📄 evolutionary biology

Why development time matters: temperature influences plasticity while nutritional stress reshapes selection and evolutionary potential

While temperature induces the most dramatic plastic changes in development time for *Drosophila melanogaster*, nutritional stress is the primary driver reshaping directional selection and evolutionary potential, demonstrating that the environmental factor causing the greatest plastic response does not necessarily dictate the trait's selection and evolution.

Original authors: Walter, G. M., Chakraborty, A., Cockerell, F., Alton, L., White, C., Hall, M. D., Sgro, C.

Published 2026-11-02

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CC BY 4.0

Original authors: Walter, G. M., Chakraborty, A., Cockerell, F., Alton, L., White, C., Hall, M. D., Sgro, C.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

In the natural world, the time it takes for an animal to grow from an egg into an adult is not just a matter of waiting; it is a critical life decision that shapes its entire future. This duration, known as development time, determines when an animal can reproduce, how large it will grow, and how well it can escape predators. Because the environment is constantly shifting, animals often have the ability to speed up or slow down this process depending on conditions like heat or food availability. This flexibility, called plasticity, allows a single species to survive in different places. However, scientists have long wondered whether this ability to change is always helpful, and whether the pressure to grow faster or slower changes when the environment becomes harsh. Understanding these rules is vital because as the planet warms and weather patterns become more erratic, the ability of populations to adapt depends on whether the traits that help them survive today will still be the right ones tomorrow.

A team of researchers set out to test these ideas using the vinegar fly, a small insect that is a common subject for studying how genes and environment interact. They raised hundreds of families of these flies in a controlled setting, exposing them to two very different temperatures and two different diets. One temperature was cool at 14 degrees Celsius, while the other was warm at 28 degrees Celsius. The diets included a standard, nutrient-rich food and a low-calorie version that simulated a shortage of resources. The goal was to see how these stressors changed how long the flies took to grow, how big they got, and most importantly, how many offspring they produced. The researchers were looking for a clear link between how fast a fly developed and its success in leaving behind the next generation.

The results revealed a surprising disconnect between how much a trait changes and how important that change is for survival. The temperature had a massive effect on the flies' growth speed. At the cooler temperature, it took the flies four to five times longer to reach adulthood compared to the warm temperature. This was a huge plastic response, a dramatic shift in behavior driven by the cold. In contrast, the low-calorie diet only extended the development time by about 20 to 30 percent. If one were to judge by the size of the change alone, the cold temperature would seem to be the dominant force shaping the flies' lives. However, the story of evolution told a different tale. While the cold made the flies wait much longer, it did not change which flies were the most successful. In the warm conditions and on the standard diet, the flies that developed the fastest were the ones that produced the most offspring. This advantage for speed held true even in the cold, where the slowest developers were still the least successful in terms of reproduction.

The real shift in the rules of survival came from the food, not the heat. When the flies were fed the low-calorie diet, the advantage of growing fast vanished. In fact, on the poor diet at the cool temperature, the flies that took longer to develop actually produced more offspring than those that hurried. The nutritional stress flipped the direction of natural selection. Instead of favoring speed, the environment now favored patience. This change was not just a temporary reaction; it was accompanied by a massive increase in the genetic variety available for evolution to work with. On the low-calorie diet, the genetic differences between families became ten to sixteen times more pronounced than on the standard diet. This means that when food is scarce, the population holds a much wider range of potential solutions to the problem of growth, giving it a greater capacity to evolve.

The researchers also looked at whether the ability to change one's growth speed in response to the environment was itself a trait that helped flies survive. They found no evidence that being more flexible was an advantage. A fly that changed its development time drastically between good and bad conditions did not necessarily have more children than a fly that stayed the same. The success of a fly depended entirely on the specific traits it expressed in the environment it was currently living in, not on how well it could adjust. Furthermore, the genetic makeup of the flies that did well on a rich diet was almost completely different from the makeup of those that did well on a poor diet. A family that grew quickly on good food did not necessarily grow quickly on bad food, suggesting that the genes controlling growth are highly sensitive to nutrition.

This study highlights a crucial lesson for understanding how life adapts to a changing world. The factor that causes the biggest visible change in an organism's body is not necessarily the one that drives its evolution. In this case, the cold temperature caused a much larger physical change in growth time than the lack of food did, yet it was the lack of food that rewrote the rules of survival and unlocked new genetic potential. For populations facing global change, this means that shifts in food availability could be more powerful drivers of evolution than temperature alone, even if the temperature changes are more obvious. The ability of a species to persist may depend less on how much it can physically alter its growth schedule and more on how the scarcity of resources reshapes the very definition of a successful life.

