Phenotype is everything you can observe about a living thing, from height and color to behavior and disease risk, and genes never write it alone. The same genes can produce strikingly different bodies depending on food, temperature, danger, or stress during life. A honey bee queen and a worker can share the same genes, with diet alone deciding which one a larva becomes.1Advances in Insect Physiology. Old Threads Make New Tapestry—Rewiring of Signalling Pathways Underlies Caste Phenotypic Plasticity in the Honey Bee, Apis mellifera L. Nature versus nurture is a false choice, because the environment works through the genes rather than around them.
Genes Set a Range and the Environment Picks the Outcome
Biologists call an organism’s set of genes its genotype. The ability of one genotype to produce different traits in different conditions is called phenotypic plasticity. Traits vary for three reasons: differences in genes, differences in environment, and random chance during development.2Symmetry. Nature, Nurture, and Noise: Developmental Instability, Fluctuating Asymmetry, and the Causes of Phenotypic Variation So even two genetically identical animals raised side by side will not turn out exactly alike.
Cuttings from a single plant are genetically identical. Grow one in shade and another in full sun, and they reach different heights and leaf sizes. Both sit within the range its genes allow.
That chance element is easy to overlook. Even with identical genes and identical conditions, small events inside cells, such as which gene happens to switch on first in a given cell, can push development in slightly different directions. It is why identical twins have different fingerprints and slightly different moles, even though they shared a womb and a genome. Chance is neither inherited nor imposed from outside, so it doesn’t belong under “environment.”
Different genotypes also respond to the same conditions in different ways. Scientists call this a gene–environment interaction.3Wikipedia. Gene–environment interaction In tiny roundworms, one strain could be the most fertile in one setting and fall behind others in another.4PubMed. Mapping phenotypic plasticity and genotype-environment interactions affecting life-history traits in Caenorhabditis elegans The same applies to people, where a diet or medicine can suit one person’s genes better than another’s.
That is why the common question “what percent of this trait is genes?” so often goes nowhere. When genes and environment interact, the effect of one depends on the other, so a fixed split means little for an individual.
Striking Examples From Bees, Water Fleas, Insects and Birds
Honey bee larvae fed a richer queen diet develop into queens, while others become workers.5Advances in Insect Physiology. Old Threads Make New Tapestry—Rewiring of Signalling Pathways Underlies Caste Phenotypic Plasticity in the Honey Bee, Apis mellifera L. A nutrient-sensing growth pathway called TOR is more active in queen-destined larvae.6PubMed Central. The Making of a Queen: TOR Pathway Is a Key Player in Diphenic Caste Development When that pathway is blocked, larvae meant to be queens grow worker bodies instead.7PubMed Central. The Making of a Queen: TOR Pathway Is a Key Player in Diphenic Caste Development
Once diet has done its work, the two castes are very different. Queens develop fully working ovaries and live far longer, while workers have underdeveloped ovaries and take on tasks like foraging and nursing. A diet difference early in a larva’s life changes body structure, behavior, and lifespan, not just size.
Water fleas grow larger bodies, longer tail spines, and head crests when they sense chemicals released by predators.8PubMed Central. Predator-induced defences in Daphnia longicephala: location of kairomone receptors and timeline of sensitive phases to trait formation The armor appears only after a delay of about two molts after first exposure.9PubMed Central. Predator-induced defences in Daphnia longicephala: location of kairomone receptors and timeline of sensitive phases to trait formation Water fleas grow by shedding their outer shell, so the new shape shows up only in shells formed after the signal registers.
Some species respond to fish chemicals only if the fish have been eating water fleas.10PubMed. The role of prey and predator identity in eliciting inducible defenses of Daphnia So the defenses react to real danger, not just the presence of fish. Genetic differences between strains mattered far less than the predator signal itself.11PubMed. The role of prey and predator identity in eliciting inducible defenses of Daphnia There is a reason the fleas don’t keep armor up all the time: spines and crests cost energy to build, and without predators that energy is better spent on reproduction.
Temperature shapes body size too. In many cold-blooded animals, individuals raised in colder conditions grow more slowly but end up larger.12PubMed. The temperature-size rule in ectotherms: simple evolutionary explanations may not be general Fruit fly larvae in warmer conditions trigger the hormone signal to stop growing at a smaller size.13PubMed Central. Temperature-size rule is mediated by thermal plasticity of critical size in Drosophila melanogaster
Organs resize with diet as well. A migrating bird increases its digestive capacity mainly by growing a bigger gut, which takes more than 1 to 2 days.14PubMed Central. Spare capacity and phenotypic flexibility in the digestive system of a migratory bird: defining the limits of animal design A bird’s gizzard grows on a high-fiber diet and shrinks again when fiber drops.15PubMed. Phenotypic flexibility of the avian gizzard: rapid, reversible and repeated changes of organ size in response to changes in dietary fibre content
These examples differ in whether they can be undone. Gizzard size goes up and down with diet. A bee’s caste is set for life once development ends. Some worms’ mouth shape is fixed irreversibly during larval growth. The timing of exposure decides which kind of change happens.
