with no predators lives much longer than its cousins on the mainland, even when both are kept safely in a zoo: the island variant's genes have been selected for slow aging, a feature useless for the mainland variety, whose genes have been selected for quick reproduction.
The development-reproduction or r-K trade-off is associated with an array of typical differences between types of organisms:
|weak||strong or well-protected|
|waste a lot of energy||energy efficient|
|less intelligent, experienced...||more intelligent, experienced...|
|have large litters||have small litters|
|reproduce at an early age||reproduce at a late age|
|fast maturation||slow maturation|
|little care for offspring||much care for offspring|
|strong sex drive||weak sex drive|
|small size at birth||large size at birth|