Table of Contents
Key Idea: What Is Discontinuous Variation?
Under modification, you already learned that the same genotype can produce different phenotypes depending on environmental influences. Discontinuous variation is one special pattern of how such differences (or, more generally, phenotypic differences) can appear in a population.
Discontinuous variation (also called qualitative variation) means that individuals fall into clearly separated categories, with no smooth intermediate forms between them. Each individual belongs to one category or another, but not “in between”.
This is in contrast to continuous variation, where traits form a smooth range (for example, body height).
In discontinuous variation:
- Phenotypes are distinct (e.g., blood group A, B, AB, or 0).
- You cannot arrange the categories by “more” or “less” of the trait on a smooth scale.
- Intermediate forms do not appear (or are extremely rare and still form their own distinct category).
- Often, few genes with clear effects and/or threshold-like environmental influences are involved.
Discontinuous variation can arise from:
- purely genetic differences,
- purely environmental modifications that flip a trait “on” or “off”,
- or a combination of both.
In this chapter we focus on discontinuous variation as a pattern of phenotypic expression and modification, not on Mendelian genetics themselves (covered elsewhere).
Characteristics of Discontinuous Variation
1. Distinct Phenotypic Classes
Traits with discontinuous variation show separate, recognizable forms. Examples (as patterns, not yet in full detail):
- Human ABO blood groups: A, B, AB, 0.
- Presence vs. absence of a trait (e.g., attached vs. free earlobes, if used as a classroom example).
- Certain flower colors in some plants: red vs. white, with no pink in between in strict monogenic traits.
Key points:
- Each individual fits neatly into one category.
- The categories are often named, not measured (e.g., “A” or “B”, not “2.4 units of blood type”).
- The pattern in a population often looks like separate piles, rather than a smooth curve.
2. Little or No Gradation Between Classes
If you were to line up the individuals by this trait, you would jump from one phenotype to another, instead of gradually moving along a smooth continuum.
For example, for blood groups, you never meet someone who is “a bit A and a bit halfway to B” outside of the defined categories; they belong to a defined blood group.
3. Often Controlled by Few Major Factors
Where genetics is involved (detailed principles are covered in Mendelian chapters):
- Often one or a few genes with major effects largely determine the phenotype.
- Different alleles (gene variants) lead to qualitatively different outcomes.
- Environmental influences may have little visible effect on the trait, or
- The environment may act like an on/off switch, pushing the phenotype across a threshold into a different category.
For modifications, the last point is especially relevant: the same genotype may develop into different discrete states if exposed to different environmental conditions.
Discontinuous Variation as a Form of Modification
In the broader chapter “Modification” you learned that environmental influences can change the phenotype without changing the genotype. Discontinuous variation is one way in which this can appear:
- The environment does not gradually shift a trait, but instead:
- Switches development into one of several alternative pathways,
- Producing clearly different phenotypic categories.
So, while many modifications cause continuous variation (e.g., gradual differences in body size due to nutrition), some cause discrete, alternative forms.
1. Reaction Norms With Discrete Outcomes
A reaction norm describes which phenotypes a genotype can produce under different environments. For discontinuous variation:
- The reaction norm is not a smooth curve, it looks more like steps.
- Below or above certain critical conditions, development jumps into a different stable state.
Conceptually:
- Same genotype
- Two (or more) different environments
- Two (or more) alternative stable phenotypes
This is often called polyphenism in developmental biology (multiple phenotypes from one genotype) and is an important special case of modification.
2. Examples of Environmentally Driven, Discontinuous Modifications
These examples illustrate the pattern, not the genetic details:
a) Caste Formation in Social Insects (e.g., bees, ants)
- Many social insects show queen and worker castes, sometimes soldiers as well.
- Often, larvae have very similar or identical genotypes.
- Differences in nutrition or hormones during development (e.g., feeding with royal jelly in honeybees) lead to:
- Queen phenotype (fertile, large, different body shape)
- Worker phenotype (usually sterile, different shape and behavior)
This is a classic discontinuous variation:
- No gradual series from “a little queen-like” to “fully queen”.