How the Environment Reaches the Genes
Much of the link runs through epigenetic changes: chemical tags on DNA and its packaging proteins that change how strongly genes are used, without altering the DNA sequence.16PubMed. Epigenetic regulation of gene expression: how the genome integrates intrinsic and environmental signals The two best-studied are DNA methylation, small methyl tags on DNA, and changes to the proteins DNA wraps around.17PubMed. Epigenetic regulation of gene expression: how the genome integrates intrinsic and environmental signals The enzymes that add and remove these tags are sensitive to nutrients and environmental chemicals, so they act as sensors.18PubMed Central. Epigenetic responses to environmental change and their evolutionary implications
Tags near a gene’s on-switch often make that gene harder for the cell’s machinery to read, turning its output down. Tags can be removed too, which is why this control can change with conditions.
Plants grown in low humidity add methyl tags to genes that build leaf pores.19PubMed Central. Low relative humidity triggers RNA-directed de novo DNA methylation and suppression of genes controlling stomatal development Those genes are turned down, and the plant makes fewer pores and loses less water.20PubMed Central. Low relative humidity triggers RNA-directed de novo DNA methylation and suppression of genes controlling stomatal development Plants unable to add the tags showed no such change.21PubMed Central. Low relative humidity triggers RNA-directed de novo DNA methylation and suppression of genes controlling stomatal development
Plants can also remember stress. Plants that survive drought can carry a memory that changes later leaves, such as pore control and waxy coating thickness, through epigenetic tags and stress hormones.22Plant Gene. Somatic drought stress memory affects leaf morpho-physiological traits of plants via epigenetic mechanisms and phytohormonal signalling
Tags are not the only route. Hormone signals, like the fly’s stop-growth signal, carry information about conditions. Nutrient sensors like TOR translate diet into growth. Microbes living in or on a body can shift traits as well, as a later section covers.
Effects That Last a Lifetime or Pass to Offspring
In rats, the care a mother gives changes methyl tags on a stress-hormone receptor gene in the offspring’s brain.23PubMed Central. Epigenetic mechanisms for the early environmental regulation of hippocampal glucocorticoid receptor gene expression in rodents and humans Parenting can therefore set lasting differences in how offspring handle stress.24PubMed Central. Epigenetic mechanisms for the early environmental regulation of hippocampal glucocorticoid receptor gene expression in rodents and humans The difference came from how the mother behaved toward her pups, such as licking and grooming, rather than from the pups’ inherited genes. Less-cared-for pups had more methyl tags on the receptor gene, so the brain made fewer receptors that help dampen the stress response. As adults, those pups reacted more strongly to stress.
Epigenetic inheritance means changes in a parent’s sperm or eggs alter traits in later generations without any DNA mutation.25PubMed. Mechanisms of transgenerational epigenetic inheritance: lessons from animal model organisms That is a much higher bar than an early-life effect, which happens inside one individual’s body and lasts their lifetime. In sheep, a father’s diet left methylation marks that appeared in his grandchildren’s sperm.26PubMed Central. Paternal diet induces transgenerational epigenetic inheritance of DNA methylation signatures and phenotypes in sheep model Finding the marks in the grandchildren’s sperm is the kind of evidence that separates inheritance from a lifetime effect. About 9 in 10 of those inherited marks kept the same direction across three generations.27PubMed Central. Paternal diet induces transgenerational epigenetic inheritance of DNA methylation signatures and phenotypes in sheep model Fathers’ stress can also affect offspring through tags and small RNA molecules in sperm.28PubMed Central. Paternal Transgenerational Epigenetic Mechanisms Mediating Stress Phenotypes of Offspring
Some changes eventually become genetic. In fruit fly experiments, a trait first triggered by stress began appearing without the stress after several generations of selection.29PubMed Central. Mechanism of evolution by genetic assimilation Selection rewired existing developmental pathways rather than needing new mutations.30PubMed. Genetic assimilation, robustness and plasticity are key processes in the development and evolution of novel traits
For people, the picture is more cautious. Most evidence for multigenerational inheritance comes from animals, and many tags are erased between generations. A grandparent’s diet affecting you is plausible but not well proven in humans. Lasting effects within one lifetime are much better supported.
