

This article is an excerpt from the Shortform book guide to "Why We Get Sick" by Randolph Nesse and George Williams. Shortform has the world's best summaries and analyses of books you should be reading.
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What does the phrase “survival of the fittest” actually mean? How does natural selection happen?
In the phrase “survival of the fittest,” the “fittest” actually refers to reproductive capabilities. Darwinian evolution says that evolution will favor the genes that allow the individual to produce the most viable offspring.
Continue reading for more information about how natural selection works.
How Natural Selection Works
How does natural selection work? Natural selection is often thought of informally as “survival of the fittest,” but this is a confusing phrase. People often have the wrong idea of what “fitness” means. It doesn’t mean fitness in the everyday sense of good health and long life.
In Darwinian terms, a higher fitness means more reproduction and a greater number of viable offspring. If a gene leads to production of more viable offspring in future generations, that gene will be enriched in the population. Likewise, if a gene codes for characteristics that result in fewer viable offspring in future generations, that gene is gradually eliminated.
Taking the phrase “survival of the fittest” again, we now see that “survival” is important for natural selection only insofar as it increases reproduction. If a gene helps an individual survive longer, but at the expense of having fewer offspring, that gene will show up less often in the gene pool.
Genes that increase lifetime reproduction will be selected for, even if they reduce the individual’s longevity.
This is one reason that we haven’t evolved away common diseases—the genes that cause gout and dementia later in life may actually increase our reproductive fitness earlier in life.
Look for Alternative Explanations
A key takeaway: when we find something that seems like an error in natural selection, more likely we are missing some important function that compensates for the deficit.
There’s a parable of Henry Ford when he asked a car engineer, “Is there anything that never goes wrong with any of these cars?” The engineer responds, “Yes, the steering column never fails.” Ford responds, “Redesign it. If it never breaks, we must be spending too much on it.”
Likewise, the body has evolved to be a set of compromises. Some traits might increase vulnerability to some diseases, but still give an overall fitness advantage.
Nuances of Natural Selection
A gene’s contribution to fitness is not determined in isolation. It’s measured in a particular species in a particular environment. Change the environment, and the gene may no longer increase fitness.

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- Why evolution hasn't rid humans of all diseases
- How reproductive fitness is more important than overall survival
- How you evolved to dislike the sound of a baby crying