Introduction: An organism's Darwinian fitness is calculated as the number of offspring it leaves behind that, themselves, survive to reproduce. In evolutionary terms, it is of no consequence if an organism is a fine, fully mature, physical specimen, or the dominant member of the herd, or even that an individual produces a lot of young but none of them survive. In the relay race of evolution, getting as many copies of your genes into the next generation as possible is the only goal. As you might imagine, there are many ways to be reproductively successful. One way is to become the dominant animal in a pack, and to monopolize mating opportunities, but another way is to be submissive and sneaky, mating with others when the dominant animal is not around to stop you. There are no moral judgements. It's just biology. Now imagine that you're an animal faced with the following choice: given limited resources, should you put them all into producing one or a few offspring, and protect them with great ferocity, or should you put a small amount of effort into a much larger number of offspring, and let them each take their chances? Should you measure out your reproductive effort over many seasons, or save it all up for a one-time mating frenzy as soon as you're able? These trade-offs relate to the r/K selection theory of life history strategies.

A mouse produces a large litter. A whale tends for a single calf.

r-selection: On one extreme are the species that are highly r-selected. r is for reproduction. Such a species puts only a small investment of resources into each offspring, but produces many such low effort babies. Such species are also generally not very invested in protecting or rearing these young. Often, the eggs are fertilized and then dispersed. The benefit of this strategy is that if resources are limited or unpredictable, you can still produce some young. However, each of these young has a high probability of mortality, and does not benefit from the protection or nurturing of a caring parent or parents. r-selected babies grow rapidly, and tend to be found in less competitive, low quality environments. Although not always the case, r-selection is more common among smaller animals with shorter lifespans and, frequently, non-overlapping generations, such as fish or insects. The young tend to be precocial (rapidly maturing) and develop early independence.

K-selection: On the other extreme are species that are highly K-selected. K refers to the carrying capacity, and means that the babies are entering a competitive world, in a population at or near its carrying capacity. K-selected reproductive strategies tend towards heavy investment in each offspring, are more common in long-lived organisms, with a longer period of maturation to adulthood, heavy parental care and nurturing, often a period of teaching the young, and with fierce protection of the babies by the parents. K-selected species produce offspring that each have a higher probability of survival to maturity. Although not always the case, K-selection is more common in larger animals, like whales or elephants, with longer lifespans and overlapping generations. The young tend to be altricial (immature, requiring extensive care).

You can see r- and K-selected strategies clearly by looking at different organisms within a phylogenetic group, such as the mammals. For example, elephants are highly K-selected, whereas mice are much more r-selected. Among the fishes, most, like the salmon, are r-selected. Some species will even inadvertently eat their own young if they are not immediately dispersed, but a few species, such as the cichlids, are K-selected and provide prolonged care and protection of the eggs and hatchlings. Even among humans, there are a range of strategies toward one or the other extreme. In one family, with ten children, for example, there is no way for the parents to put as much time, energy, or resources into all of them as could be done with an only child. But, with humans, it gets complicated by the fact that others, including siblings, grandparents, blood-relatives, and the larger community all play a role in the nurturing and education of children.

Even plants are capable of r- and K-selected reproductive strategies. Wind pollinated species produce much more pollin that insect pollinated ones, for example, because the pollin has to be carried at random by the wind to a receptive female flower. Eggs too, can be r- or K-selected. The amount of nutrient energy placed in an egg gives it a lesser or greater ability to survive in adverse conditions. One can even compare the reproductive stragies of males and females within a species, when sperm and egg represent different levels of energy investment. Often sperm are resource poor, and produced in large quantities, while eggs are resource rich and produced in smaller numbers. This can lead to differences in behavior between the sexes, often with the result that the female is the choosier sex when it comes to reproduction. This trend is further extended if the female also carries the young (in the case of internal fertilization) or has a greater role in parental care once the babies are born. There are some interesting exceptions that illustrate the rule. Male seahorses are the choosier sex, and they are the ones that incubate the young. In a small fish called the stickleback, the male is also choosier, it is believed, because the female lays her eggs in a nest he constructed and then leaves. The male guards the nest and tends the young for an extended period.

It should be noted that r- and K-selection are the extremes at both ends of a continuum and that most species fall somewhere inbetween.

Which survivorship curve is most r-selected? Which is most K-selected?

More information:

http://www.bio.miami.edu/tom/courses/bil160/bil160goods/16_rKselection.html

https://en.wikipedia.org/wiki/R/K_selection_theory