What the Dutch Famine Shows About Human Health
Babies in the womb during the 1944–45 Dutch famine had higher adult risk of type 2 diabetes and heart disease.31International Journal of Environmental Health Research. Lessons learned from 25 years of research into long-term consequences of prenatal exposure to the 1944-1945 Dutch famine Exposure in mid-pregnancy was linked to early signs of kidney and blood vessel damage in adulthood.32International Journal of Environmental Health Research. Lessons learned from 25 years of research into long-term consequences of prenatal exposure to the 1944-1945 Dutch famine As adults, their blood showed higher glucose and certain amino acids.33PubMed Central. Adults prenatally exposed to the Dutch Famine exhibit a metabolic signature associated with a broad spectrum of common diseases Their blood chemistry closely resembled that of people about to develop diabetes or a heart attack.34PubMed Central. Adults prenatally exposed to the Dutch Famine exhibit a metabolic signature associated with a broad spectrum of common diseases A large part of the link, though, ran through higher body weight.35PubMed Central. Adults prenatally exposed to the Dutch Famine exhibit a metabolic signature associated with a broad spectrum of common diseases
The womb is a sensitive window because organs and metabolism are being built. Signals of scarcity may set the body to store energy, and that setting clashes with later plenty. Timing during pregnancy changed which organs were affected.
The famine also works as a natural experiment. It began and ended at known times, so researchers could compare people who were in the womb during it with those born just before or after. Because the groups came from the same population, a difference in adult health points to the timing of exposure. Researchers can’t randomly assign famine, which is why this kind of study relies on such events.
For everyday health, genes are not destiny for common diseases. Nutrition in pregnancy has effects lasting decades, yet adult habits like weight control can still change risk. The risk is raised, not guaranteed.
When Changing With the Environment Backfires
Under stress, a plastic response can push a trait further from what would help survival.36Functional Ecology. Adaptive versus non‐adaptive phenotypic plasticity and the potential for contemporary adaptation in new environments Some amaranth crops responded to higher heat in ways that weakened their structure, growth and seed production.37bioRxiv. High plasticity increases phenotype-environment mismatch leading to sub-optimal performance in underutilized crop species A response that evolved to help in familiar conditions may overshoot when conditions are pushed beyond anything the species experienced before. Plasticity is a tool tuned to past conditions, not a guarantee of the right answer.
Responses also often hit a physical limit in extreme conditions.38PubMed Central. Adaptation, Plasticity, and Extinction in a Changing Environment: Towards a Predictive Theory A body can’t make its tissues tolerate unlimited heat or dryness. Beyond that ceiling, further change in the environment no longer produces a helpful adjustment, which is why gradual warming can be absorbed for a while and then suddenly cause harm.
Many organisms time life events by cues like temperature that used to predict the season.39PubMed Central. Strong phenotypic plasticity limits potential for evolutionary responses to climate change As climate grows more variable, these cues become less reliable.40PubMed Central. Strong phenotypic plasticity limits potential for evolutionary responses to climate change When a population lacks genetic variety in how it responds, its ability to adapt and survive can weaken.41PubMed Central. Strong phenotypic plasticity limits potential for evolutionary responses to climate change
Microbes and Organisms Shaping Their Own Environment
The microbes an animal carries act as part of its environment. Fruit flies given microbes from two different field sites differed greatly in reproduction, fat levels, stress tolerance and sleep.42PubMed Central. Environmental microbes promote phenotypic plasticity in reproduction and sleep behaviour Clearing the microbes erased those differences.43PubMed Central. Environmental microbes promote phenotypic plasticity in reproduction and sleep behaviour That shows the microbes themselves caused the changes.
Many animals also depend on resident microbes for normal growth, including steps that help the body use food. Disrupting a host’s microbes can upset normal development.44PubMed Central. Environmental disruption of host–microbe co-adaptation as a potential driving force in evolution An animal can then develop abnormally even with healthy genes and plenty of food, so a body is shaped partly by organisms living in or on it.
The influence runs the other way too. Organisms change their surroundings, such as when plants shift when they flower or sprout, which then reshapes the conditions they and their offspring face.45PubMed. Niche construction through phenological plasticity: life history dynamics and ecological consequences Beavers’ dams change the waters their young grow up in, and earthworms alter the soil they live in. Human diets shape gut microbes, which affect health. Offspring inherit a modified world along with their genes.