- Individuals become queen or worker, a clear categorical difference stemming from environmental modification.
b) Seasonal Morphs in Some Insects
Some butterflies or insects develop different seasonal forms (e.g., summer vs. winter morphs):
- Temperature or day length during development can trigger:
- A dark morph vs. a light morph,
- Different wing patterns,
- Different body sizes or shapes.
- Again, the same genotype can lead to two or more clearly distinct phenotypes, depending on environmental cues.
This yields discontinuous variation within the species: individuals cluster into distinct morphs rather than forming a continuous gradient.
c) Environmentally Determined Sex in Some Species
(Genotypic sex determination is treated elsewhere; here we focus on the pattern.)
In many fish, reptiles, and some invertebrates:
- Sex can be determined by environmental conditions, such as temperature during early development (temperature-dependent sex determination).
- The outcome is often discrete: individuals become male or female, not something in between, despite having very similar genetic backgrounds.
From a variation perspective, this is also a discontinuous pattern:
- Traits linked to sex (gonads, secondary sexual traits, behavior) form clear categories,
- Triggered in part by environmental modifications.
Distinguishing Discontinuous From Continuous Variation (in the Context of Modification)
It is useful to contrast the two main patterns, especially for environmentally influenced traits:
Continuous Variation (Context: Modification)
- Many small steps between extremes.
- Examples (as patterns):
- Body mass of animals depending on food supply.
- Leaf size variation of the same plant species in sunny vs. shady spots.
- Environment gradually shifts the phenotype.
- Reaction norm looks like a smooth line or curve.
Discontinuous Variation (Context: Modification)
- Few, clearly separated phenotypic classes.
- Examples (as patterns):
- Worker vs. queen castes in insects.
- Different seasonal morphs.
- Environmentally determined sex categories.
- Environment pushes development into alternative stable states.
- Reaction norm looks like steps between discrete phenotypes.
In both cases, the genotype remains unchanged; the environment influences how the genetic potential is expressed.
Population-Level View of Discontinuous Variation
When looking at a population, discontinuous variation appears in characteristic ways.
1. Frequency of Categories
Instead of measuring a continuous trait and seeing a bell-shaped curve, you would count how many individuals fall into each category.
For example (simplified pattern):
- 60% workers
- 5% queens
- 35% soldiers
Or:
- 70% summer morph
- 30% winter morph
Such data are usually shown in:
- Bar charts (one bar per category),
- or tables of counts or percentages, not in a smooth frequency curve.
2. Environmental Shifts Can Change Category Proportions
For modification-based discontinuous variation:
- Changing the environment (e.g., food availability, temperature, day length) can change the relative numbers in each category,
- Without changing the underlying gene frequencies in the population.
Examples (in pattern):
- Warmer incubation temperatures might produce more individuals of one sex than the other in a species with temperature-dependent sex determination.
- Abundant high-quality food might increase the proportion of queen larvae that actually become queens.
This is a key difference from genetic changes: the same generation can show different category proportions just by different environmental exposures.
Practical Considerations and Significance
1. Importance in Developmental Biology and Ecology
Discontinuous variation based on environmental modification shows that:
- Development is flexible and responsive to the environment.
- Organisms can switch between alternative phenotypes depending on conditions.
- This flexibility (e.g., queen/workers, seasonal morphs) can be adaptive, letting the same genotype fit into different ecological roles.
2. Relevance for Experiments on Modification
When studying modification experimentally (details of methods are covered elsewhere), discontinuous variation is often used because:
- The outcome is easy to score (e.g., queen vs. worker).
- Researchers can test how environmental factors (like food, temperature, light) influence the frequency of each category.
This allows clear demonstrations that environment alone, acting on a stable genotype, can produce different, discrete phenotypes.
Summary
- Discontinuous variation is a pattern where phenotypes form distinct categories with no gradual intermediates.
- In the context of modification, the same genotype can develop into different discrete phenotypes when exposed to different environmental conditions.
- Typical features:
- Clear-cut classes (e.g., queen vs. worker, seasonal morphs, environment-determined sexes),
- Often few major developmental pathways,
- Environmental factors often act like switches, not like fine‑tuning knobs.
- At the population level, you count how many individuals are in each category, and environmental changes can shift these proportions without altering genotypes.