Technical Summary: Development Time Plasticity, Selection, and Evolutionary Potential Under Temperature and Nutritional Stress

Problem Statement
Global change imposes multidimensional environmental stressors, particularly warming temperatures and unpredictable food availability, which threaten natural populations. While phenotypic plasticity allows organisms to track environmental shifts within a generation, the limits of plasticity and the subsequent reliance on evolutionary responses depend on how selection acts on heritable variation under realistic stressor combinations. Development time is a central life-history trait influencing body size, reproductive timing, and fitness. However, direct estimates of how selection on development time varies across combinations of environmental stressors are rare. Consequently, it remains unclear whether the stressor causing the largest plastic response in a trait is also the one that most significantly alters its selection pressures and evolutionary potential.

Methodology
The study utilized Drosophila melanogaster to investigate the interplay between temperature and nutritional stress.

Population Design: A large, mass-bred population was created by combining flies from eight locations across Victoria, Australia, to maximize genetic variation. From this, 750 full-sibling families were established using a nested half-sibling breeding design.
Experimental Setup: The researchers selected the 34 fastest and 34 slowest developing families from an initial screening (Experiment 1) and subjected them to a full-factorial design in Experiment 2. The treatments included two temperatures (14°C and 28°C) and two diets (standard and low-calorie, representing a 25% dilution).
Measurements:
- Development Time: Measured from egg to adult eclosion.
- Fitness Proxy: Productivity (number of adult offspring produced by individual females) was measured, along with egg-to-adult viability at the family level.
- Morphology: Body mass and wing size (centroid size) were recorded for individual females.
Statistical Analysis:
- Plasticity: Assessed using linear mixed models to determine the magnitude of phenotypic shifts across treatments.
- Selection: Directional and non-linear selection gradients were estimated using generalized linear mixed models (GLMMs) with a zero-inflated negative binomial distribution to model productivity. Selection was analyzed both at the individual level (fecundity selection) and the family level (genotypic selection including viability).
- Genetic Variation: Genotype-by-environment (G×E) interactions were quantified using Bayesian linear mixed models (MCMCglmm) to estimate broad-sense heritability ( $H^2$ ), evolvability ( $I_G$ ), and genetic correlations across the four treatment combinations.

Key Results

Differential Plastic Responses: Temperature induced the largest plastic changes in development time, causing a 4–5-fold increase in duration at 14°C compared to 28°C. Nutritional stress (low-calorie diet) caused a more modest increase, extending development by approximately 20–30%.
Divergent Selection Pressures: Contrary to the magnitude of plasticity, diet—not temperature—determined the direction of selection on development time.
- On the standard diet, faster development was associated with higher fitness at both temperatures.
- On the low-calorie diet, the advantage of fast development disappeared. At 14°C, slower-developing females produced more offspring.
- Selection for larger wing size was stronger under nutritional stress, while selection on body mass was generally non-significant or favored intermediate sizes at 28°C.
Genetic Variation and G×E: Nutritional stress significantly increased the evolvability of development time (10–16-fold higher on low-calorie diets compared to standard diets) and broad-sense heritability. Genetic correlations between diets were close to zero (e.g., -0.12 at 14°C), indicating strong G×E interactions where genotypes ranking high in development speed on one diet did not necessarily rank high on the other. Genetic correlations across temperatures were also weaker under nutritional stress.
Plasticity and Fitness: There was no evidence that the degree of plasticity in development time (the ability to change development time across environments) was itself associated with fitness. Fitness was determined by the trait values expressed within specific environments rather than the plastic response itself.

Significance and Claims
The paper argues that the stressor causing the greatest plastic change in a functional trait is not necessarily the one that shapes its selection and evolution. Specifically:

Nutritional Stress as a Primary Driver: While temperature drives the magnitude of phenotypic change, nutritional stress reshapes the adaptive landscape by altering directional selection and exposing substantial genetic variation.
Context-Dependent Evolution: The evolutionary potential of development time is highly context-dependent. The near-zero genetic correlations between diets suggest that evolutionary responses to selection on development time will be constrained if environments fluctuate between high and low nutrition, as selection favors different genotypes in each.
Decoupling of Plasticity and Selection: The study demonstrates that large plastic responses (induced by temperature) do not guarantee alignment with selection pressures. Conversely, smaller plastic responses (induced by diet) can coincide with major shifts in selection and increased evolvability.
Implications for Global Change: Predicting population responses to climate change requires understanding how multiple stressors interact to alter not just mean phenotypes, but the selective landscape and the availability of genetic variance. The authors conclude that nutritional conditions may play a more prominent role than temperature in shaping contemporary selection and evolutionary potential for life-history traits, even in warming environments.

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