You've probably heard it a million times in descriptions of evolution and natural selection. Charles Darwin even liked to say it. But the phrase "survival of the fittest" is wrong, and understanding why can help us better understand what it means to be human.
Survival's Origins
Darwin uses the phrase "survival of the fittest" in chapter four of On the Origin of Species to describe the process of natural selection. But he did not coin the phrase. It was borrowed from English philosopher Herbert Spencer, who first talked about survival of the fittest in his Principles of Sociology. "The term 'natural selection,'" wrote Darwin in The Origin, "is in some respects a bad one, as it seems to imply conscious choice." Referring to the process as "survival of the fittest," Darwin thought, helped clarify things. But the famed naturalist's appropriated turn of phrase turned out to be rather inappropriate, itself.
Princeton biological anthropologist Alan Mann told io9 that in most cases, "survival of the fittest" has been replaced by the term "reproduction of the fittest," or "differential selection." This holds particularly true in discussions concerning mammals — humans, especially. Mann says there are two main reasons for this.
One: for an organism to reproduce, it is implied that it must first live long enough (i.e. survive) to do so. And two: the phrase "survival of the fittest" paints a mental image of what Mann characterizes as "the tooth and claw of bloody nature" — as though every organism in a particular area is perpetually fighting for the ability to survive. In this context, "fitness" can be misinterpreted as an ideal evolutionary goal. "But Evolution acts to produce function, not perfection" says Mann. Moreover, "fitness" should properly refer not so much to characteristics like strength or speed, but rather an animal's ability to produce viable offspring.
On Fish and Humans
Where a phrase like "survival of the fittest" becomes relevant, says Mann, is in discussions about what is known in ecology as "selection theory," or ideas about the trade-off between the quantity and quality of an organism's offspring.
Fish, for example, can produce and fertilize thousands of eggs during a mating session; but the number of fertilized eggs that are eaten, killed, or die in some other way before reaching sexual maturity is huge. This "make as many as you can" reproductive strategy is called "r-selection." Large numbers of offspring are produced, but the vast majority of them perish. And this strategy, says Mann, does, in some ways, follow the concept of "survival" of the fittest.
Say you have a newborn fish that is a prey species to a larger, predacious fish. In most fish species, there is little-to-no parental care, so that animal has predator-avoidance behaviors built into its neurological system. When a young fish sees the shadow of predator nearby, or feels the water current of a larger fish, it begins to exhibit predator-avoidance behavior. For many fish, says Mann, this means either swimming very fast, or swimming in a zig-zag fashion. But of course, predacious fish have also evolved mechanisms to catch prey. He continues:
So if the prey fish is going zig zag zig zag zig zag, and the predator fish has evolved mechanisms to go zig zag zig zag zig zag, that particular prey becomes lunch. If however there is a biological variation, and instead of the prey fish join zig zag zig zag zig zag, it goes zig zag zig zag zig zig, it lives another day. So, on that level, survival of the fittest has some meaning.
But other animals, and mammals in particular, employ a reproductive strategy dubbed "K-selection." They produce fewer young, so their strategy is based on cultivating behaviors like postnatal protection and nurturing. These learned behaviors ensure their smaller number of offspring will reach reproductive maturity.
Human Behavior and Evolution
"Fitness" refers not to how long an organism lives, but how successful it is at reproducing. And "survival of the fittest" fails to encompass the subtleties of natural selection in mammals, which Mann points out often involve learned behaviors.
"One of the things that's happened in human evolution," he says, "is the time from birth to reproductive maturity and adulthood has been prolonged." This, he continues, probably holds true for most large-bodied mammals (think elephants, for example, or great apes). "When you think about that kind of biological change, it's really pretty difficult to understand, unless there is some adaptive advantage in allowing the young to internalize more behaviors."
In other words, increasing the age of sexual maturity makes little sense in the absence of some other evolutionary adaptation that makes it possible for offspring to develop safely over a longer period of time. This insight is crucial for understanding humans (and, arguably, mammals in general) not just in a biological light, but a cultural one, as well.
Consider, for example, that a typical pregnant human usually gives birth to just one child, occasionally two, and very rarely more than that. As a result, human parental investment in offspring is huge. An infant is raised, often by more than one family member, through a very long childhood development and dependency period. This not only ensures that the offspring will reach reproductive maturity, but that it has time to, as Mann puts it, "learn more appropriate behaviors, become better socialized into their society, and by this way become more successful and therefore capable of producing more offspring of their own."
It's therefore likely that the behavioral repertoires of humans, apes and other mammals have become remarkably complex because of the adaptive advantage they've provided as the time between birth and reproductive maturity has increased. On one hand, this allows for evolutionary fitness to be maintained. At the same time, however, it allows room for the possibility of sexually mature, adult animals (who have very clearly "survived," to reproductive age) who do not actually reproduce — once again highlighting the important distinction between "survival" of the fittest and "reproduction" of the fittest.
Among humans, not having children is often a culturally motivated choice, rather than a biological limitation (though both are often at play). People choose not to have children in order to pursue a career, or to raise only a small number of children. Others forego having children for so long that, when they finally decide to conceive, they encounter complications during childbirth. Despite a prolonged maturation period, these individuals are surviving to maturity without a problem. Evolved social mechanisms have played a large part in making that survival possible. But those same mechanisms can also lead to humans not reproducing, in which case their biological fitness would be considered to be very low.
Ultimately, "survival of the fittest" is necessary, but not always sufficient, for the survival of the species.