Table of Contents
Understanding Continuous Variation
In this chapter, we focus on continuous variation as a special pattern of phenotypic diversity. You have already learned what modification is in general and how environment can influence the phenotype without altering the genotype. Here we look specifically at traits that show a continuous range of forms.
What Is Continuous Variation?
Continuous variation describes traits that do not fall into sharply separated classes, but instead form a smooth spectrum of values. Between the smallest and largest observable forms, all or almost all intermediate forms occur.
Examples in humans:
- Body height
- Body weight (under stable conditions)
- Skin color
- Many physiological measures, e.g. blood pressure, resting pulse
If you measure such traits in a large population and plot a frequency diagram (histogram), you typically get a bell-shaped curve: many individuals are near the average, and fewer have very small or very large values.
Key features of continuous variation:
- No clear-cut “types” (unlike blood groups, which are A, B, AB, 0, etc.).
- Differences between individuals are often small and gradual.
- Trait values can usually be measured on a scale (e.g. cm, kg, °C).
Continuous vs. Discontinuous (Discrete) Variation
To understand continuous variation, it helps to contrast it briefly with discontinuous variation (covered in detail elsewhere):
- Discontinuous variation
- Traits fall into distinct categories (e.g. flower color red/white, blood group A/B/AB/0).
- Usually controlled by single genes with large effect, often with simple Mendelian inheritance.
- Environment often has little influence on which category an individual belongs to.
- Continuous variation
- Traits form a continuum of values.
- Usually determined by many genes and the environment.
- Each gene contributes only a small amount to the final phenotype.
Continuous variation is therefore typical of quantitative traits.
Quantitative Traits and Polygenic Inheritance
Traits showing continuous variation are often called quantitative traits because they can be measured numerically. In many cases, continuous variation arises from polygenic inheritance:
- Polygenic inheritance means that many genes (loci) influence the same trait.
- Each gene contributes a small, often additive effect.
- The more such genes are involved, the smoother the distribution of the trait in the population.
Simplified example:
- Imagine 3 genes (A, B, C), each with 2 alleles: a “plus” allele (
A,B,C) that increases height slightly and a “minus” allele (a,b,c) that does not. - Each plus allele adds a small fixed amount to the trait value.
- Individuals can then have from 0 to 6 plus alleles.
- The more plus alleles, the greater the trait expression.
In a large, randomly mating population, the combinations of plus and minus alleles are distributed such that many individuals have medium numbers of plus alleles and fewer individuals have very low or very high numbers. This alone already creates a bell-shaped spread of phenotypes.
Important: We do not see the genotypes directly – we see their combined effect as a gradual range of phenotypes.
Role of the Environment in Continuous Variation
Continuous variation is typically strongly influenced by environmental factors, making it a classic example of modification:
- The genotype sets a reaction norm (reaction range): the range of phenotypes that are possible under different environmental conditions.
- The environment determines where within this range the actual phenotype ends up.
For example, body height:
- Genetically, an individual might have the potential to become 170–180 cm tall.
- Nutritional status, diseases in childhood, and other environmental influences can shift the actual height up or down within this range.
- In a population, this environmental variability combines with genetic variability, broadening and smoothing the distribution of heights.
In continuous variation, therefore, observed differences between individuals usually reflect:
- Genetic differences (different combinations of many small-effect alleles), and
- Environmental differences (nutrition, climate, lifestyle, etc.), and
- Often interactions between genes and environment.
Continuous Variation and Normal Distribution
Many continuously varying traits in large populations approximate a normal distribution (bell-shaped curve). This arises because:
- Numerous independent, small genetic effects add up.
- Numerous independent, small environmental influences add as well.
When a trait is influenced by many small additive factors, the overall distribution tends to become symmetric around an average value, with:
- Most individuals near the mean.
- Fewer individuals at the extremes.
This is why body height or many physiological traits show bell-shaped distributions.
However, not all continuous traits are perfectly normal:
- Some may be skewed (e.g. more small individuals than large ones).
- Some may be multimodal (more than one peak) if there are subpopulations with different allele frequencies or different environments.
Recognizing such patterns is an important step in understanding how genetic and environmental factors structure variation.
Threshold Traits: Hidden Continuous Liability
Some traits appear discrete (affected vs. unaffected), but are actually influenced by underlying continuous variation in risk or “liability”. These are called threshold traits.
Concept:
- Many genes and environmental factors contribute continuously to a hidden “liability value” for a condition (e.g. risk of developing a disease).
- Only when this value exceeds a threshold does the trait become visible (e.g. the disease appears).
- Below the threshold: the phenotype looks normal; above the threshold: the individual is affected.
This illustrates:
- A continuous underlying predisposition.
- A discontinuous visible outcome (yes/no).
Such traits help bridge the understanding between clearly continuous variation and apparently discrete outcomes.
Measuring Continuous Variation
Because continuous traits are quantitative, they are studied with statistical methods. Core concepts:
- Mean (average):
$$\bar{x} = \frac{1}{n} \sum_{i=1}^{n} x_i$$
where $x_i$ is the measured value of individual $i$, and $n$ is the number of individuals measured. - Variance and standard deviation:
These describe how spread out the values are around the mean.
A large standard deviation means high variability; a small one means individuals are more similar.
For genetic studies of continuous variation, two additional ideas are important (detailed treatment belongs in later chapters):
- Heritability (in the broad sense): proportion of total phenotypic variance that is due to genetic differences in a particular population and environment.
- Environmental variance: part of the phenotypic variance caused by environmental differences.
Formally (schematically, without going into methodological details):
$$
V_P = V_G + V_E
$$
where:
- $V_P$ = phenotypic variance (observed variation in the trait)
- $V_G$ = genetic variance
- $V_E$ = environmental variance
This decomposition is central for understanding how much of continuous variation is genetically vs. environmentally determined, always for a specific population in a specific environment.
Biological and Practical Significance of Continuous Variation
Continuous variation is fundamental for several reasons:
- Basis for natural selection
- Natural selection acts on phenotypic differences within populations.
- For many traits important for survival and reproduction (e.g. size, fecundity, stress resistance), selection “sees” a continuous range of values.
- Small, cumulative differences can gradually shift the average of a population over generations.
- Importance in breeding and agriculture
- Yield, growth rate, milk production, fat content, disease resistance, etc., are usually quantitative traits.
- Breeding programs aim to shift the mean of these traits by selecting individuals with favorable values.
- Because many genes and the environment are involved, progress is typically gradual, not abrupt.
- Relevance for human health
- Many medically important characteristics – e.g. blood pressure, body mass index, cholesterol levels – show continuous variation.
- The risk for many common diseases (e.g. type 2 diabetes, coronary heart disease) depends on a complex interaction of many genes with small effects and environmental/lifestyle factors.
- Understanding continuous variation allows for concepts like “risk factors,” “polygenic risk,” and “prevention” to be meaningfully framed.
Summary
- Continuous variation refers to traits that form a smooth range of phenotypes with many intermediate forms.
- It usually arises from polygenic inheritance combined with environmental influences.
- Many such traits approximate a normal distribution in large populations.
- Threshold traits show that a continuously varying liability can lead to a discrete (yes/no) outcome.
- Continuous variation is central to natural selection, breeding, and understanding complex human traits and diseases.