The most widely cited example of stabilizing selection is human birth weight, where babies born at intermediate weights historically had the highest survival rates, while very small and very large newborns faced greater risks. But birth weight is just one case. Stabilizing selection shows up across the natural world whenever individuals with average traits survive and reproduce better than those at the extremes.
In stabilizing selection, the bell curve of a trait in a population stays centered in the same place, but it gets narrower over time. The average doesn’t shift. Instead, the extremes get pruned. This stands in contrast to directional selection, which pushes the whole curve toward one end, and disruptive selection, which favors both extremes and hollows out the middle.
Gall Size in Goldenrod Flies
One of the cleanest examples of stabilizing selection in nature involves a small fly called Eurosta solidaginis that lays its eggs inside the stems of tall goldenrod plants. The plant responds by forming a round growth called a gall, and the fly larva develops inside it. The size of that gall is partly determined by the fly’s own genes, which makes it a heritable trait that natural selection can act on.
Here’s where the selection pressure comes from two directions at once. In forested areas, birds (mainly downy woodpeckers) peck open the largest galls to eat the larvae inside. Larger galls are easier to find and worth the effort. At the same time, a parasitoid wasp called Eurytoma gigantea targets the smallest galls, drilling through the thin walls to lay its own eggs inside, killing the fly larva in the process. The wasp’s egg-laying organ is too short to reach larvae in bigger galls.
The result is a vise grip on gall size. Fly larvae in large galls get eaten by birds. Fly larvae in small galls get parasitized by wasps. Larvae in medium-sized galls survive at the highest rates and pass on their genes to the next generation. Over time, this keeps the population’s average gall diameter stable and reduces the variation on either side. It’s a textbook case because the two opposing forces are easy to identify and measure independently.
Human Birth Weight
For most of human history, birth weight followed the same pattern. Babies born too small were more vulnerable to infection, had trouble maintaining body temperature, and faced higher mortality. Babies born too large risked complications during delivery, including obstructed labor, which was often fatal for both mother and child before modern obstetrics. The sweet spot was somewhere in the middle, roughly 7 to 8 pounds, where survival rates peaked.
This is a case where stabilizing selection is now weakening. Cesarean sections can safely deliver babies who would have been too large to pass through the birth canal, and neonatal intensive care dramatically improves outcomes for very small or premature infants. Biologists studying this shift have pointed out that C-sections essentially remove the disadvantage that very large-headed babies once faced during birth. Over many generations, this could shift the pattern from stabilizing selection toward directional selection favoring larger babies. If C-sections keep more genes for large heads and narrow hips in the population, the need for surgical delivery could increase in a self-reinforcing cycle. Whether this is actually happening at a measurable pace is still debated, but the logic illustrates how modern medicine can alter evolutionary pressures that existed for millennia.
Cricket Calls and Trait Combinations
Research on black field crickets in Australia provides a more detailed look at how stabilizing selection shapes multiple traits at once. Male crickets produce advertisement calls to attract females, and these calls have several measurable properties: pitch, duration, rhythm, and so on. Researchers found strong stabilizing selection acting on specific combinations of call traits, meaning that males with extreme versions of those combinations left fewer offspring.
The study revealed something interesting about genetics. Trait combinations under the strongest stabilizing selection had the least genetic variation remaining in the population, exactly what theory predicts. Selection had been grinding down the diversity in those traits for generations. Meanwhile, other trait combinations that weren’t under strong selection still harbored much higher genetic variation. This pattern explains a long-standing puzzle in evolutionary biology: why populations maintain so much genetic diversity even when stabilizing selection should, in theory, erode it. The answer is that selection hits some trait dimensions hard while leaving others relatively untouched, and those less-constrained dimensions store the bulk of the population’s genetic variety.
How Stabilizing Selection Differs From Other Types
All three major types of natural selection act on the same bell-shaped distribution of traits in a population, but they reshape it in different ways. Directional selection shifts the peak of the curve in one direction. If larger body size helps an animal survive, smaller individuals are weeded out and the average body size increases over generations. Disruptive selection does the opposite of stabilizing: it favors both extremes and works against the middle, sometimes splitting a population into two distinct groups.
Stabilizing selection is the most common form observed in natural populations. A 2017 analysis of contemporary humans using data from the UK Biobank found widespread evidence of stabilizing selection across multiple traits, though the strength of selection was relatively weak compared to estimates from other species. This makes intuitive sense. Most organisms are already reasonably well adapted to their environment, so the average value of most traits is already near the optimum. Selection mostly just needs to keep things where they are by trimming the outliers.
Why Stabilizing Selection Matters
Stabilizing selection explains why many species look remarkably consistent over long stretches of time. If the environment stays relatively stable, the optimal trait value doesn’t change, and selection keeps pulling the population back toward that optimum generation after generation. New mutations constantly introduce variation, but stabilizing selection counteracts this by reducing the survival or reproduction of individuals who land too far from the average. This balance between mutation adding variation and selection removing it is what maintains the characteristic range of a trait in a population.
The practical consequence is that stabilizing selection constrains future evolution. Research on crickets showed that as selection depletes genetic variance in particular trait directions, it limits the population’s ability to evolve in those directions later. If the environment suddenly changes and a new optimum appears, populations that have been under strong stabilizing selection for a long time may have less raw material to work with. They’ve traded evolutionary flexibility for current fitness, a tradeoff that works beautifully in stable environments but becomes a liability when conditions shift.