Table of Contents
Modification refers to non-heritable changes in the traits (phenotype) of an organism that are caused by environmental influences, not by alterations in the DNA sequence. In other words: the genotype stays the same, but the way it is “expressed” changes depending on conditions.
This chapter focuses on:
- how and why the same genotype can lead to different phenotypes
- how to distinguish modifications from true genetic differences
- the biological meaning of phenotypic flexibility
Details on continuous vs. discontinuous variation and phenotypic sex determination are treated in their own subsections; here we look at the overarching idea of modification.
Genotype, Phenotype, and the Role of the Environment
Every trait results from an interaction between:
- the genetic information (genotype)
- the environment (external factors such as temperature, nutrition, light, toxins, social conditions, etc.)
- internal conditions (e.g. hormones, developmental stage)
In pure modification, the DNA sequence is unchanged. Environmental conditions influence:
- which genes are turned on or off
- how strongly they are expressed
- developmental processes that depend on these gene activities
This produces different phenotypes from the same genotype.
Important:
- Modifications are usually not inherited by offspring (in the classical genetic sense).
- The offspring shows its own reaction to the environment according to its genotype.
Typical Features of Modifications
Modifications tend to show a set of characteristic properties that help distinguish them from genetic changes (mutations):
- Reversibility:
Many modifications disappear when the environmental condition is removed or reversed. - Example: Muscle size increases with training and decreases again when training stops.
- No change in allele frequencies:
In a population, modification does not alter which alleles are present or how common they are; only the appearance of individuals changes. - Predictable dependence on conditions:
The form of the trait often follows a regular rule in relation to an environmental factor (e.g. more light → darker pigment; more food → larger body size). - Occurs in many individuals at once:
If the environment changes for a whole group, many individuals show similar phenotypic shifts. - Usually not transmitted to the next generation:
The offspring will only show the modified form if it experiences similar environmental conditions.
Reaction Norm (Norm of Reaction)
Each genotype has a reaction norm (also called norm of reaction): the range of phenotypes it can produce across different environments.
You can think of it as a function:
- Input: environmental condition (e.g. temperature, nutrient level, light intensity)
- Output: resulting phenotype (e.g. body size, color intensity, growth rate)
For a simple trait, such a relationship can often be drawn as a curve:
- on the x-axis: environment (e.g. temperature)
- on the y-axis: phenotype (e.g. body length)
Different genotypes have different reaction norms:
- Some are stable: phenotype changes very little across environments (low phenotypic plasticity).
- Some are flexible: phenotype changes strongly with environment (high phenotypic plasticity).
Modification is the actual, observed change within that reaction norm.
Phenotypic Plasticity
The ability of an organism to produce different phenotypes from the same genotype in response to different environmental conditions is called phenotypic plasticity.
Key points:
- Plasticity is itself genetically determined:
The genes specify how sensitive a trait is to the environment. - Plasticity can be:
- Adaptive: The environmentally induced phenotype increases survival or reproduction in that environment.
- Non-adaptive: The change is just a side effect with no clear advantage.
- Plastic traits can be:
- Morphological: e.g. leaf shape, body size, presence of defensive structures.
- Physiological: e.g. enzyme activities, metabolic rates.
- Behavioral: e.g. foraging strategies, social behavior.
In this chapter, “modification” is mainly used for morphology and physiology, but the same principle applies to behavior.
Classic Examples of Modification
Environmental Effects on Size and Shape
Many organisms show size differences purely from environmental conditions:
- Nutrition:
- Individuals with abundant, high-quality food grow larger and may mature earlier.
- Undernourished individuals stay smaller, even with the same genes.
- Density and crowding:
High population density can slow growth, reduce size, or alter body form due to competition and stress.
In such cases:
- The size difference is a modification.
- The genetic potential for growth capacity is still there in all individuals.
Temperature Effects on Development
Temperature affects biochemical reactions and developmental patterns:
- Developmental speed:
Warm conditions speed up development in many ectotherms (insects, amphibians, reptiles); cool conditions slow it down. - Morphological traits:
- Coloration intensity can depend on developmental temperature.
- Shape proportions (e.g. limb length) may shift with temperature during growth.
The underlying genotype sets limits and rules, but the actual form is a product of this genotype–temperature interaction.
Effects of Light in Plants
Plants react strongly to light intensity and light direction:
- Form and structure of leaves:
- Sun leaves are often thicker, smaller, and more robust.
- Shade leaves are thinner, larger, and more delicate.
- Overall plant form:
- Low light → elongated, pale growth (etiolation).
- High light → compact, well-branched form with intense green coloration.
These differences are usually reversible if light conditions change early enough in development and are therefore typical modifications.
Seasonal and Environmental Color Changes
Certain animals change color with season or background:
- Seasonal coat changes (e.g. light summer coat vs. thick winter coat).
- Pigment adjustments influenced by light intensity or background color.
The underlying genetic program allows for such responses, but the environment triggers which phenotype appears at a given time.
Modification vs. Mutation and Adaptation
It is important to distinguish:
- Modification
- Phenotypic change.
- Environment-induced.
- DNA sequence unchanged.
- Typically not inherited.
- Mutation
- Change in DNA sequence itself.
- Can be inherited.
- May affect the reaction norm and thus future modifications.
- Adaptation (in the evolutionary sense)
- A trait that has become common in a population due to natural selection over generations.
- Often encoded genetically.
- May include an evolved capacity for specific modifications (plasticity).
Modification acts at the level of individual life.
Mutation and adaptation act at the level of populations across generations.
Biological Significance of Modification
Modifications and phenotypic plasticity have several important consequences:
- Short-term adjustment to variable environments:
Individuals can cope with changing conditions without requiring genetic change in every generation. - Buffering against environmental stress:
Plastic responses can maintain core functions (e.g. metabolism, reproduction) under suboptimal conditions. - Influence on selection:
- Plasticity can hide or reveal genetic differences, affecting which genotypes are favored by natural selection.
- Under some circumstances, environmentally induced phenotypes can later become genetically fixed if mutations lock in a beneficial form (a process often discussed as “genetic assimilation”).
- Relevance for experiments and breeding:
When interpreting trait differences between individuals or lines, it is necessary to separate: - variation due to genotype
- variation due to modification by the environment
Recognizing Modifications in Practice
In simple experimental designs, modifications can be identified by:
- Common garden experiments:
Individuals of different origin are raised under identical conditions. - Remaining differences are mostly genetic.
- Disappearing differences were largely modifications.
- Reciprocal transplant or environment-switch experiments:
Same genotype placed in different environments. - If the phenotype changes, the difference is due to modification.
At an introductory level, the key diagnostic question is:
- Does the trait difference persist across generations when the environment is standardized?
- If yes, it suggests a genetic difference.
- If no, it points to modification.
Summary
- Modification is a non-heritable, environment-induced change in phenotype.
- The genotype remains unchanged; what changes is how it is expressed under particular conditions.
- Each genotype has a reaction norm, describing which phenotypes it can produce across environments; the degree of change is called phenotypic plasticity.
- Modifications are often reversible, predictably related to environmental factors, and do not alter gene frequencies in the population.
- They allow organisms to adjust quickly to changing environments and must be distinguished from mutations and evolutionary adaptations.